Role of allosteric switches and adaptor domains in long-distance cross-talk and transient tunnel formation

Activity Regulation of a Glutamine Amidotransferase Bienzyme Complex by Substrate-Induced Subunit Interface Expansion

Glutamine amidotransferases are multienzyme machineries in which reactive ammonia is generated by a glutaminase and then transferred through a sequestered protein tunnel to a synthase active site for incorporation into diverse metabolites. To avoid wasteful metabolite consumption, there is a requirement for synchronized catalysis, but any generally applicable mechanistic insight is still lacking. As synthase activity depends on glutamine turnover, we investigated possible mechanisms controlling glutaminase catalysis using aminodeoxychorismate synthase involved in folate biosynthesis as a model. By analyzing this system in distinct states of catalysis, we found that incubation with glutamine leads to a subunit interface expansion by one-third of its original area. These changes completely enclose the glutaminase active site for sequestered catalysis and the subsequent transport of volatile ammonia to the synthase active site. In view of similar rearrangements in other glutamine amidotransferases, our observations may provide a general mechanism for the catalysis synchronization of this multienzyme family.

A high throughput assay for phosphoribosylformylglycinamidine synthase

Metabolic reprogramming of purine biosynthesis is a hallmark of cancer metabolism and represents a critical vulnerability. The enzyme phosphoribosylformylglycinamidine synthase (PFAS) catalyzes the fourth step in de novo purine biosynthesis and has been demonstrated to be prognostic for survival of liver cancer. Despite the importance of this protein as a drug target, there are no known specific inhibitors of PFAS activity. Here, we describe a new continuous, spectrophotometric assay for the synthase domain of PFAS that is amenable to high-throughput screening (HTS). This mechanism-based fluorescent assay makes use of the acid phosphatase substrate, 6,8-difluoro-4-methylumbelliferyl phosphate (DiFMUP). PFAS catalyzes the turnover of DiFMUP with a KM of 108 ± 7 µM. After optimization and miniaturization of the assay for 1,536-well format, we conducted a pilot HTS using the LOPAC1280 library. The assay performed extremely well, with an average Z' of 0.94 ± 0.02, average signal to noise of 5.01 ± 0.06, excellent inter plate correlation, and a hit rate of 1.18 %. This assay provides a critically needed tool to advance the study of PFAS enzymology and will be foundational for the discovery of small molecule inhibitors both as functional probes and for the basis of new drug development.

Conformational switches in human RNA binding proteins involved in neurodegeneration

Conformational switching in RNA binding proteins (RBPs) is crucial for regulation of RNA processing and transport. Dysregulation or mutations in RBPs and broad RNA processing abnormalities are related to many human diseases including neurodegenerative disorders. Here, we review the role of protein-RNA conformational switches in RBP-RNA complexes. RBP-RNA complexes exhibit wide range of conformational switching depending on the RNA molecule and its ability to induce conformational changes in its partner RBP. We categorize the conformational switches into three groups: rigid body, semi-flexible and full flexible. We also investigate conformational switches in large cellular assemblies including ribosome, spliceosome and RISC complexes. In addition, the role of intrinsic disorder in RBP-RNA conformational switches is discussed. We have also discussed the effect of different disease-causing mutations on conformational switching of proteins associated with neurodegenerative diseases. We believe that this study will enhance our understanding on the role of protein-RNA conformational switches. Furthermore, the availability of a large number of atomic structures of RBP-RNA complexes in near future would facilitate to create a complete repertoire of human RBP-RNA conformational switches.

Substrate-Induced Dynamic Regulation of the Catalytic Loop in Assisting Allosteric Communication in Formylglycinamidine Synthetase

Bifunctional enzymes that execute tandem chemical reactions progress through orchestrated conformational states to achieve chemical synchronization. In these allosterically regulated systems, specific stimuli, such as substrate and cofactor binding, determine reactivity. Here, we employ a combination of steady-state and time-resolved fluorescence methods to monitor the conformational dynamics of a catalytic loop in formylglycinamidine synthetase, an enzyme that catalyzes a crucial step toward the synthesis of precursors of DNA and RNA. We show that the catalytic loop harbors adaptive structural elements that change secondary structure in response to substrate binding and, thereby, enable allosteric cues to the 25 Å distal NH3-producing site. To exclusively track the conformational changes in the loop, a fluorescent unnatural amino acid was introduced into the 1300-amino acid protein, allowing for a unique signal that was not masked by the indigenous fluorescent amino acids. The study highlights the role of flexible small elements that act as triggers of the allosteric cycle and maps states that are essential for function.

Dissecting the CRISPR Cas1-Cas2 Protospacer Binding and Selection Mechanism by Using Molecular Dynamics Simulations

Cas1 and Cas2 are highly conserved proteins among the clustered regularly interspaced short palindromic repeat Cas (CRISPR-Cas) systems and play a crucial role in protospacer selection and integration. According to the double-forked CRISPR Cas1-Cas2 complex, we conducted extensive all-atom molecular dynamics simulations to investigate the protospacer DNA binding and recognition. Our findings revealed that single-point amino acid mutations in Cas1 or in Cas2 had little impact on the binding of the protospacer, both in the binding and precatalytic states. In contrast, multiple-point amino acid mutations, particularly G74A, P80L, and V89A mutations on Cas2 and Cas2' proteins (m-multiple1 system), significantly affected the protospacer binding and selection. Notably, mutations on Cas2 and Cas2' led to an increased number of hydrogen bonds (#HBs) between Cas2&Cas2' and the dsDNA in the m-multiple1 system compared with the wild-type system. And the strong electrostatic interactions between Cas1-Cas2 and the protospacer DNA (psDNA) in the m-multiple1 system again suggested the increase in the binding affinity of protospacer acquisition. Specifically, mutations in Cas2 and Cas2' can remotely make the protospacer adjacent motif complementary (PAMc) sequences better in recognition by the two active sites, while multiple mutations K211E, P202Q, P212L, R138L, V134A, A286T, P282H, and P294H on Cas1a/Cas1b&Cas1a'/Cas1b' (m-multiple2 system) decrease the #HBs and the electrostatic interactions and make the PAMc worse in recognition compared with the wild-type system.

Diverse strategies adopted by nature for regulating purine biosynthesis via fine-tuning of purine metabolic enzymes

Purine nucleotides, generated by de novo synthesis and salvage pathways, are essential for metabolism and act as building blocks of genetic material. To avoid an imbalance in the nucleotide pool, nature has devised several strategies to regulate/tune the catalytic performance of key purine metabolic enzymes. Here, we discuss some recent examples, such as stress-regulating alarmones that bind to select pathway enzymes, huge ensembles like dynamic metabolons and self-assembled filaments that highlight the layered fine-control prevalent in the purine metabolic pathway to fulfill requisite purine demands. Examples of enzymes that turn-on only under allosteric control, are regulated via long-distance communication that facilitates transient conduits have additionally been explored.

Mechanistic Insights into the Long-range Allosteric Regulation of KRAS Via Neurofibromatosis Type 1 (NF1) Scaffold Upon SPRED1 Loading

Allosteric regulation is the most direct and efficient way of regulating protein function, wherein proteins transmit the perturbations at one site to another distinct functional site. Deciphering the mechanism of allosteric regulation is of vital importance for the comprehension of both physiological and pathological events in vivo as well as the rational allosteric drug design. However, it remains challenging to elucidate dominant allosteric signal transduction pathways, especially for large and multi-component protein machineries where long-range allosteric regulation exits. One of the quintessential examples having long-range allosteric regulation is the ternary complex, SPRED1-RAS-neurofibromin type 1 (NF1, a RAS GTPase-activating protein), in which SPRED1 facilitates RAS-GTP hydrolysis by interacting with NF1 at a distal, allosteric site from the RAS binding site. To address the underlying mechanism, we performed extensive Gaussian accelerated molecular dynamics simulations and Markov state model analysis of KRAS-NF1 complex in the presence and absence of SPRED1. Our findings suggested that SPRED1 loading allosterically enhanced KRAS-NF1 binding, but hindered conformational transformation of the NF1 catalytic center for RAS hydrolysis. Moreover, we unveiled the possible allosteric pathways upon SPRED1 binding through difference contact network analysis. This study not only provided an in-depth mechanistic insight into the allosteric regulation of KRAS by SPRED1, but also shed light on the investigation of long-range allosteric regulation among complex macromolecular systems.

Prediction and confirmation of a switch-like region within the N-terminal domain of hSIRT1

Many proteins display conformational changes resulting from allosteric regulation. Often only a few residues are crucial in conveying these structural and functional allosteric changes. These regions that undergo a significant change in structure upon receiving an input signal, such as molecular recognition, are defined as switch-like regions. Identifying these key residues within switch-like regions can help elucidate the mechanism of allosteric regulation and provide guidance for synthetic regulation. In this study, we combine a novel computational workflow with biochemical methods to identify a switch-like region in the N-terminal domain of human SIRT1 (hSIRT1), a lysine deacetylase that plays important roles in regulating cellular pathways. Based on primary sequence, computational methods predicted a region between residues 186-193 in hSIRT1 to exhibit switch-like behavior. Mutations were then introduced in this region and the resulting mutants were tested for allosteric reactions to resveratrol, a known hSIRT1 allosteric regulator. After fine-tuning the mutations based on comparison of known secondary structures, we were able to pinpoint M193 as the residue essential for allosteric regulation, likely by communicating the allosteric signal. Mutation of this residue maintained enzyme activity but abolished allosteric regulation by resveratrol. Our findings suggest a method to predict switch-like regions in allosterically regulated enzymes based on the primary sequence. If further validated, this could be an efficient way to identify key residues in enzymes for therapeutic drug targeting and other applications.

Tunnel Architectures in Enzyme Systems that Transport Gaseous Substrates

Molecular tunnels regulate delivery of substrates/intermediates in enzymes which either harbor deep-seated reaction centers or are for transport of reactive/toxic intermediates that need to be specifically delivered. Here, we focus on the importance of structural diversity in tunnel architectures, especially for the gaseous substrate translocation, in rendering differential substrate preferences and directionality. Two major types of tunnels have been discussed, one that transports stable gases from the environment to the active site, namely, external gaseous (EG) tunnels, and the other that transports molecules between active sites, namely, internal gaseous (IG) tunnels. Aspects as to how the gaseous tunnels have shaped during the course of evolution and their potential to modulate the substrate flow and enzymatic function are examined. In conclusion, the review highlights our perspective on the pulsation mechanism that could facilitate unidirectional translocation of the gaseous molecules through buried tunnels.

Out-of-Equilibrium Biophysical Chemistry: The Case for Multidimensional, Integrated Single-Molecule Approaches

Out-of-equilibrium processes are ubiquitous across living organisms and all structural hierarchies of life. At the molecular scale, out-of-equilibrium processes (for example, enzyme catalysis, gene regulation, and motor protein functions) cause biological macromolecules to sample an ensemble of conformations over a wide range of time scales. Quantifying and conceptualizing the structure–dynamics to function relationship is challenging because continuously evolving multidimensional energy landscapes are necessary to describe nonequilibrium biological processes in biological macromolecules. In this perspective, we explore the challenges associated with state-of-the-art experimental techniques to understanding biological macromolecular function. We argue that it is time to revisit how we probe and model functional out-of-equilibrium biomolecular dynamics. We suggest that developing integrated single-molecule multiparametric force–fluorescence instruments and using advanced molecular dynamics simulations to study out-of-equilibrium biomolecules will provide a path towards understanding the principles of and mechanisms behind the structure–dynamics to function paradigm in biological macromolecules.

Allosteric regulation in CRISPR/Cas1-Cas2 protospacer acquisition mediated by DNA and Cas2

Cas1 and Cas2 are highly conserved proteins across clustered-regularly-interspaced-short-palindromic-repeat-Cas systems and play a significant role in protospacer acquisition. Based on crystal structure of twofold symmetric Cas1-Cas2 in complex with dual-forked protospacer DNA (psDNA), we conducted all-atom molecular dynamics simulations to study the psDNA binding, recognition, and response to cleavage on the protospacer-adjacent-motif complementary sequence, or PAMc, of Cas1-Cas2. In the simulation, we noticed that two active sites of Cas1 and Cas1’ bind asymmetrically to two identical PAMc on the psDNA captured from the crystal structure. For the modified psDNA containing only one PAMc, as that to be recognized by Cas1-Cas2 in general, our simulations show that the non-PAMc association site of Cas1-Cas2 remains destabilized until after the stably bound PAMc being cleaved at the corresponding association site. Thus, long-range correlation appears to exist upon the PAMc cleavage between the two active sites (∼10 nm apart) on Cas1-Cas2, which can be allosterically mediated by psDNA and Cas2 and Cas2’ in bridging. To substantiate such findings, we conducted repeated runs and further simulated Cas1-Cas2 in complex with synthesized psDNA sequences psL and psH, which have been measured with low and high frequency in acquisition, respectively. Notably, such intersite correlation becomes even more pronounced for the Cas1-Cas2 in complex with psH but remains low for the Cas1-Cas2 in complex with psL. Hence, our studies demonstrate that PAMc recognition and cleavage at one active site of Cas1-Cas2 may allosterically regulate non-PAMc association or even cleavage at the other site, and such regulation can be mediated by noncatalytic Cas2 and DNA protospacer to possibly support the ensued psDNA acquisition.

Deciphering protein microenvironment by using a cysteine specific switch-ON fluorescent probe

Fluorescent probes provide an unparalleled opportunity to visualize and quantify dynamic events. Here, we employ a medium-size, cysteine specific coumarin based switch-ON fluorescent probe 'L' to track protein unfolding profiles and accessibility of cysteine residues in proteins. It was established that 'L' is highly selective and exhibits no artifact due to interaction with other bystander species. 'L' is able to gauge subtle changes in protein microenvironment and proved to be effective in delineating early unfolding events that are difficult to otherwise discern by classic techniques such as circular dichroism. By solving the X-ray structure of TadA and probing the temperature dependent fluorescence-ON response with native TadA and its cysteine mutants, it was revealed that unfolding occurs in a stage-wise manner and the regions that are functionally important form compact sub-domains and unfold at later stages. Our results assert that probe 'L' serves as an efficient tool to monitor subtle changes in protein structure and can be employed as a generic dye to study processes such as protein unfolding.

Human de novo purine biosynthesis

The focus of this review is the human de novo purine biosynthetic pathway. The pathway enzymes are enumerated, as well as the reactions they catalyze and their physical properties. Early literature evidence suggested that they might assemble into a multi-enzyme complex called a metabolon. The finding that fluorescently-tagged chimeras of the pathway enzymes form discrete puncta, now called purinosomes, is further elaborated in this review to include: a discussion of their assembly; the role of ancillary proteins; their locus at the microtubule/mitochondria interface; the elucidation that at endogenous levels, purinosomes function to channel intermediates from phosphoribosyl pyrophosphate to AMP and GMP; and the evidence for the purinosomes to exist as a protein condensate. The review concludes with a consideration of probable signaling pathways that might promote the assembly and disassembly of the purinosome, in particular the identification of candidate kinases given the extensive phosphorylation of the enzymes. These collective findings substantiate our current view of the de novo purine biosynthetic metabolon whose properties will be representative of how other metabolic pathways might be organized for their function.

A Novel Aminoacyl-tRNA Synthetase Appended Domain Can Supply the Core Synthetase with Its Amino Acid Substrate

The structural organization and functionality of aminoacyl-tRNA synthetases have been expanded through polypeptide additions to their core aminoacylation domain. We have identified a novel domain appended to the methionyl-tRNA synthetase (MetRS) of the intracellular pathogen Mycoplasma penetrans. Sequence analysis of this N-terminal region suggests the appended domain is an aminotransferase, which we demonstrate here. The aminotransferase domain of MpMetRS is capable of generating methionine from its α-keto acid analog, 2-keto-4-methylthiobutyrate (KMTB). The methionine thus produced can be subsequently attached to cognate tRNA^Met in the MpMetRS aminoacylation domain. Genomic erosion in the Mycoplasma species has impaired many canonical biosynthetic pathways, causing them to rely on their host for numerous metabolites. It is still unclear if this bifunctional MetRS is a key part of pathogen life cycle or is a neutral consequence of the reductive evolution experienced by Mycoplasma species.