A role for flexible loops in enzyme catalysis

A role for conformational changes in enzyme catalysis

The role played by conformational changes in enzyme catalysis is controversial. In addition to examining specific enzymes, studying formal models can help identify the conditions under which conformational changes promote catalysis. Here, we present a model demonstrating how conformational changes can break a generic trade-off due to the conflicting requirements of successive steps in catalytic cycles, namely high specificity for the transition state to accelerate the chemical transformation and low affinity for the products to favor their release. The mechanism by which the trade-off is broken is a transition between conformations with different affinities for the substrate. The role of the effector that induces the transition is played by a substrate "handle," a part of the substrate that is not chemically transformed but whose interaction with the enzyme is nevertheless essential to rapidly complete the catalytic cycle. A key element of the model is the formalization of the constraints causing the trade-off that the presence of multiple states breaks, which we attribute to the strong chemical similarity between successive reaction states-substrates, transition states, and products. For the sake of clarity, we present our model for irreversible one-step unimolecular reactions. In this context, we demonstrate how the different forms that chemical similarities between reaction states can take impose limits on the overall catalytic turnover. We first analyze catalysts without internal degrees of freedom and then show how two-state catalysts can overcome their limitations. Our results recapitulate previous proposals concerning the role of conformational changes and substrate handles in a formalism that makes explicit the constraints that elicit these features. In addition, our approach establishes links with studies in the field of heterogeneous catalysis, where the same trade-offs are observed and where overcoming them is a well-recognized challenge.

Activation and friction in enzymatic loop opening and closing dynamics

Protein loop dynamics have recently been recognized as central to enzymatic activity, specificity and stability. However, the factors controlling loop opening and closing kinetics have remained elusive. Here, we combine molecular dynamics simulations with string-method determination of complex reaction coordinates to elucidate the molecular mechanism and rate-limiting step for WPD-loop dynamics in the PTP1B enzyme. While protein conformational dynamics is often represented as diffusive motion hindered by solvent viscosity and internal friction, we demonstrate that loop opening and closing is activated. It is governed by torsional rearrangement around a single loop peptide group and by significant friction caused by backbone adjustments, which can dynamically trap the loop. Considering both torsional barrier and time-dependent friction, our calculated rate constants exhibit very good agreement with experimental measurements, reproducing the change in loop opening kinetics between proteins. Furthermore, we demonstrate the applicability of our results to other enzymatic loops, including the M20 DHFR loop, thereby offering prospects for loop engineering potentially leading to enhanced designs.

A precise swaying map for how promiscuous cellobiose-2-epimerase operate bi-reaction

Promiscuous enzymes play a crucial role in organism survival and new reaction mining. However, comprehensive mapping of the catalytic and regulatory mechanisms hasn't been well studied due to the characteristic complexity. The cellobiose 2-epimerase from Caldicellulosiruptor saccharolyticus (CsCE) with complex epimerization and isomerization was chosen to comprehensively investigate the promiscuous mechanisms. Here, the catalytic frame of ring-opening, cis-enediol mediated catalysis and ring-closing was firstly determined. To map the full view of promiscuous CE, the structure of CsCE complex with the isomerized product glucopyranosyl-β1,4-fructose was determined. Combined with computational calculation, the promiscuity was proved a precise cooperation of the double subsites, loop rearrangement, and intermediate swaying. The flexible loop was like a gear, whose structural reshaping regulates the sway of the intermediates between the two subsites of H377-H188 and H377-H247, and thus regulates the catalytic directions. The different protonated states of cis-enediol intermediate catalyzed by H188 were the key point for the catalysis. The promiscuous enzyme tends to utilize all elements at hand to carry out the promiscuous functions.

Characterization and Structural Analyses of Enolase from Shrimp (Litopenaeus vannamei)

Transcriptome analysis had recognized enolase from shrimp Litopenaeus vannamei (L. vannamei), which is termed LvEnolase, as one of the allergens, but its amino acid sequence and protein structure have been lacking. In this study, natural LvEnolase was isolated from L. vannamei and characterized for the first time. The full-length cDNA sequence of LvEnolase was effectively cloned, which encoded 434 amino acid residues. The crystal structure of LvEnolase was successfully determined at a resolution of 2.5 Å by X-ray crystallography (PDB: 8UEL). Notably, it was observed that near the active center, a loop exists in either an open or closed state, and the open loop was associated with the product release phase. Furthermore, enzyme activity assays were conducted to validate the catalytic capabilities of purified LvEnolase. These findings significantly enhance our comprehension of the enolase family and provide valuable support for delving into the functions and characteristics of LvEnolase.

Harnessing generative AI to decode enzyme catalysis and evolution for enhanced engineering

Enzymes, as paramount protein catalysts, occupy a central role in fostering remarkable progress across numerous fields. However, the intricacy of sequence-function relationships continues to obscure our grasp of enzyme behaviors and curtails our capabilities in rational enzyme engineering. Generative artificial intelligence (AI), known for its proficiency in handling intricate data distributions, holds the potential to offer novel perspectives in enzyme research. Generative models could discern elusive patterns within the vast sequence space and uncover new functional enzyme sequences. This review highlights the recent advancements in employing generative AI for enzyme sequence analysis. We delve into the impact of generative AI in predicting mutation effects on enzyme fitness, catalytic activity and stability, rationalizing the laboratory evolution of de novo enzymes, and decoding protein sequence semantics and their application in enzyme engineering. Notably, the prediction of catalytic activity and stability of enzymes using natural protein sequences serves as a vital link, indicating how enzyme catalysis shapes enzyme evolution. Overall, we foresee that the integration of generative AI into enzyme studies will remarkably enhance our knowledge of enzymes and expedite the creation of superior biocatalysts.

Comprehensive assessment of protein loop modeling programs on large-scale datasets: prediction accuracy and efficiency

Protein loops play a critical role in the dynamics of proteins and are essential for numerous biological functions, and various computational approaches to loop modeling have been proposed over the past decades. However, a comprehensive understanding of the strengths and weaknesses of each method is lacking. In this work, we constructed two high-quality datasets (i.e. the General dataset and the CASP dataset) and systematically evaluated the accuracy and efficiency of 13 commonly used loop modeling approaches from the perspective of loop lengths, protein classes and residue types. The results indicate that the knowledge-based method FREAD generally outperforms the other tested programs in most cases, but encountered challenges when predicting loops longer than 15 and 30 residues on the CASP and General datasets, respectively. The ab initio method Rosetta NGK demonstrated exceptional modeling accuracy for short loops with four to eight residues and achieved the highest success rate on the CASP dataset. The well-known AlphaFold2 and RoseTTAFold require more resources for better performance, but they exhibit promise for predicting loops longer than 16 and 30 residues in the CASP and General datasets. These observations can provide valuable insights for selecting suitable methods for specific loop modeling tasks and contribute to future advancements in the field.

Biochemical and molecular characterization of a novel glycerol dehydratase from Klebsiella pneumoniae 2e with high tolerance against crude glycerol impurities

BACKGROUND

The direct bioconversion of crude glycerol, a byproduct of biodiesel production, into 1,3-propanediol by microbial fermentation constitutes a remarkably promising value-added applications. However, the low activity of glycerol dehydratase, which is the key and rate-limiting enzyme in the 1,3-propanediol synthetic pathway, caused by crude glycerol impurities is one of the main factors affecting the 1,3-propanediol yield. Hence, the exploration of glycerol dehydratase resources suitable for crude glycerol bioconversion is required for the development of 1,3-propanediol-producing engineered strains.

RESULTS

In this study, the novel glycerol dehydratase 2eGDHt, which has a tolerance against crude glycerol impurities from Klebsiella pneumoniae 2e, was characterized. The 2eGDHt exhibited the highest activity toward glycerol, with Km and Vm values of 3.42 mM and 58.15 nkat mg-1, respectively. The optimum pH and temperature for 2eGDHt were 7.0 and 37 °C, respectively. 2eGDHt displayed broader pH stability than other reported glycerol dehydratases. Its enzymatic activity was increased by Fe2+ and Tween-20, with 294% and 290% relative activities, respectively. The presence of various concentrations of the crude glycerol impurities, including NaCl, methanol, oleic acid, and linoleic acid, showed limited impact on the 2eGDHt activity. In addition, the enzyme activity was almost unaffected by the presence of an impurity mixture that mimicked the crude glycerol environment. Structural analyses revealed that 2eGDHt possesses more coil structures than reported glycerol dehydratases. Moreover, molecular dynamics simulations and site-directed mutagenesis analyses implied that the existence of unique Val744 from one of the increased coil regions played a key role in the tolerance characteristic by increasing the protein flexibility.

CONCLUSIONS

This study provides insight into the mechanism for enzymatic action and the tolerance against crude glycerol impurities, of a novel glycerol dehydratase 2eGDHt, which is a promising glycerol dehydratase candidate for biotechnological conversion of crude glycerol into 1,3-PDO.

Triosephosphate Isomerase: The Crippling Effect of the P168A/I172A Substitution at the Heart of an Enzyme Active Site

The P168 and I172 side chains sit at the heart of the active site of triosephosphate isomerase (TIM) and play important roles in the catalysis of the isomerization reaction. The phosphodianion of substrate glyceraldehyde 3-phosphate (GAP) drives a conformational change at the TIM that creates a steric interaction with the P168 side chain that is relieved by the movement of P168 that carries the basic E167 side chain into a clamp that consists of the hydrophobic I172 and L232 side chains. The P168A/I172A substitution at TIM from Trypanosoma brucei brucei (TbbTIM) causes a large 120,000-fold decrease in kcat for isomerization of GAP that eliminates most of the difference in the reactivity of TIM compared to the small amine base quinuclidinone for deprotonation of catalyst-bound GAP. The I172A substitution causes a > 2-unit decrease in the pKa of the E167 carboxylic acid in a complex to the intermediate analog PGA, but the P168A substitution at the I172A variant has no further effect on this pKa. The P168A/I172A substitutions cause a 5-fold decrease in Km for the isomerization of GAP from a 0.9 kcal/mol stabilization of the substrate Michaelis complexes. The results show that the P168 and I172 side chains play a dual role in destabilizing the ground-state Michaelis complex to GAP and in promoting stabilization of the transition state for substrate isomerization. This is consistent with an important role for these side chains in an induced fit reaction mechanism [Richard, J. P. (2022) Enabling Role of Ligand-Driven Conformational Changes in Enzyme Evolution. Biochemistry 61, 1533-1542].

Harnessing Generative AI to Decode Enzyme Catalysis and Evolution for Enhanced Engineering

Enzymes, as paramount protein catalysts, occupy a central role in fostering remarkable progress across numerous fields. However, the intricacy of sequence-function relationships continues to obscure our grasp of enzyme behaviors and curtails our capabilities in rational enzyme engineering. Generative artificial intelligence (AI), known for its proficiency in handling intricate data distributions, holds the potential to offer novel perspectives in enzyme research. By applying generative models, we could discern elusive patterns within the vast sequence space and uncover new functional enzyme sequences. This review highlights the recent advancements in employing generative AI for enzyme sequence analysis. We delve into the impact of generative AI in predicting mutation effects on enzyme fitness, activity, and stability, rationalizing the laboratory evolution of de novo enzymes, decoding protein sequence semantics, and its applications in enzyme engineering. Notably, the prediction of enzyme activity and stability using natural enzyme sequences serves as a vital link, indicating how enzyme catalysis shapes enzyme evolution. Overall, we foresee that the integration of generative AI into enzyme studies will remarkably enhance our knowledge of enzymes and expedite the creation of superior biocatalysts.

Supramolecular Regulation of Catalytic Activity in Molecularly Responsive Catalysts

Some enzymes switch between an open form and a closed form. We report a molecularly tuned catalyst that accommodates a substrate and a signal molecule simultaneously. Binding of the signal molecule helps direct the reactive group of the substrate to the catalytic group and enhances the catalytic activity. Subtle structural changes in either the substrate or the signal molecule are readily detected. The switching mechanism also allows the catalytic reaction to be turned on and off reversibly by specific molecular signals.

History of advances in enzyme kinetic methods: From minutes to milliseconds

The last review on transient-state kinetic methods in The Enzymes was published three decades ago (Johnson, K.A., 1992. The Enzymes, XX, 1-61). In that review the foundations were laid out for the logic behind the design and interpretation of experiments. In the intervening years the instrumentation has improved mainly in providing better sample economy and shorter dead times. More significantly, in 1992 we were just introducing methods for fitting data based on numerical integration of rate equations, but the software was slow and difficult to use. Today, advances in numerical integration methods for data fitting have led to fast and dynamic software, making it easy to fit data without simplifying approximations. This approach overcomes multiple disadvantages of traditional data fitting based on equations derived by analytical integration of rate equations, requiring simplifying approximations. Mechanism-based fitting using computer simulation resolves mechanisms by accounting for the concentration dependence of the rates and amplitudes of the reaction to find a set of intrinsic rate constants that reproduce the experimental data, including details about how the experiment was performed in modeling the data. Rather than discuss how to design and interpret rapid-quench and stopped-flow experiments individually, we now focus on how to fit them simultaneously so that the quench-flow data anchor the interpretation of fluorescence signals. Computer simulation streamlines the fitting of multiple experiments globally to yield a single unifying model to account for all available data.

Improvement of optimum pH and specific activity of pectate lyase from Bacillus RN.1 using loop replacement

Background: Alkaline pectate lyase plays an important role in papermaking, biological refining and wastewater treatment, but its industrial applications are largely limited owing to its low activity and poor alkali resistance. Methods: The alkaline pectate lyase BspPel from Bacillus RN.1 was heterologously expressed in Escherichia coli BL21 (DE3) and its activity and alkali resistance were improved by loop replacement. Simultaneously, the effect of R260 on enzyme alkaline tolerance was also explored. Results: Recombinant pectate lyase (BspPel-th) showed the highest activity at 60°C and pH 11.0, and showed significant stability over a wide pH range (3.0-11.0). The specific enzyme activity after purification was 139.4 U/mg, which was 4.4 times higher than that of the wild-type enzyme. BspPel-th has good affinity for apple pectin, since the V _max and K _m were 29 μmol/min. mL and 0.46 mol/L, respectively. Molecular dynamics simulation results showed that the flexibility of the loop region of BspPel-th was improved. Conclusion: The modified BspPel-th has considerable potential for industrial applications with high pH processes.

Enzyme dynamics-a brief review

Debate has been simmering for some years regarding the importance of internal thermal motions of enzymes to catalysis. Recent developments in protein design may bring resolution of the more contentious points a little closer.

Design and interpretation of experiments to establish enzyme pathway and define the role of conformational changes in enzyme specificity

We describe the experimental methods and analysis to define the role of enzyme conformational changes in specificity based on published studies using DNA polymerases as an ideal model system. Rather than give details of how to perform transient-state and single-turnover kinetic experiments, we focus on the rationale of the experimental design and interpretation. We show how initial experiments to measure k_cat and k_cat/K_m can accurately quantify specificity but do not define its underlying mechanistic basis. We describe methods to fluorescently label enzymes to monitor conformational changes and to correlate fluorescence signals with rapid-chemical-quench flow assays to define the steps in the pathway. Measurements of the rate of product release and of the kinetics of the reverse reaction complete the kinetic and thermodynamic description of the full reaction pathway. This analysis showed that the substrate-induced change in enzyme structure from an open to a closed state was much faster than rate-limiting chemical bond formation. However, because the reverse of the conformational change was much slower than chemistry, specificity is governed solely by the product of the binding constant for the initial weak substrate binding and the rate constant for the conformational change (k_cat/K_m=K₁k₂) so that the specificity constant does not include k_cat. The enzyme conformational change leads to a closed complex in which the substrate is bound tightly and is committed to the forward reaction. In contrast, an incorrect substrate is bound weakly, and the rate of chemistry is slow, so the mismatch is released from the enzyme rapidly. Thus, the substrate-induced-fit is the major determinant of specificity. The methods outlined here should be applicable to other enzyme systems.

Enhanced conformational exploration of protein loops using a global parameterization of the backbone geometry

Flexible loops are paramount to protein functions, with action modes ranging from localized dynamics contributing to the free energy of the system, to large amplitude conformational changes accounting for the repositioning whole secondary structure elements or protein domains. However, generating diverse and low energy loops remains a difficult problem. This work introduces a novel paradigm to sample loop conformations, in the spirit of the hit-and-run (HAR) Markov chain Monte Carlo technique. The algorithm uses a decomposition of the loop into tripeptides, and a novel characterization of necessary conditions for Tripeptide Loop Closure to admit solutions. Denoting m the number of tripeptides, the algorithm works in an angular space of dimension 12 m. In this space, the hyper-surfaces associated with the aforementioned necessary conditions are used to run a HAR-like sampling technique. On classical loop cases up to 15 amino acids, our parameter free method compares favorably to previous work, generating more diverse conformational ensembles. We also report experiments on a 30 amino acids long loop, a size not processed in any previous work.

Shorter Alkanesulfonate Carbon Chains Destabilize the Active Site Architecture of SsuD for Desulfonation

Bacteria have evolved to utilize alternative organosulfur sources when sulfur is limiting. The SsuE/SsuD and MsuE/MsuD enzymes expressed when sulfur sources are restricted, are responsible for providing specific bacteria with sulfur in the form of alkanesulfonates. In this study, we evaluated why two structurally and functionally similar FMNH₂-dependent monooxygenase enzymes (MsuD and SsuD) are needed for the acquisition of alkanesulfonates in some bacteria. In desulfonation assays, MsuD was able to utilize the entire range of alkanesulfonates (C1-C10). However, SsuD was not able to utilize smaller alkanesulfonate substrates. Interestingly, SsuD had a similar binding affinity for methanesulfonate (MES) (15 ± 1 μM) as MsuD (12 ± 1 μM) even though SsuD was not able to catalyze the desulfonation of the MES substrate. SsuD and MsuD showed decreased proteolytic susceptibility in the presence of FMNH₂ with MES and octanesulfonate (OCS). Tighter loop closure was observed for the MsuD/FMNH₂ complex with MES and OCS compared to SsuD under comparable conditions. Analysis of the SsuD/FMNH₂/MES structure using accelerated molecular dynamics simulations found three different conformations for MES, demonstrating the instability of the bound structure. Even when MES was bound in a similar fashion to OCS within the active site, the smaller alkane chain resulted in a shift of FMNH₂ so that it was no longer in a position to catalyze the desulfonation of MES. The active site of SsuD requires a longer alkane chain to maintain the appropriate architecture for desulfonation.

Increasing the O2 Resistance of the [FeFe]-Hydrogenase CbA5H through Enhanced Protein Flexibility

The high turnover rates of [FeFe]-hydrogenases under mild conditions and at low overpotentials provide a natural blueprint for the design of hydrogen catalysts. However, the unique active site (H-cluster) degrades upon contact with oxygen. The [FeFe]-hydrogenase fromClostridium beijerinckii (CbA5H) is characterized by the flexibility of its protein structure, which allows a conserved cysteine to coordinate to the active site under oxidative conditions. Thereby, intrinsic cofactor degradation induced by dioxygen is minimized. However, the protection from O₂ is only partial, and the activity of the enzyme decreases upon each exposure to O₂. By using site-directed mutagenesis in combination with electrochemistry, ATR-FTIR spectroscopy, and molecular dynamics simulations, we show that the kinetics of the conversion between the oxygen-protected inactive state (cysteine-bound) and the oxygen-sensitive active state can be accelerated by replacing a surface residue that is very distant from the active site. This sole exchange of methionine for a glutamate residue leads to an increased resistance of the hydrogenase to dioxygen. With our study, we aim to understand how local modifications of the protein structure can have a crucial impact on protein dynamics and how they can control the reactivity of inorganic active sites through outer sphere effects.

Natural Evolution Provides Strong Hints about Laboratory Evolution of Designer Enzymes

Laboratory evolution combined with computational enzyme design provides the opportunity to generate novel biocatalysts. Nevertheless, it has been challenging to understand how laboratory evolution optimizes designer enzymes by introducing seemingly random mutations. A typical enzyme optimized with laboratory evolution is the abiological Kemp eliminase, initially designed by grafting active site residues into a natural protein scaffold. Here, we relate the catalytic power of laboratory-evolved Kemp eliminases to the statistical energy ([Formula: see text]) inferred from their natural homologous sequences using the maximum entropy model. The [Formula: see text] of designs generated by directed evolution is correlated with enhanced activity and reduced stability, thus displaying a stability-activity trade-off. In contrast, the [Formula: see text] for mutants in catalytic-active remote regions (in which remote residues are important for catalysis) is strongly anticorrelated with the activity. These findings provide an insight into the role of protein scaffolds in the adaption to new enzymatic functions. It also indicates that the valley in the [Formula: see text] landscape can guide enzyme design for abiological catalysis. Overall, the connection between laboratory and natural evolution contributes to understanding what is optimized in the laboratory and how new enzymatic function emerges in nature, and provides guidance for computational enzyme design.

Enabling Role of Ligand-Driven Conformational Changes in Enzyme Evolution

Many enzymes that show a large specificity in binding the enzymatic transition state with a higher affinity than the substrate utilize substrate binding energy to drive protein conformational changes to form caged substrate complexes. These protein cages provide strong stabilization of enzymatic transition states. Using part of the substrate binding energy to drive the protein conformational change avoids a similar strong stabilization of the Michaelis complex and irreversible ligand binding. A seminal step in the development of modern enzyme catalysts was the evolution of enzymes that couple substrate binding to a conformational change. These include enzymes that function in glycolysis (triosephosphate isomerase), the biosynthesis of lipids (glycerol phosphate dehydrogenase), the hexose monophosphate shunt (6-phosphogluconate dehydrogenase), and the mevalonate pathway (isopentenyl diphosphate isomerase), catalyze the final step in the biosynthesis of pyrimidine nucleotides (orotidine monophosphate decarboxylase), and regulate the cellular levels of adenine nucleotides (adenylate kinase). The evolution of enzymes that undergo ligand-driven conformational changes to form active protein-substrate cages is proposed to proceed by selection of variants, in which the selected side chain substitutions destabilize a second protein conformer that shows compensating enhanced binding interactions with the substrate. The advantages inherent to enzymes that incorporate a conformational change into the catalytic cycle provide a strong driving force for the evolution of flexible protein folds such as the TIM barrel. The appearance of these folds represented a watershed event in enzyme evolution that enabled the rapid propagation of enzyme activities within enzyme superfamilies.

Complex Loop Dynamics Underpin Activity, Specificity, and Evolvability in the (βα)8 Barrel Enzymes of Histidine and Tryptophan Biosynthesis

Enzymes are conformationally dynamic, and their dynamical properties play an important role in regulating their specificity and evolvability. In this context, substantial attention has been paid to the role of ligand-gated conformational changes in enzyme catalysis; however, such studies have focused on tremendously proficient enzymes such as triosephosphate isomerase and orotidine 5'-monophosphate decarboxylase, where the rapid (μs timescale) motion of a single loop dominates the transition between catalytically inactive and active conformations. In contrast, the (βα)₈-barrels of tryptophan and histidine biosynthesis, such as the specialist isomerase enzymes HisA and TrpF, and the bifunctional isomerase PriA, are decorated by multiple long loops that undergo conformational transitions on the ms (or slower) timescale. Studying the interdependent motions of multiple slow loops, and their role in catalysis, poses a significant computational challenge. This work combines conventional and enhanced molecular dynamics simulations with empirical valence bond simulations to provide rich details of the conformational behavior of the catalytic loops in HisA, PriA, and TrpF, and the role of their plasticity in facilitating bifunctionality in PriA and evolved HisA variants. In addition, we demonstrate that, similar to other enzymes activated by ligand-gated conformational changes, loops 3 and 4 of HisA and PriA act as gripper loops, facilitating the isomerization of the large bulky substrate ProFAR, albeit now on much slower timescales. This hints at convergent evolution on these different (βα)₈-barrel scaffolds. Finally, our work reemphasizes the potential of engineering loop dynamics as a tool to artificially manipulate the catalytic repertoire of TIM-barrel proteins.

An Enzyme with High Catalytic Proficiency Utilizes Distal Site Substrate Binding Energy to Stabilize the Closed State but at the Expense of Substrate Inhibition

Understanding the factors that underpin the enormous catalytic proficiencies of enzymes is fundamental to catalysis and enzyme design. Enzymes are, in part, able to achieve high catalytic proficiencies by utilizing the binding energy derived from nonreacting portions of the substrate. In particular, enzymes with substrates containing a nonreacting phosphodianion group coordinated in a distal site have been suggested to exploit this binding energy primarily to facilitate a conformational change from an open inactive form to a closed active form, rather than to either induce ground state destabilization or stabilize the transition state. However, detailed structural evidence for the model is limited. Here, we use β-phosphoglucomutase (βPGM) to investigate the relationship between binding a phosphodianion group in a distal site, the adoption of a closed enzyme form, and catalytic proficiency. βPGM catalyzes the isomerization of β-glucose 1-phosphate to glucose 6-phosphate via phosphoryl transfer reactions in the proximal site, while coordinating a phosphodianion group of the substrate(s) in a distal site. βPGM has one of the largest catalytic proficiencies measured and undergoes significant domain closure during its catalytic cycle. We find that side chain substitution at the distal site results in decreased substrate binding that destabilizes the closed active form but is not sufficient to preclude the adoption of a fully closed, near-transition state conformation. Furthermore, we reveal that binding of a phosphodianion group in the distal site stimulates domain closure even in the absence of a transferring phosphoryl group in the proximal site, explaining the previously reported β-glucose 1-phosphate inhibition. Finally, our results support a trend whereby enzymes with high catalytic proficiencies involving phosphorylated substrates exhibit a greater requirement to stabilize the closed active form.

Engineering of the Recombinant Expression and PEGylation Efficiency of the Therapeutic Enzyme Human Thymidine Phosphorylase

Human thymidine phosphorylase (HsTP) is an enzyme with important implications in the field of rare metabolic diseases. Defective mutations of HsTP lead to mitochondrial neurogastrointestinal encephalomyopathy (MNGIE), a disease with a high unmet medical need that is associated with severe neurological and gastrointestinal complications. Current efforts focus on the development of an enzyme replacement therapy (ERT) using the Escherichia coli ortholog (EcTP). However, bacterial enzymes are counter-indicated for human therapeutic applications because they are recognized as foreign by the human immune system, thereby eliciting adverse immune responses and raising significant safety and efficacy risks. Thus, it is critical to utilize the HsTP enzyme as starting scaffold for pre-clinical drug development, thus de-risking the safety concerns associated with the use of bacterial enzymes. However, HsTP expresses very poorly in E. coli, whereas its PEGylation, a crucial chemical modification for achieving long serum persistence of therapeutic enzymes, is highly inefficient and negatively affects its catalytic activity. Here we focused on the engineering of the recombinant expression profile of HsTP in E. coli cells, as well as on the optimization of its PEGylation efficiency aiming at the development of an alternative therapeutic approach for MNGIE. We show that phylogenetic and structural analysis of proteins can provide important insights for the rational design of N’-terminus-truncation constructs which exhibit significantly improved recombinant expression levels. In addition, we developed and implemented a criteria-driven rational surface engineering strategy for the substitution of arginine-to-lysine and lysine-to-arginine residues to achieve more efficient, homogeneous and reproducible PEGylation without negatively affecting the enzymatic catalytic activity upon PEGylation. Collectively, our proposed strategies provide an effective way to optimize enzyme PEGylation and E. coli recombinant expression and are likely applicable for other proteins and enzymes.

Protein-Ribofuranosyl Interactions Activate Orotidine 5'-Monophosphate Decarboxylase for Catalysis

The role of a global, substrate-driven, enzyme conformational change in enabling the extraordinarily large rate acceleration for orotidine 5'-monophosphate decarboxylase (OMPDC)-catalyzed decarboxylation of orotidine 5'-monophosphate (OMP) is examined in experiments that focus on the interactions between OMPDC and the ribosyl hydroxyl groups of OMP. The D37 and T100' side chains of OMPDC interact, respectively, with the C-3' and C-2' hydroxyl groups of enzyme-bound OMP. D37G and T100'A substitutions result in 1.4 kcal/mol increases in the activation barrier ΔG^⧧ for catalysis of decarboxylation of the phosphodianion-truncated substrate 1-(β-d-erythrofuranosyl)orotic acid (EO) but result in larger 2.1-2.9 kcal/mol increases in ΔG^⧧ for decarboxylation of OMP and for phosphite dianion-activated decarboxylation of EO. This shows that these substitutions reduce transition-state stabilization by the Q215, Y217, and R235 side chains at the dianion binding site. The D37G and T100'A substitutions result in <1.0 kcal/mol increases in ΔG^⧧ for activation of OMPDC-catalyzed decarboxylation of the phosphoribofuranosyl-truncated substrate FO by phosphite dianions. Experiments to probe the effect of D37 and T100' substitutions on the kinetic parameters for d-glycerol 3-phosphate and d-erythritol 4-phosphate activators of OMPDC-catalyzed decarboxylation of FO show that ΔG^⧧ for sugar phosphate-activated reactions is increased by ca. 2.5 kcal/mol for each -OH interaction eliminated by D37G or T100'A substitutions. We conclude that the interactions between the D37 and T100' side chains and ribosyl or ribosyl-like hydroxyl groups are utilized to activate OMPDC for catalysis of decarboxylation of OMP, EO, and FO.

Evolution of dynamical networks enhances catalysis in a designer enzyme

Activation heat capacity is emerging as a crucial factor in enzyme thermoadaptation, as shown by the non-Arrhenius behaviour of many natural enzymes. However, its physical origin and relationship to the evolution of catalytic activity remain uncertain. Here we show that directed evolution of a computationally designed Kemp eliminase reshapes protein dynamics, which gives rise to an activation heat capacity absent in the original design. These changes buttress transition-state stabilization. Extensive molecular dynamics simulations show that evolution results in the closure of solvent-exposed loops and a better packing of the active site. Remarkably, this gives rise to a correlated dynamical network that involves the transition state and large parts of the protein. This network tightens the transition-state ensemble, which induces a negative activation heat capacity and non-linearity in the activity-temperature dependence. Our results have implications for understanding enzyme evolution and suggest that selectively targeting the conformational dynamics of the transition-state ensemble by design and evolution will expedite the creation of novel enzymes.

Current approaches to flexible loop modeling

Loops are key components of protein structures, involved in many biological functions. Due to their conformational variability, the structural investigation of loops is a difficult topic, requiring a combination of experimental and computational methods. This paper provides a brief overview of current computational approaches to flexible loop modeling, and presents the main ingredients of the most standard protocols. Despite great progress in recent years, accurately modeling the conformational variability of long flexible loops remains a challenging problem. Future advances in this field will likely come from a tight coupling of experimental and computational techniques, which would enable a better understanding of the relationships between loop sequence, structural flexibility, and functional roles. In fine, accurate loop modeling will open the road to loop design problems of interest for applications in biomedicine and biotechnology.

Inhibitors of β-Lactamases. New Life of β-Lactam Antibiotics

β-Lactam antibiotics account for about 60% of all produced antibiotics. Due to a high activity and minimal side effects, they are the most commonly used class of antibacterial drugs for the treatment of various infectious diseases of humans and animals, including severe hospital infections. However, the emergence of bacteria resistant to β-lactams has led to the clinical inefficiency of these antibiotics, and as a result, their use in medicine has been limited. The search for new effective ways for overcoming the resistance to β-lactam antibiotics is an essential task. The major mechanism of bacterial resistance is the synthesis of β-lactamases (BLs) that break the antibiotic β-lactam ring. Here, we review specific inhibitors of serine β-lactamases and metallo-β-lactamases and discuss approaches for creating new inhibitors that would prolong the "life" of β-lactams.

Recent research and development of NDM-1 inhibitors

Bacteria carrying New Delhi metallo-β-lactamase-1 (New Delhi metallo-β-lactamase, NDM-1) resistance gene is a new type of "superbug", which can hydrolyze almost all β-lactam antibiotics, rapidly spread among the same species and even spread among different species. NDM-1 belongs to the class B1 broad-spectrum enzyme of β-lactamase. The two positively charged zinc ions in the active center have electrostatic interaction with the hydroxyl ions in them to seize the hydrogen atom near the water molecule to form a bridging ring water molecule, which strengthens its nucleophilicity and attacks the carbonyl group on the lactam ring; thus, catalyzing the hydrolysis of β-lactam antibiotics. Since NDM-1 has an open active site and unique electrostatic structure, it essentially provides a wider range of substrate specificity. Due to its flexible hydrolysis mechanism and more and more variants also aggravate the threat of drug-resistant bacteria infection, there is still no effective inhibitor in clinic, which is a serious threat to human health and public health safety. The electron-rich substituents of NDM-1 inhibitors coordinate with two positively charged zinc ions in the active center of the enzyme through ion-dipole interaction to produce NDM-1 inhibitory activity. In this review, the research progress of NDM-1 enzyme and its inhibitors in the past 5 years was reviewed. The crystal structure, active center structure, surrounding important amino acid residues, newly discovered inhibitors and their action mechanism are classified and summarized in detail, which can be used as a reference for the development of effective drugs against drug-resistant bacteria targeting NDM-1.

Protein Loop Conformational Free Energy Changes via an Alchemical Path without Reaction Coordinates

We introduce a method called restrain-free energy perturbation-release 2.0 (R-FEP-R 2.0) to estimate conformational free energy changes of protein loops via an alchemical path. R-FEP-R 2.0 is a generalization of the method called restrain-free energy perturbation-release (R-FEP-R) that can only estimate conformational free energy changes of protein side chains but not loops. The reorganization of protein loops is a central feature of many biological processes. Unlike other advanced sampling algorithms such as umbrella sampling and metadynamics, R-FEP-R and R-FEP-R 2.0 do not require predetermined collective coordinates and transition pathways that connect the two endpoint conformational states. The R-FEP-R 2.0 method was applied to estimate the conformational free energy change of a β-turn flip in the protein ubiquitin. The result obtained by R-FEP-R 2.0 agrees with the benchmarks very well. We also comment on problems commonly encountered when applying umbrella sampling to calculate protein conformational free energy changes.

Rational engineering of BaLal_16 from a novel Bacillus amyloliquefaciens strain to improve catalytic performance

_L-amino acid ligases (Lals) are promising biocatalysts for the synthesis of dipeptides with special biological properties. However, their poor (or broad) substrate specificity limits their industrial applications. To address this problem, a molecular engineering method for Lals was developed to enhance their catalytic performance. Based on substrate channeling, entrances to the active site for different substrates were identified, and the "gate" located around the active site pocket, which plays an essential role in substrate recognition, was then engineered to facilitate acceptance of _L-Gln. Two mutants (L110Y and N108F/L110Y) were discovered to display significantly increased catalytic activity toward _L-Ala and _L-Gln in the biosynthesis of Ala-Gln. The catalytic efficiency (k_cat/ K_m) of the L110Y and N108F/L110Y mutants was improved by 2.64-fold and 4.06-fold, respectively, compared with that of the wild type. N108F/L110Y was then further applied for batch production of Ala-Gln, which showed that the released Pi yield was 694.47 μM, which was an increase of approximately 21.4 %, and the yield of Ala-Gln was approximately 2.59 mM^-1 L^-1 mg^-1. Collectively, these findings suggest the potential practical application of this method in the rational design of Lals for increased catalytic performance.

Characterisation of plasmodial transketolases and identification of potential inhibitors: an in silico study

BACKGROUND

Plasmodial transketolase (PTKT) enzyme is one of the novel pharmacological targets being explored as potential anti-malarial drug target due to its functional role and low sequence identity to the human enzyme. Despite this, features contributing to such have not been exploited for anti-malarial drug design. Additionally, there are no anti-malarial drugs targeting PTKTs whereas the broad activity of these inhibitors against PTKTs from other Plasmodium spp. is yet to be reported. This study characterises different PTKTs [Plasmodium falciparum (PfTKT), Plasmodium vivax (PvTKT), Plasmodium ovale (PoTKT), Plasmodium malariae (PmTKT) and Plasmodium knowlesi (PkTKT) and the human homolog (HsTKT)] to identify key sequence and structural based differences as well as the identification of selective potential inhibitors against PTKTs.

METHODS

A sequence-based study was carried out using multiple sequence alignment, phylogenetic tree calculations and motif discovery analysis. Additionally, TKT models of PfTKT, PmTKT, PoTKT, PmTKT and PkTKT were modelled using the Saccharomyces cerevisiae TKT structure as template. Based on the modelled structures, molecular docking using 623 South African natural compounds was done. The stability, conformational changes and detailed interactions of selected compounds were accessed viz all-atom molecular dynamics (MD) simulations and binding free energy (BFE) calculations.

RESULTS

Sequence alignment, evolutionary and motif analyses revealed key differences between plasmodial and the human TKTs. High quality homodimeric three-dimensional PTKTs structures were constructed. Molecular docking results identified three compounds (SANC00107, SANC00411 and SANC00620) which selectively bind in the active site of all PTKTs with the lowest (better) binding affinity ≤ - 8.5 kcal/mol. MD simulations of ligand-bound systems showed stable fluctuations upon ligand binding. In all systems, ligands bind stably throughout the simulation and form crucial interactions with key active site residues. Simulations of selected compounds in complex with human TKT showed that ligands exited their binding sites at different time steps. BFE of protein-ligand complexes showed key residues involved in binding.

CONCLUSIONS

This study highlights significant differences between plasmodial and human TKTs and may provide valuable information for the development of novel anti-malarial inhibitors. Identified compounds may provide a starting point in the rational design of PTKT inhibitors and analogues based on these scaffolds.

Enhancing the enthalpic contribution of hydrogen bonds by solvent shielding

In biological systems, polar interactions are heavily burdened by high desolvation penalties resulting from strong solute-solvent interactions. As a consequence thereof, enthalpic contributions of hydrogen bonds to the free energy of binding are severely diminished. However, this effect is strongly attenuated for interactions within solvent-shielded areas of proteins. In microcalorimetric experiments, we show that the bacterial lectin FimH utilizes conformational adaptions to effectively shield its binding site from solvent. The transition into a lower dielectric environment results in an enthalpic benefit of approximately -13 kJ mol^-1 for mannoside binding. However, this effect can be abrogated, if the hydrogen bond network within the binding site is disturbed by deoxygenation of the ligand. Conformational adaption leading to reduced local dielectric constants could represent a general mechanism for proteins to enable enthalpy-driven recognition of polar ligands.

Membrane Lipids Assist Catalysis by CTP: Phosphocholine Cytidylyltransferase

While most of the articles in this issue review the workings of integral membrane enzymes, in this review, we describe the catalytic mechanism of an enzyme that contains a soluble catalytic domain but appears to catalyze its reaction on the membrane surface, anchored and assisted by a separate regulatory amphipathic helical domain and inter-domain linker. Membrane partitioning of CTP: phosphocholine cytidylyltransferase (CCT), a key regulatory enzyme of phosphatidylcholine metabolism, is regulated chiefly by changes in membrane phospholipid composition, and boosts the enzyme's catalytic efficiency >200-fold. Catalytic enhancement by membrane binding involves the displacement of an auto-inhibitory helix from the active site entrance-way and promotion of a new conformational ensemble for the inter-domain, allosteric linker that has an active role in the catalytic cycle. We describe the evidence for close contact between membrane lipid, a compact allosteric linker, and the CCT active site, and discuss potential ways that this interaction enhances catalysis.

Harnessing Conformational Plasticity to Generate Designer Enzymes

Recent years have witnessed an explosion of interest in understanding the role of conformational dynamics both in the evolution of new enzymatic activities from existing enzymes and in facilitating the emergence of enzymatic activity de novo on scaffolds that were previously non-catalytic. There are also an increasing number of examples in the literature of targeted engineering of conformational dynamics being successfully used to alter enzyme selectivity and activity. Despite the obvious importance of conformational dynamics to both enzyme function and evolvability, many (although not all) computational design approaches still focus either on pure sequence-based approaches or on using structures with limited flexibility to guide the design. However, there exist a wide variety of computational approaches that can be (re)purposed to introduce conformational dynamics as a key consideration in the design process. Coupled with laboratory evolution and more conventional existing sequence- and structure-based approaches, these techniques provide powerful tools for greatly expanding the protein engineering toolkit. This Perspective provides an overview of evolutionary studies that have dissected the role of conformational dynamics in facilitating the emergence of novel enzymes, as well as advances in computational approaches that allow one to target conformational dynamics as part of enzyme design. Harnessing conformational dynamics in engineering studies is a powerful paradigm with which to engineer the next generation of designer biocatalysts.

Orotidine 5'-Monophosphate Decarboxylase: The Operation of Active Site Chains Within and Across Protein Subunits

The D37 and T100′ side chains of orotidine 5′-monophosphate decarboxylase (OMPDC) interact with the C-3′ and C-2′ ribosyl hydroxyl groups, respectively, of the bound substrate. We compare the intra-subunit interactions of D37 with the inter-subunit interactions of T100′ by determining the effects of the D37G, D37A, T100′G, and T100′A substitutions on the following: (a) k_cat and k_cat/K_m values for the OMPDC-catalyzed decarboxylations of OMP and 5-fluoroorotidine 5′-monophosphate (FOMP) and (b) the stability of dimeric OMPDC relative to the monomer. The D37G and T100′A substitutions resulted in 2 kcal mol^–1 increases in ΔG^† for k_cat/K_m for the decarboxylation of OMP, while the D37A and T100′G substitutions resulted in larger 4 and 5 kcal mol^–1 increases, respectively, in ΔG^†. The D37G and T100′A substitutions both resulted in smaller 2 kcal mol^–1 decreases in ΔG^† for the decarboxylation of FOMP compared to that of OMP. These results show that the D37G and T100′A substitutions affect the barrier to the chemical decarboxylation step while the D37A and T100′G substitutions also affect the barrier to a slow, ligand-driven enzyme conformational change. Substrate binding induces the movement of an α-helix (G′98–S′106) toward the substrate C-2′ ribosyl hydroxy bound at the main subunit. The T100′G substitution destabilizes the enzyme dimer by 3.5 kcal mol^–1 compared to the monomer, which is consistent with the known destabilization of α-helices by the internal Gly side chains [Serrano, L., et al. (1992) Nature, 356, 453–455]. We propose that the T100′G substitution weakens the α-helical contacts at the dimer interface, which results in a decrease in the dimer stability and an increase in the barrier to the ligand-driven conformational change.

Hydrogen deuterium exchange defines catalytically linked regions of protein flexibility in the catechol O-methyltransferase reaction

Human catechol O-methyltransferase (COMT) has emerged as a model for understanding enzyme-catalyzed methyl transfer from S-adenosylmethionine (AdoMet) to small-molecule catecholate acceptors. Mutation of a single residue (tyrosine 68) behind the methyl-bearing sulfonium of AdoMet was previously shown to impair COMT activity by interfering with methyl donor-acceptor compaction within the activated ground state of the wild type enzyme [J. Zhang, H. J. Kulik, T. J. Martinez, J. P. Klinman, Proc. Natl. Acad. Sci. U.S.A. 112, 7954-7959 (2015)]. This predicts the involvement of spatially defined protein dynamical effects that further tune the donor/acceptor distance and geometry as well as the electrostatics of the reactants. Here, we present a hydrogen/deuterium exchange (HDX)-mass spectrometric study of wild type and mutant COMT, comparing temperature dependences of HDX against corresponding kinetic and cofactor binding parameters. The data show that the impaired Tyr68Ala mutant displays similar breaks in Arrhenius plots of both kinetic and HDX properties that are absent in the wild type enzyme. The spatial resolution of HDX below a break point of 15-20 °C indicates changes in flexibility across ∼40% of the protein structure that is confined primarily to the periphery of the AdoMet binding site. Above 20 °C, Tyr68Ala behaves more like WT in HDX, but its rate and enthalpic barrier remain significantly altered. The impairment of catalysis by Tyr68Ala can be understood in the context of a mutationally induced alteration in protein motions that becomes manifest along and perpendicular to the primary group transfer coordinate.

The role of ligand-gated conformational changes in enzyme catalysis

Structural and biochemical studies on diverse enzymes have highlighted the importance of ligand-gated conformational changes in enzyme catalysis, where the intrinsic binding energy of the common phosphoryl group of their substrates is used to drive energetically unfavorable conformational changes in catalytic loops, from inactive open to catalytically competent closed conformations. However, computational studies have historically been unable to capture the activating role of these conformational changes. Here, we discuss recent experimental and computational studies, which can remarkably pinpoint the role of ligand-gated conformational changes in enzyme catalysis, even when not modeling the loop dynamics explicitly. Finally, through our joint analyses of these data, we demonstrate how the synergy between theory and experiment is crucial for furthering our understanding of enzyme catalysis.

An Atomistic Understanding of Allosteric Inhibition of Glutamate Racemase: a Dampening of Native Activation Dynamics

Glutamate racemases (GR) are members of the family of bacterial enzymes known as cofactor-independent racemases and epimerases and catalyze the stereoinversion of glutamate. D-amino acids are universally important for the proper construction of viable bacterial cell walls, and thus have been repeatedly validated as attractive targets for novel antimicrobial drug design. Significant aspects of the mechanism of this challenging stereoinversion remain unknown. The current study employs a combination of MD and QM/MM computational approaches to show that the GR from H. pylori must proceed via a pre-activation step, which is dependent on the enzyme's flexibility. This mechanism is starkly different from previously proposed mechanisms. These findings have immediate pharmaceutical relevance, as the H. pylori GR enzyme is a very attractive allosteric drug target. The results presented in this study offer a distinctly novel understanding of how AstraZeneca's lead series of inhibitors cripple the H. pylori GR's native motions, via prevention of this critical chemical pre-activation step. Our experimental studies, using SPR, fluorescence and NMR WaterLOGSY, show that H. pylori GR is not inhibited by the uncompetitive mechanism originally put forward by Lundqvist et al.. The current study supports a deep connection between native enzyme motions and chemical reactivity, which has strong relevance to the field of allosteric drug discovery.

Substrate-to-Product Conversion Facilitates Active Site Loop Opening in Yeast Enolase: A Molecular Dynamics Study

Yeast enolase serves as a prototype for metalloenzymes with labile, catalytic active site metal ions and is important for glycolysis and fermentation processes. Herein, microsecond molecular dynamics simulations of the protein-substrate and protein-product complexes are conducted to elucidate the mechanism of the opening of catalytically important active site loops. These simulations indicate that conversion of substrate to product is accompanied by diminished metal coordination and hydrogen-bonding interactions, as well as enhanced loop flexibility. Moreover, free energy simulations show that the loop opening is endergonic when substrate is bound but exergonic when product is bound. Thus, the conversion to product weakens the association of the loop with the ligand and binding site, thereby facilitating the loop opening after catalysis and enabling product release. These insights about active site loop motions in enzyme catalysis may be useful in guiding enzyme design efforts.

Uncovering the Role of Key Active-Site Side Chains in Catalysis: An Extended Brønsted Relationship for Substrate Deprotonation Catalyzed by Wild-Type and Variants of Triosephosphate Isomerase

We report results of detailed empirical valence bond simulations that model the effect of several amino acid substitutions on the thermodynamic (ΔG°) and kinetic activation (ΔG^⧧) barriers to deprotonation of dihydroxyacetone phosphate (DHAP) and d-glyceraldehyde 3-phosphate (GAP) bound to wild-type triosephosphate isomerase (TIM), as well as to the K12G, E97A, E97D, E97Q, K12G/E97A, I170A, L230A, I170A/L230A, and P166A variants of this enzyme. The EVB simulations model the observed effect of the P166A mutation on protein structure. The E97A, E97Q, and E97D mutations of the conserved E97 side chain result in ≤1.0 kcal mol^-1 decreases in the activation barrier for substrate deprotonation. The agreement between experimental and computed activation barriers is within ±1 kcal mol^-1, with a strong linear correlation between ΔG^⧧ and ΔG° for all 11 variants, with slopes β = 0.73 (R² = 0.994) and β = 0.74 (R² = 0.995) for the deprotonation of DHAP and GAP, respectively. These Brønsted-type correlations show that the amino acid side chains examined in this study function to reduce the standard-state Gibbs free energy of reaction for deprotonation of the weak α-carbonyl carbon acid substrate to form the enediolate phosphate reaction intermediate. TIM utilizes the cationic side chain of K12 to provide direct electrostatic stabilization of the enolate oxyanion, and the nonpolar side chains of P166, I170, and L230 are utilized for the construction of an active-site cavity that provides optimal stabilization of the enediolate phosphate intermediate relative to the carbon acid substrate.

Remodeling of the interdomain allosteric linker upon membrane binding of CCTα pulls its active site close to the membrane surface

The rate-limiting step in the biosynthesis of the major membrane phospholipid, phosphatidylcholine, is catalyzed by CTP:phosphocholine cytidylyltransferase (CCT), which is regulated by reversible membrane binding of a long amphipathic helix (domain M). The M domain communicates with the catalytic domain via a conserved ∼20-residue linker, essential for lipid activation of CCT. Previous analysis of this region (denoted as the αE_C/J) using MD simulations, cross-linking, mutagenesis, and solvent accessibility suggested that membrane binding of domain M promotes remodeling of the αE_C/J into a more compact structure that is required for enzyme activation. Here, using tryptophan fluorescence quenching, we show that the allosteric linker lies superficially on the membrane surface. Analyses with truncated CCTs show that the αE_C/J can interact with lipids independently of the M domain. We observed strong FRET between engineered tryptophans in the αE_C/J and vesicles containing dansyl-phosphatidylethanolamine that depended on the native J sequence. These data are incompatible with the extended conformation of the αE helix observed in the previously determined crystal structure of inactive CCT but support a bent αE helix conformation stabilized by J segment interactions. Our results suggest that the membrane-adsorbed, folded allosteric linker may partially cover the active site cleft and pull it close to the membrane surface, where cytidyl transfer can occur efficiently in a relatively anhydrous environment.

Role of the Carboxylate in Enzyme-Catalyzed Decarboxylation of Orotidine 5'-Monophosphate: Transition State Stabilization Dominates Over Ground State Destabilization

Kinetic parameters k_ex (s^–1) and k_ex/K_d (M^–1 s^–1) are reported for exchange for deuterium in D₂O of the C-6 hydrogen of 5-fluororotidine 5′-monophosphate (FUMP) catalyzed by the Q215A, Y217F, and Q215A/Y217F variants of yeast orotidine 5′-monophosphate decarboxylase (ScOMPDC) at pD 8.1, and by the Q215A variant at pD 7.1–9.3. The pD rate profiles for wildtype ScOMPDC and the Q215A variant are identical, except for a 2.5 log unit downward displacement in the profile for the Q215A variant. The Q215A, Y217F and Q215A/Y217F substitutions cause 1.3–2.0 kcal/mol larger increases in the activation barrier for wildtype ScOMPDC-catalyzed deuterium exchange compared with decarboxylation, because of the stronger apparent side chain interaction with the transition state for the deuterium exchange reaction. The stabilization of the transition state for the OMPDC-catalyzed deuterium exchange reaction of FUMP is ca. 19 kcal/mol smaller than the transition state for decarboxylation of OMP, and ca. 8 kcal/mol smaller than for OMPDC-catalyzed deprotonation of FUMP to form the vinyl carbanion intermediate common to OMPDC-catalyzed reactions OMP/FOMP and UMP/FUMP. We propose that ScOMPDC shows similar stabilizing interactions with the common portions of decarboxylation and deprotonation transition states that lead to formation of this vinyl carbanion intermediate, and that there is a large ca. (19–8) = 11 kcal/mol stabilization of the former transition state from interactions with the nascent CO₂ of product. The effects of Q215A and Y217F substitutions on k_cat/K_m for decarboxylation of OMP are expressed mainly as an increase in K_m for the reactions catalyzed by the variant enzymes, while the effects on k_ex/K_d for deuterium exchange are expressed mainly as an increase in k_ex. This shows that the Q215 and Y217 side chains stabilize the Michaelis complex to OMP for the decarboxylation reaction, compared with the complex to FUMP for the deuterium exchange reaction. These results provide strong support for the conclusion that interactions which stabilize the transition state for ScOMPDC-catalyzed decarboxylation at a nonpolar enzyme active site dominate over interactions that destabilize the ground-state Michaelis complex.

Identification of non-conserved residues essential for improving the hydrocarbon-producing activity of cyanobacterial aldehyde-deformylating oxygenase

BACKGROUND

Cyanobacteria produce hydrocarbons corresponding to diesel fuels by means of aldehyde-deformylating oxygenase (ADO). ADO catalyzes a difficult and unusual reaction in the conversion of aldehydes to hydrocarbons and has been widely used for biofuel production in metabolic engineering; however, its activity is low. A comparison of the amino acid sequences of highly active and less active ADOs will elucidate non-conserved residues that are essential for improving the hydrocarbon-producing activity of ADOs.

RESULTS

Here, we measured the activities of ADOs from 10 representative cyanobacterial strains by expressing each of them in Escherichia coli and quantifying the hydrocarbon yield and amount of soluble ADO. We demonstrated that the activity was highest for the ADO from Synechococcus elongatus PCC 7942 (7942ADO). In contrast, the ADO from Gloeobacter violaceus PCC 7421 (7421ADO) had low activity but yielded high amounts of soluble protein, resulting in a high production level of hydrocarbons. By introducing 37 single amino acid substitutions at the non-conserved residues of the less active ADO (7421ADO) to make its sequence more similar to that of the highly active ADO (7942ADO), we found 20 mutations that improved the activity of 7421ADO. In addition, 13 other mutations increased the amount of soluble ADO while maintaining more than 80% of wild-type activity. Correlation analysis showed a solubility-activity trade-off in ADO, in which activity was negatively correlated with solubility.

CONCLUSIONS

We succeeded in identifying non-conserved residues that are essential for improving ADO activity. Our results may be useful for generating combinatorial mutants of ADO that have both higher activity and higher amounts of the soluble protein in vivo, thereby producing higher yields of biohydrocarbons.

Thermodynamic analysis of remote substrate binding energy in 3α-hydroxysteroid dehydrogenase/carbonyl reductase catalysis

The binding energy of enzyme and substrate is used to lower the activation energy for the catalytic reaction. 3α-HSD/CR uses remote binding interactions to accelerate the reaction of androsterone with NAD⁺. Here, we examine the enthalpic and entropic components of the remote binding energy in the 3α-HSD/CR-catalyzed reaction of NAD⁺ with androsterone versus the substrate analogs, 2-decalol and cyclohexanol, by analyzing the temperature-dependent kinetic parameters through steady-state kinetics. The effects of temperature on k_cat/K_m for 3α-HSD/CR acting on androsterone, 2-decalol, and cyclohexanol show the reactions are entropically favorable but enthalpically unfavorable. Thermodynamic analysis from the temperature-dependent values of K_m and k_cat shows the binding of the E-NAD⁺ complex with either 2-decalol or cyclohexanol to form the ternary complex is endothermic and entropy-driven, and the subsequent conversion to the transition state is both enthalpically and entropically unfavorable. Hence, solvation entropy may play an important role in the binding process through both the desolvation of the solute molecules and the release of bound water molecules from the active site into bulk solvent. As compared to the thermodynamic parameters of 3α-HSD/CR acting on cyclohexanol, the hydrophobic interaction of the B-ring of steroids with the active site of 3α-HSD/CR contributes to catalysis by increasing exclusively the entropy of activation (ΔTΔS^‡ = 1.8 kcal/mol), while the BCD-ring of androsterone significantly lowers ΔΔH^‡ by 10.4 kcal/mol with a slight entropic penalty of -1.9 kcal/mol. Therefore, the remote non-reacting sites of androsterone may induce a conformational change of the substrate binding loop with an entropic cost for better interaction with the transition state to decrease the enthalpy of activation, significantly increasing catalytic efficiency.