Synthesis and utility of 14C-labeled nicotinamide cofactors

Evolution of Optimized Hydride Transfer Reaction and Overall Enzyme Turnover in Human Dihydrofolate Reductase

Evolution of dihydrofolate reductase (DHFR) has been studied using the enzyme from Escherichia coli DHFR (ecDHFR) as a model, but less studies have used the enzyme from Homo sapiens DHFR (hsDHFR). Each enzyme maintains a short and narrow distribution of hydride donor-acceptor distances (DAD) at the tunneling ready state (TRS). Evolution of the enzyme was previously studied in ecDHFR where three key sites were identified as important to the catalyzed reaction. The corresponding sites in hsDHFR are F28, 62-PEKN, and 26-PPLR. Each of these sites was studied here through the creation of mutant variants of the enzyme and measurements of the temperature dependence of the intrinsic kinetic isotope effects (KIEs) on the reaction. F28 is mutated first to M (F28M) and then to the L of the bacterial enzyme (F28L). The KIEs of the F28M variant are larger and more temperature-dependent than wild-type (WT), suggesting a broader and longer average DAD at the TRS. To more fully mimic ecDHFR, we also study a triple mutant of the human enzyme (F32L-PP26N-PEKN62G). Remarkably, the intrinsic KIEs, while larger in magnitude, are temperature-independent like the WT enzymes. We also construct deletion mutations of hsDHFR removing both the 62-PEKN and 26-PPLR sequences. The results mirror those described previously for insertion mutants of ecDHFR. Taken together, these results suggest a balancing act during DHFR evolution between achieving an optimal TRS for hydride transfer and preventing product inhibition arising from the different intercellular pools of NADPH and NADP⁺ in prokaryotic and eukaryotic cells.

The Chinese medicinal herbs of spleen-meridian property regulate body temperature in yeast-induced fever rats

BACKGROUND

According to traditional Chinese medicine (TCM) theory, the herbal property is the most important guiding principle of ancient medication in China. The classification of warm- and cold-stimulating TCM is defined mainly based on the effects of herbs in regulating body temperature; however, the underlying mechanism of such distinction has not been fully identified.

METHODS

Here, four commonly used spleen-meridian herbs, Ginseng Radix and Astragali Radix as typical warm-stimulating herbs, and Nelumbinis Semen and Coicis Semen as typical cold-stimulating herbs, were selected to test their effects in regulating body temperature, as well as its triggered thermo-regulatory factors and energy related metabolites, in yeast-induced fever rats.

RESULTS

The intake of Astragali Radix increased body temperature in yeast-induced fever rats; while Coicis Semen showed cooling effects in such rats. In parallel, the levels of cAMP, PGE₂ and thermo-related metabolites, including choline, creatine, alanine, lactate and leucine, in the blood of yeast-induced rats were increased significantly by the intake of Astragali Radix. Oppositely, the cold-stimulating herbs, Nelumbinis Semen and Coicis Semen, showed cooling effects by increasing certain metabolites, e.g. histidine, tyrosine, lipid, myo-inositol, as well as AVP level.

CONCLUSION

Here, we compared different effects of warm and cooling spleen-meridian herbs in the regulation of body temperature. By providing an intuitive comparison of thermo-regulatory factors and related metabolites after intake of selected herbs, the mechanism behind the warm and cooling effects of specific herbs were revealed.

The chemistry of the vitamin B3 metabolome

The functional cofactors derived from vitamin B3 are nicotinamide adenine dinucleotide (NAD⁺), its phosphorylated form, nicotinamide adenine dinucleotide phosphate (NADP⁺) and their reduced forms (NAD(P)H). These cofactors, together referred as the NAD(P)(H) pool, are intimately implicated in all essential bioenergetics, anabolic and catabolic pathways in all forms of life. This pool also contributes to post-translational protein modifications and second messenger generation. Since NAD⁺ seats at the cross-road between cell metabolism and cell signaling, manipulation of NAD⁺ bioavailability through vitamin B3 supplementation has become a valuable nutritional and therapeutic avenue. Yet, much remains unexplored regarding vitamin B3 metabolism. The present review highlights the chemical diversity of the vitamin B3-derived anabolites and catabolites of NAD⁺ and offers a chemical perspective on the approaches adopted to identify, modulate and measure the contribution of various precursors to the NAD(P)(H) pool.

Examinations of the Chemical Step in Enzyme Catalysis

Advances in computational and experimental methods in enzymology have aided comprehension of enzyme-catalyzed chemical reactions. The main difficulty in comparing computational findings to rate measurements is that the first examines a single energy barrier, while the second frequently reflects a combination of many microscopic barriers. We present here intrinsic kinetic isotope effects and their temperature dependence as a useful experimental probe of a single chemical step in a complex kinetic cascade. Computational predictions are tested by this method for two model enzymes: dihydrofolate reductase and thymidylate synthase. The description highlights the significance of collaboration between experimentalists and theoreticians to develop a better understanding of enzyme-catalyzed chemical conversions.

The Effect of Protein Mass Modulation on Human Dihydrofolate Reductase

Dihydrofolate reductase (DHFR) from Escherichia coli has long served as a model enzyme with which to elucidate possible links between protein dynamics and the catalyzed reaction. Such physical properties of its human counterpart have not been rigorously studied so far, but recent computer-based simulations suggest that these two DHFRs differ significantly in how closely coupled the protein dynamics and the catalyzed C-H → C hydride transfer step are. To test this prediction, two contemporary probes for studying the effect of protein dynamics on catalysis were combined here: temperature dependence of intrinsic kinetic isotope effects (KIEs), which are sensitive to the physical nature of the chemical step, and protein mass modulation, which slows down fast dynamics (femto- to picosecond time scale) throughout the protein. The intrinsic H/T KIEs of human DHFR, like those of E. coli DHFR, are shown to be temperature-independent in the range from 5 to 45 °C, indicating fast sampling of donor and acceptor distances (DADs) at the reaction's transition state (or tunneling ready state, TRS). Mass modulation of these enzymes through isotopic labeling with (13)C, (15)N, and (2)H at nonexchangeable hydrogens yields an 11% heavier enzyme. The additional mass has no effect on the intrinsic KIEs of the human enzyme. This finding indicates that the mass modulation of the human DHFR affects neither DAD distribution nor the DAD's conformational sampling dynamics. Furthermore, reduction in the enzymatic turnover number and the dissociation rate constant for the product indicate that the isotopic substitution affects kinetic steps that are not the catalyzed C-H → C hydride transfer. The findings are discussed in terms of fast dynamics and their role in catalysis, the comparison of calculations and experiments, and the interpretation of isotopically modulated heavy enzymes in general.

Network of remote and local protein dynamics in dihydrofolate reductase catalysis

Molecular dynamics calculations and bionformatic studies of dihydrofolate reductase (DHFR) have suggested a network of coupled motions across the whole protein that is correlated to the reaction coordinate. Experimental studies demonstrated that distal residues G121, M42 and F125 in E. coli DHFR participate in that network. The missing link in our understanding of DHFR catalysis is the lack of a mechanism by which such remote residues can affect the catalyzed chemistry at the active site. Here, we present a study of the temperature dependence of intrinsic kinetic isotope effects (KIEs) that indicates synergism between a remote residue in that dynamic network, G121, and the active site's residue I14. The intrinsic KIEs for the I14A-G121V double mutant showed steeper temperature dependence (ΔE_a(T-H)) than expected from comparison of the wild type and two single mutants. That effect was non-additive, i.e., ΔE_a(T-H)G121V +ΔE_{a(T-H) I14A} < ΔE_{a(T-H) double mutant}, which indicates a synergism between the two residues. This finding links the remote residues in the network under investigation to the enzyme's active site, providing a mechanism by which these residues can be coupled to the catalyzed chemistry. This experimental evidence validates calculations proposing that both remote and active site residues constitute a network of coupled promoting motions correlated to the bond activation step (C-H→C hydride transfer in this case). Additionally, the effect of I14A and G121V mutations on single turnover rates was additive rather than synergistic. Although single turnover rate measurements are more readily available and thus more popular than assessing intrinsic kinetic isotope effects, the current finding demonstrates that for these rates, which in DHFR reflect several microscopic rate constants, can fall short of revealing the nature of the C-H bond activation per se.

Determination of concentration and activity of immobilized enzymes

Methods that directly measure the concentration of surface-immobilized biomolecules are scarce. More commonly, the concentration of the soluble molecule is measured before and after immobilization, and the bound concentration is assessed by elimination, assuming that all bound molecules are active. An assay was developed for measuring the active site concentration, activity, and thereby the catalytic turnover rate (kcat) of an immobilized dihydrofolate reductase as a model system. The new method yielded a similar first-order rate constant, kcat, to that of the same enzyme in solution. The findings indicate that the activity of the immobilized enzyme, when separated from the surface by the DNA spacers, has not been altered. In addition, a new immobilization method that leads to solution-like activity of the enzyme on the surface is described. The approaches developed here for immobilization and for determining the concentration of an immobilized enzyme are general and can be extended to other enzymes, receptors, and antibodies.

Escherichia coli dihydrofolate reductase catalyzed proton and hydride transfers: temporal order and the roles of Asp27 and Tyr100

The reaction catalyzed by Escherichia coli dihydrofolate reductase (ecDHFR) has become a model for understanding enzyme catalysis, and yet several details of its mechanism are still unresolved. Specifically, the mechanism of the chemical step, the hydride transfer reaction, is not fully resolved. We found, unexpectedly, the presence of two reactive ternary complexes [enzyme:NADPH:7,8-dihydrofolate (E:NADPH:DHF)] separated by one ionization event. Furthermore, multiple kinetic isotope effect (KIE) studies revealed a stepwise mechanism in which protonation of the DHF precedes the hydride transfer from the nicotinamide cofactor (NADPH) for both reactive ternary complexes of the WT enzyme. This mechanism was supported by the pH- and temperature-independent intrinsic KIEs for the C-H→C hydride transfer between NADPH and the preprotonated DHF. Moreover, we showed that active site residues D27 and Y100 play a synergistic role in facilitating both the proton transfer and subsequent hydride transfer steps. Although D27 appears to have a greater effect on the overall rate of conversion of DHF to tetrahydrofolate, Y100 plays an important electrostatic role in modulating the pKa of the N5 of DHF to enable the preprotonation of DHF by an active site water molecule.

Extension and limits of the network of coupled motions correlated to hydride transfer in dihydrofolate reductase

Enzyme catalysis has been studied extensively, but the role of enzyme dynamics in the catalyzed chemical conversion is still an enigma. The enzyme dihydrofolate reductase (DHFR) is often used as a model system to assess a network of coupled motions across the protein that may affect the catalyzed chemical transformation. Molecular dynamics simulations, quantum mechanical/molecular mechanical studies, and bioinformatics studies have suggested the presence of a “global dynamic network” of residues in DHFR. Earlier studies of two DHFR distal mutants, G121V and M42W, indicated that these residues affect the chemical step synergistically. While this finding was in accordance with the concept of a network of functional motions across the protein, two residues do not constitute a network. To better define the extent and limits of the proposed network, the current work studied two remote residues predicted to be part of the same network: W133 and F125. The effect of mutations in these residues on the nature of the chemical step was examined via measurements of the temperature-dependence of the intrinsic kinetic isotope effects (KIEs) and other kinetic parameters, and double mutants were used to tie the findings to G121 and M42. The findings indicate that residue F125, which was implicated by both calculations and bioinformatic methods, is a part of the same global dynamic network as G121 and M42, while W133, implicated only by bioinformatics, is not. These findings extend our understanding of the proposed network and the relations between functional and genomic couplings. Delineating that network illuminates the need to consider remote residues and protein structural dynamics in the rational design of drugs and of biomimetic catalysts.

Preservation of protein dynamics in dihydrofolate reductase evolution

The hydride transfer reaction catalyzed by dihydrofolate reductase (DHFR) is a model for examining how protein dynamics contribute to enzymatic function. The relationship between functional motions and enzyme evolution has attracted significant attention. Recent studies on N23PP Escherichia coli DHFR (ecDHFR) mutant, designed to resemble parts of the human enzyme, indicated a reduced single turnover rate. NMR relaxation dispersion experiments with that enzyme showed rigidification of millisecond Met-20 loop motions (Bhabha, G., Lee, J., Ekiert, D. C., Gam, J., Wilson, I. A., Dyson, H. J., Benkovic, S. J., and Wright, P. E. (2011) Science 332, 234-238). A more recent study of this mutant, however, indicated that fast motions along the reaction coordinate are actually more dispersed than for wild-type ecDHFR (WT). Furthermore, a double mutant (N23PP/G51PEKN) that better mimics the human enzyme seems to restore both the single turnover rates and narrow distribution of fast dynamics (Liu, C. T., Hanoian, P., French, T. H., Hammes-Schiffer, S., and Benkovic, S. J. (2013) Proc. Natl. Acad. Sci. U.S.A. 110, 10159-11064). Here, we measured intrinsic kinetic isotope effects for both N23PP and N23PP/G51PEKN double mutant DHFRs over a temperature range. The findings indicate that although the C-H→C transfer and dynamics along the reaction coordinate are impaired in the altered N23PP mutant, both seem to be restored in the N23PP/G51PEKN double mutant. This indicates that the evolution of G51PEKN, although remote from the Met-20 loop, alleviated the loop rigidification that would have been caused by N23PP, enabling WT-like H-tunneling. The correlation between the calculated dynamics, the nature of C-H→C transfer, and a phylogenetic analysis of DHFR sequences are consistent with evolutionary preservation of the protein dynamics to enable H-tunneling from well reorganized active sites.

Synthesis of radiolabeled nicotinamide cofactors from labeled pyridines: versatile probes for enzyme kinetics

(14)C-labeled nicotinamide cofactors are widely employed in biomedical investigations, for example, to delineate metabolic pathways, to elucidate enzymatic mechanisms, and as substrates in kinetic isotope effect (KIE) experiments. The (14)C label has generally been located remote from the reactive position, frequently at the adenine ring. Rising costs of commercial precursors and disruptions in the availability of enzymes required for established syntheses have recently made the preparation of labeled nicotinamides such as [Ad-(14)C]NADPH unviable. Here, we report the syntheses and characterization of several alternatives: [carbonyl-(14)C]NADPH, 4R-[carbonyl-(14)C, 4-(2)H]NADPH, and [carbonyl-(14)C, 4-(2)H(2)]NADPH. The new procedures use [carbonyl-(14)C]nicotinamide as starting material, because it is significantly cheaper than other commercial (14)C precursors of NADPH, and require only one commercially available enzyme to prepare NAD(P)(+) and NAD(P)H. The proximity of carbonyl-(14)C to the reactive center raises the risk of an inopportune (14)C isotope effect. This concern has been alleviated via competitive KIE measurements with Escherichia coli dihydrofolate reductase (EcDHFR) that use this specific carbonyl-(14)C NADPH. A combination of binding isotope effect and KIE measurements yielded no significant (12)C/(14)C isotope effect at the amide carbonyl (KIE=1.003±0.004). The reported procedure provides a high-yield, high-purity, and cost-effective alternative to labeled nicotinamide cofactors synthesized by previously published routes.

Effects of the donor-acceptor distance and dynamics on hydride tunneling in the dihydrofolate reductase catalyzed reaction

A significant contemporary question in enzymology involves the role of protein dynamics and hydrogen tunneling in enhancing enzyme catalyzed reactions. Here, we report a correlation between the donor-acceptor distance (DAD) distribution and intrinsic kinetic isotope effects (KIEs) for the dihydrofolate reductase (DHFR) catalyzed reaction. This study compares the nature of the hydride-transfer step for a series of active-site mutants, where the size of a side chain that modulates the DAD (I14 in E. coli DHFR) is systematically reduced (I14V, I14A, and I14G). The contributions of the DAD and its dynamics to the hydride-transfer step were examined by the temperature dependence of intrinsic KIEs, hydride-transfer rates, activation parameters, and classical molecular dynamics (MD) simulations. Results are interpreted within the framework of the Marcus-like model where the increase in the temperature dependence of KIEs arises as a direct consequence of the deviation of the DAD from its distribution in the wild type enzyme. Classical MD simulations suggest new populations with larger average DADs, as well as broader distributions, and a reduction in the population of the reactive conformers correlated with the decrease in the size of the hydrophobic residue. The more flexible active site in the mutants required more substantial thermally activated motions for effective H-tunneling, consistent with the hypothesis that the role of the hydrophobic side chain of I14 is to restrict the distribution and dynamics of the DAD and thus assist the hydride-transfer. These studies establish relationships between the distribution of DADs, the hydride-transfer rates, and the DAD's rearrangement toward tunneling-ready states. This structure-function correlation shall assist in the interpretation of the temperature dependence of KIEs caused by mutants far from the active site in this and other enzymes, and may apply generally to C-H→C transfer reactions.

Engineering the respiratory complex I to energy-converting NADPH:ubiquinone oxidoreductase

The respiratory complex I couples the electron transfer from NADH to ubiquinone with a translocation of protons across the membrane. Its nucleotide-binding site is made up of a unique Rossmann fold to accommodate the binding of the substrate NADH and of the primary electron acceptor flavin mononucleotide. Binding of NADH includes interactions of the hydroxyl groups of the adenosine ribose with a conserved glutamic acid residue. Structural analysis revealed that due to steric hindrance and electrostatic repulsion, this residue most likely prevents the binding of NADPH, which is a poor substrate of the complex. We produced several variants with mutations at this position exhibiting up to 200-fold enhanced catalytic efficiency with NADPH. The reaction of the variants with NAD(P)H is coupled with proton translocation in an inhibitor-sensitive manner. Thus, we have created an energy-converting NADPH:ubiquinone oxidoreductase, an activity so far not found in nature. Remarkably, the oxidation of NAD(P)H by the variants leads to an enhanced production of reactive oxygen species.

Triple isotopic labeling and kinetic isotope effects: exposing H-transfer steps in enzymatic systems

Kinetic isotope effect (KIE) studies can provide insight into the mechanism and kinetics of specific chemical steps in complex catalytic cascades. Recent results from hydrogen KIE measurements have examined correlations between enzyme dynamics and catalytic function, leading to a surge of studies in this area. Unfortunately, most enzymatic H-transfer reactions are not rate limiting, and the observed KIEs do not reliably reflect the intrinsic KIEs on the chemical step of interest. Given their importance to understanding the chemical step under study, accurate determination of the intrinsic KIE from the observed data is essential. In 1975, Northrop developed an elegant method to assess intrinsic KIEs from their observed values [Northrop, D. B. (1975) Steady-state analysis of kinetic isotope effects in enzymic reactions. Biochemistry 14, 2644-2651]. The Northrop method involves KIE measurements using all three hydrogen isotopes, where one of them serves as the reference isotope. This method has been successfully used with different combinations of observed KIEs over the years, but criteria for a rational choice of reference isotope have never before been experimentally determined. Here we compare different reference isotopes (and hence distinct experimental designs) using the reduction of dihydrofolate and dihydrobiopterin by two dissimilar enzymes as model reactions. A number of isotopic labeling patterns have been applied to facilitate the comparative study of reference isotopes. The results demonstrate the versatility of the Northrop method and that such experiments are limited only by synthetic techniques, availability of starting materials, and the experimental error associated with the use of distinct combinations of isotopologues.

The role of enzyme dynamics and tunnelling in catalysing hydride transfer: studies of distal mutants of dihydrofolate reductase

Residues M42 and G121 of Escherichia coli dihydrofolate reductase (ecDHFR) are on opposite sides of the catalytic centre (15 and 19 A away from it, respectively). Theoretical studies have suggested that these distal residues might be part of a dynamics network coupled to the reaction catalysed at the active site. The ecDHFR mutant G121V has been extensively studied and appeared to have a significant effect on rate, but only a mild effect on the nature of H-transfer. The present work examines the effect of M42W on the physical nature of the catalysed hydride transfer step. Intrinsic kinetic isotope effects (KIEs), their temperature dependence and activation parameters were studied. The findings presented here are in accordance with the environmentally coupled hydrogen tunnelling. In contrast to the wild-type (WT), fluctuations of the donor-acceptor distance were required, leading to a significant temperature dependence of KIEs and deflated intercepts. A comparison of M42W and G121V to the WT enzyme revealed that the reduced rates, the inflated primary KIEs and their temperature dependences resulted from an imperfect potential surface pre-arrangement relative to the WT enzyme. Apparently, the coupling of the enzyme's dynamics to the reaction coordinate was altered by the mutation, supporting the models in which dynamics of the whole protein is coupled to its catalysed chemistry.

Coordinated effects of distal mutations on environmentally coupled tunneling in dihydrofolate reductase

One of the most intriguing questions in modern enzymology is whether enzyme dynamics evolved to enhance the catalyzed chemical transformation. In this study, dihydrofolate reductase, a small monomeric protein that catalyzes a single C-H-C transfer, is used as a model system to address this question. Experimental and computational studies have proposed a dynamic network that includes two residues remote from the active site (G121 and M42). The current study compares the nature of the H-transfer step of the WT enzyme, two single mutants, and their double mutant. The contribution of quantum mechanical tunneling and enzyme dynamics to the H-transfer step was examined by determining intrinsic kinetic isotope effects, their temperature dependence, and activation parameters. Different patterns of environmentally coupled tunneling were found for these four enzymes. The findings indicate that the naturally evolved WT dihydrofolate reductase requires no donor-acceptor distance fluctuations (no gating). Both single mutations affect the rearrangement of the system before tunneling, so some gating is required, but the overall nature of the environmentally coupled tunneling appears similar to that of the WT enzyme. The double mutation, on the other hand, seems to cause a major change in the nature of H transfer, leading to poor reorganization and substantial gating. These findings support the suggestion that these distal residues synergistically affect the H transfer at the active site of the enzyme. This observation is in accordance with the notion that these remote residues are part of a dynamic network that is coupled to the catalyzed chemistry.

Effects of a distal mutation on active site chemistry

Previous studies of Escherichia coli dihydrofolate reductase (ecDHFR) have demonstrated that residue G121, which is 19 A from the catalytic center, is involved in catalysis, and long distance dynamical motions were implied. Specifically, the ecDHFR mutant G121V has been extensively studied by various experimental and theoretical tools, and the mutation's effect on kinetic, structural, and dynamical features of the enzyme has been explored. This work examined the effect of this mutation on the physical nature of the catalyzed hydride transfer step by means of intrinsic kinetic isotope effects (KIEs), their temperature dependence, and activation parameters as described previously for wild type ecDHFR [Sikorski, R. S., et al. (2004) J. Am. Chem. Soc. 126, 4778-4779]. The temperature dependence of initial velocities was used to estimate activation parameters. Isotope effects on the preexponential Arrhenius factors, and the activation energy, could be rationalized by an environmentally coupled hydrogen tunneling model, similar to the one used for the wild-type enzyme. Yet, in contrast to that in the wild type, fluctuations of the donor-acceptor distance were now required. Secondary (2 degrees ) KIEs were also measured for both H- and D-transfer, and as in the case of the wild-type enzyme, no coupled motion was detected. Despite these similarities, the reduced rates, the slightly inflated primary (1 degrees ) KIEs, and their temperature dependence, together with relatively deflated 2 degrees KIEs, indicate that the potential surface prearrangement was not as ideal as for the wild-type enzyme. These findings support theoretical studies suggesting that the G121V mutation led to a different conformational ensemble of reactive states and less effective rearrangement of the potential surface but has an only weak effect on H-tunneling.

Purification, analysis, and preservation of reduced nicotinamide adenine dinucleotide 2'-phosphate

Nicotinamide-containing cofactors are ubiquitous in biological systems. Consequently, numerous assays have been developed to study such systems that involve a variety of derivatives and isotopically labeled forms of these cofactors. Often the nicotinamide ring is labeled at the C-4 position which is directly involved in the hydride transfer chemistry catalyzed by nicotinamide-dependent enzymes. A label remote from the reactive center is often also required to follow the course of a reaction or the location of the cofactor. Since the necessary labeling pattern can be as unique as the designed experiment, these cofactors need to be synthesized, analyzed, and, most preferably, preserved. The micro-scale preservation of reduced nicotinamides has long been a challenge due to the inherent lability of the reduced cofactors. Furthermore, it has been found that the reduced 2'-phosphorylated cofactor is even less stable (i.e., reduced nicotinamide adenine dinucleotide phosphate (NADPH) is more labile than reduced nicotinamide adenine dinucleotide). Presented here are methods that were established to purify nicotinamide cofactors via reverse-phase high-performance liquid chromatography (HPLC) and, most importantly, to stabilize NADPH under optimal conditions for long-term storage. Additionally, an analytical HPLC method which achieves 7-min resolution between oxidized and reduced cofactors was developed. This method also results in over 4-min resolution of these nicotinamide cofactors from various derivatives of folic acid. This analysis affords a new analytical assay for the dihydrofolate reductase-catalyzed reaction and several dehydrogenases involved in folic acid metabolism.

Tunneling and Coupled Motion in the Escherichia coli Dihydrofolate Reductase Catalysis

H-transfer was studied in the complex kinetic cascade of dihydrofolate reductase. Intrinsic kinetic isotope effects, their temperature dependence, and other temperature-dependent parameters indicated H-tunneling, but no 1 degrees to 2 degrees coupled motion. The data also suggested environmentally coupled tunneling and commitment to catalysis on pre-steady-state isotope effects.