On the role of the conserved aspartate in the hydrolysis of the phosphocysteine intermediate of the low molecular weight tyrosine phosphatase

The usual rate-determining step in the catalytic mechanism of the low molecular weight tyrosine phosphatases involves the hydrolysis of a phosphocysteine intermediate. To explain this hydrolysis, general base-catalyzed attack of water by the anion of a conserved aspartic acid has sometimes been invoked. However, experimental measurements of solvent deuterium kinetic isotope effects for this enzyme do not reveal a rate-limiting proton transfer accompanying dephosphorylation. Moreover, base activation of water is difficult to reconcile with the known gas-phase proton affinities and solution phase pK(a)'s of aspartic acid and water. Alternatively, hydrolysis could proceed by a direct nucleophilic attack by a water molecule. To understand the hydrolysis mechanism, we have used high-level density functional methods of quantum chemistry combined with continuum electrostatics models of the protein and the solvent. Our calculations do not support a catalytic activation of water by the aspartate. Instead, they indicate that the water oxygen directly attacks the phosphorus, with the aspartate residue acting as a H-bond acceptor. In the transition state, the water protons are still bound to the oxygen. Beyond the transition state, the barrier to proton transfer to the base is greatly diminished; the aspartate can abstract a proton only after the transition state, a result consistent with experimental solvent isotope effects for this enzyme and with established precedents for phosphomonoester hydrolysis.     

Role of protein dynamics in reaction rate enhancement by enzymes

An integrated view of protein structure, dynamics, and function is emerging, where proteins are considered as dynamically active assemblies and internal motions are closely linked to function such as enzyme catalysis. Further, the motion of solvent bound to external regions of protein impacts internal motions and, therefore, protein function. Recently, we discovered a network of protein vibrations in enzyme cyclophilin A, coupled to its catalytic activity of peptidyl-prolyl cis-trans isomerization. Detailed studies suggest that this network, extending from surface regions to active site, is a conserved part of enzyme structure and has a role in promoting catalysis. In this report, theoretical investigations of concerted conformational fluctuations occurring on microsecond and longer time scales within the discovered network are presented. Using a new technique, kinetic energy was added to protein vibrational modes corresponding to conformational fluctuations in the network. The results reveal that protein dynamics promotes catalysis by altering transition state barrier crossing behavior of reaction trajectories. An increase in transmission coefficient and number of productive trajectories with increasing amounts of kinetic energy in vibrational modes is observed. Variations in active site enzyme-substrate interactions near transition state are found to be correlated with barrier recrossings. Simulations also showed that energy transferred from first solvation shell to surface residues impacts catalysis through network fluctuations. The detailed characterization of network presented here indicates that protein dynamics plays a role in rate enhancement by enzymes. Therefore, coupled networks in enzymes have wide implications in understanding allostericity and cooperative effects, as well as protein engineering and drug design.     

The role of the putative catalytic base in the phosphoryl transfer reaction in a protein kinase: first-principles calculations

Protein kinases are important enzymes controlling the majority of cellular signaling events via a transfer of the gamma-phosphate of ATP to a target protein. Even after many years of study, the mechanism of this reaction is still poorly understood. Among many factors that may be responsible for the 1011-fold rate enhancement due to this enzyme, the role of the conserved aspartate (Asp166) has been given special consideration. While the essential presence of Asp166 has been established by mutational studies, its function is still debated. The general base catalyst role assigned to Asp166 on the basis of its position in the active site has been brought into question by the pH dependence of the reaction rate, isotope measurements, and pre-steady-state kinetics. Recent semiempirical calculations have added to the controversy surrounding the role of Asp166 in the catalytic mechanism. No major role for Asp166 has been found in these calculations, which have predicted the reaction process consisting of an early transfer of a substrate proton onto the phosphate group. These conclusions were inconsistent with experimental observations. To address these differences between experimental results and theory with a more reliable computational approach and to provide a theoretical platform for understanding catalysis in this important enzyme family, we have carried out first-principles structural and dynamical calculations of the reaction process in cAPK kinase. To preserve the essential features of the reaction, representations of all of the key conserved residues (82 atoms) were included in the calculation. The structural calculations were performed using the local basis density functional (DFT) approach with both hybrid B3LYP and PBE96 generalized gradient approximations. This kind of calculation has been shown to yield highly accurate structural information for a large number of systems. The optimized reactant state structure is in good agreement with X-ray data. In contrast to semiempirical methods, the lowest energy product state places the substrate proton on Asp166. First-principles molecular dynamics simulations provide additional support for the stability of this product state. The latter also demonstrate that the proton transfer to Asp166 occurs at a point in the reaction where bond cleavage at the PO bridging position is already advanced. This mechanism is further supported by the calculated structure of the transition state in which the substrate hydroxyl group is largely intact. A metaphoshate-like structure is present in the transition state, which is consistent with the X-ray structures of transition state mimics. On the basis of the calculated structure of the transition state, it is estimated to be 85% dissociative. Our analysis also indicates an increase in the hydrogen bond strength between Asp166 and substrate hydroxyl and a small decrease in the bond strength of the latter in the transition state. In summary, our calculations demonstrate the importance of Asp166 in the enzymatic mechanism as a proton acceptor. However, the proton abstraction from the substrate occurs late in the reaction process. Thus, in the catalytic mechanism of cAPK protein kinase, Asp166 plays a role of a "proton trap" that locks the transferred phosphoryl group to the substrate. These results resolve prior inconsistencies between theory and experiment and bring new understanding of the role of Asp166 in the protein kinase catalytic mechanism.     

Proton-coupled electron transfer in soybean lipoxygenase: dynamical behavior and temperature dependence of kinetic isotope effects

The dynamical behavior and the temperature dependence of the kinetic isotope effects (KIEs) are examined for the proton-coupled electron transfer reaction catalyzed by the enzyme soybean lipoxygenase. The calculations are based on a vibronically nonadiabatic formulation that includes the quantum mechanical effects of the active electrons and the transferring proton, as well as the motions of all atoms in the complete solvated enzyme system. The rate constant is represented by the time integral of a probability flux correlation function that depends on the vibronic coupling and on time correlation functions of the energy gap and the proton donor-acceptor mode, which can be calculated from classical molecular dynamics simulations of the entire system. The dynamical behavior of the probability flux correlation function is dominated by the equilibrium protein and solvent motions and is not significantly influenced by the proton donor-acceptor motion. The magnitude of the overall rate is strongly influenced by the proton donor-acceptor frequency, the vibronic coupling, and the protein/solvent reorganization energy. The calculations reproduce the experimentally observed magnitude and temperature dependence of the KIE for the soybean lipoxygenase reaction without fitting any parameters directly to the experimental kinetic data. The temperature dependence of the KIE is determined predominantly by the proton donor-acceptor frequency and the distance dependence of the vibronic couplings for hydrogen and deuterium. The ratio of the overlaps of the hydrogen and deuterium vibrational wavefunctions strongly impacts the magnitude of the KIE but does not significantly influence its temperature dependence. For this enzyme reaction, the large magnitude of the KIE arises mainly from the dominance of tunneling between the ground vibronic states and the relatively large ratio of the overlaps between the corresponding hydrogen and deuterium vibrational wavefunctions. The weak temperature dependence of the KIE is due in part to the dominance of the local component of the proton donor-acceptor motion.     

Significant kinetic solvent isotope effects in folding of the catalytic RNA from the hepatitis delta virus

The exchange of deuterium for hydrogen in water often produces solvent kinetic isotope effects (KSIEs) on the rate constants associated with enzyme reactions, including those catalyzed by RNA. Recently, KSIEs have been used to show that proton transfer occurs in the rate-limiting step of cleavage by the hepatitis delta virus (HDV) ribozyme and other catalytic RNAs. To test the underlying assumption that KSIEs are related to the chemistry step of ribozyme-mediated cleavage reactions, we developed fluorescence resonance energy transfer assays to measure KSIEs on the rate constants of conformational changes associated with substrate binding and dissociation by a trans-acting HDV ribozyme. We observe comparable KSIEs ( approximately 2-2.5-fold) of rate constants of conformational change and cleavage, while proton inventory experiments are consistent with a shift in the ensemble of transition states upon increase of D2O in the solvent. Taken together, these results challenge the common assumption that pL profiles of RNA-catalyzed reactions yielding a pKa and KSIE necessarily provide evidence for an ionization (chemistry) step to be rate-limiting. They also suggest that an unusual proton inventory may provide a signature for a conformational change contributing to the rate-limiting step.     

Uncoupled forms of tyrosine hydroxylase unmask kinetic isotope effects on chemical steps

Tyrosine hydroxylase (TyrH) catalyzes the hydroxylation of tyrosine to dihydroxyphenylalanine. In the proposed mechanism, a ferryl-oxo species attacks the aromatic ring of tyrosine, forming a cationic intermediate. However, no significant isotope effect is found for wild-type TyrH when 3,5-2H2-tyrosine is used as a substrate. The isotope effect has now been determined with 3,5-2H2-tyrosine using mutant forms of TyrH in which the oxidation of the pterin is uncoupled from hydroxylation of the amino acid. Three mutant enzymes exhibit significant inverse deuterium isotope effects and inverse solvent isotope effects. A proton inventory for the E326A enzyme is consistent with a normal solvent isotope effect of 2.4 on an unproductive step. The results support the proposed mechanism and demonstrate the utility of using mutant proteins with branched pathways to reveal isotope effects which are masked in the wild-type enzyme.     

Reaction of imidazole with toluene-4-sulfonate salts of substituted phenyl N-methylpyridinium-4-carboxylate esters: special base catalysis by imidazole

The reaction of imidazole in aqueous solution with toluene-4-sulfonate salts of substituted phenyl N-methylpyridinium-4-carboxylate esters obeys the rate law: k(obs) - k(background) = k2[Im] + k3[Im]2 where [Im] is the imidazole concentration present as free base. The parameters k2 and k3 fit Br?nsted type free energy correlations against the pKa of the leaving phenol with betaLg values of -0.65 and -0.42 respectively. The imidazolysis is insensitive to catalysis by general bases and yet k3 for the 3-cyanophenyl ester possesses a deuterium oxide solvent isotope effect of 4.43 consistent with rate limiting proton transfer. A special catalytic function is proposed for decomposition of the tetrahedral addition intermediate (T+/-) via k3 whereby the catalytic imidazole interacts electrophilically with the leaving phenolate ion and removes a proton from the nitrogen in the rate limiting step with subsequent non-rate limiting ArO-C bond fission. This is consistent with the change in effective charge on the leaving oxygen in the transition structure of k3 which is more positive (-0.42) than that expected (-0.60) for the equilibrium formation of the zwitterion intermediate. The catalytic function at the leaving oxygen is likely to be an electrophilic role of the NH as a hydrogen bond donor. In the k2 step the deuterium oxide solvent isotope effect of 1.51 for the 3-cyanophenyl ester and the betaLg of -0.65 are consistent with rate limiting expulsion of the phenolate ion from the T+/- intermediate. The absence of general base catalysis of imidazolysis rules out the established mechanism for aminolysis of esters where T+/- is stabilised by a standard rate limiting proton transfer. The kinetically equivalent term for k3 where T- reacts with the imidazolium ion as an acid catalyst would require this step to be rate limiting and involve proton transfer not consistent with departure of the good aryl oxide leaving group.     

Simulation of slow reaction with quantum character: Neutral hydrolysis of carboxylic ester

By computer simulation, using both quantum and classical dynamics, we determined the rate constant and the kinetic isotope effect of the rate-determining step in the neutral hydrolysis of p-methoxyphenyl dichloroacetate in aqueous solution. This step involves a proton transfer concerted with the formation of a C O bond. A method of biased sampling was used; the Gibbs free energy of the biased configuration from which proton transfer is likely to occur was determined by a combination of semiempirical quantum calculations and thermodynamic integration. The proton dynamics was modeled with the quantum-dynamical density matrix evolution method that includes nonadiabatic pathways. The proton dynamics is driven by a fluctuating proton potential that was derived from a classical molecular dynamics simulation of the system including solvent. The calculated rate constant of 3×10⁻² s⁻¹ agrees within the error of the calculation with the experimentally observed value of 2.78×10⁻³. The calculated pseudo-first-order kinetic isotope effect of 3.9 is in good agreement with the experimentally observed value of 3.2. The results show the feasibility of computational approaches to slow reactions in complex environments, where proton transfer with an essential quantum-dynamical nature is the rate-limiting step. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 886–895, 1999     

Isotope Effects in the Solvolysis of Sterically Hindered Arenesulfonyl Chlorides

Solvent isotope effects in the ethanolysis of sterically hindered arenesulfonyl chlorides ruled out a proton transfer in the rate‐determining step and agreed with a S_N2 mechanism involving at least a second solvent molecule in the transition state (TS). The lack of a secondary kinetic isotope effect in the o‐alkyl groups allows us to disregard the possible contribution of σ–π hyperconjugation. The measured activation parameters are consistent with a S_N2 mechanism involving the participation of solvent molecules in the TS, possibly forming a cyclic TS through a chain of solvent molecules.     

Accumulation of tetrahedral intermediates in cholinesterase catalysis: a secondary isotope effect study

In a previous communication, kinetic β-deuterium secondary isotope effects were reported that support a mechanism for substrate-activated turnover of acetylthiocholine by human butyrylcholinesterase (BuChE) wherein the accumulating reactant state is a tetrahedral intermediate ( Tormos , J. R. ; et al. J. Am. Chem. Soc. 2005 , 127 , 14538 - 14539 ). In this contribution additional isotope effect experiments are described with acetyl-labeled acetylthiocholines (CL(3)COSCH(2)CH(2)N(+)Me(3); L = H or D) that also support accumulation of the tetrahedral intermediate in Drosophila melanogaster acetylcholinesterase (DmAChE) catalysis. In contrast to the aforementioned BuChE-catalyzed reaction, for this reaction the dependence of initial rates on substrate concentration is marked by pronounced substrate inhibition at high substrate concentrations. Moreover, kinetic β-deuterium secondary isotope effects for turnover of acetylthiocholine depended on substrate concentration, and gave the following: (D3)k(cat)/K(m) = 0.95 ± 0.03, (D3)k(cat) = 1.12 ± 0.02 and (D3)βk(cat) = 0.97 ± 0.04. The inverse isotope effect on k(cat)/K(m) is consistent with conversion of the sp(2)-hybridized substrate carbonyl in the E + A reactant state into a quasi-tetrahedral transition state in the acylation stage of catalysis, whereas the markedly normal isotope effect on k(cat) is consistent with hybridization change from sp(3) toward sp(2) as the reactant state for deacylation is converted into the subsequent transition state. Transition states for Drosophila melanogaster AChE-catalyzed hydrolysis of acetylthiocholine were further characterized by measuring solvent isotope effects and determining proton inventories. These experiments indicated that the transition state for rate-determining decomposition of the tetrahedral intermediate is stabilized by multiple protonic interactions. Finally, a simple model is proposed for the contribution that tetrahedral intermediate stabilization provides to the catalytic power of acetylcholinesterase.     

Hydride transfer in liver alcohol dehydrogenase: quantum dynamics, kinetic isotope effects, and role of enzyme motion.

The quantum dynamics of the hydride transfer reaction catalyzed by liver alcohol dehydrogenase (LADH) are studied with real-time dynamical simulations including the motion of the entire solvated enzyme. The electronic quantum effects are incorporated with an empirical valence bond potential, and the nuclear quantum effects of the transferring hydrogen are incorporated with a mixed quantum/classical molecular dynamics method in which the transferring hydrogen nucleus is represented by a three-dimensional vibrational wave function. The equilibrium transition state theory rate constants are determined from the adiabatic quantum free energy profiles, which include the free energy of the zero point motion for the transferring nucleus. The nonequilibrium dynamical effects are determined by calculating the transmission coefficients with a reactive flux scheme based on real-time molecular dynamics with quantum transitions (MDQT) surface hopping trajectories. The values of nearly unity for these transmission coefficients imply that nonequilibrium dynamical effects such as barrier recrossings are not dominant for this reaction. The calculated deuterium and tritium kinetic isotope effects for the overall rate agree with experimental results. These simulations elucidate the fundamental nature of the nuclear quantum effects and provide evidence of hydrogen tunneling in the direction along the donor-acceptor axis. An analysis of the geometrical parameters during the equilibrium and nonequilibrium simulations provides insight into the relation between specific enzyme motions and enzyme activity. The donor-acceptor distance, the catalytic zinc-substrate oxygen distance, and the coenzyme (NAD(+)/NADH) ring angles are found to strongly impact the activation free energy barrier, while the donor-acceptor distance and one of the coenzyme ring angles are found to be correlated to the degree of barrier recrossing. The distance between VAL-203 and the reactive center is found to significantly impact the activation free energy but not the degree of barrier recrossing. This result indicates that the experimentally observed effect of mutating VAL-203 on the enzyme activity is due to the alteration of the equilibrium free energy difference between the transition state and the reactant rather than nonequilibrium dynamical factors. The promoting motion of VAL-203 is characterized in terms of steric interactions involving THR-178 and the coenzyme.     

Evidence for direct attack by hydroxide in phosphodiester hydrolysis

Phosphodiester hydrolysis has been the subject of intense study due to its importance in biology. Despite the numerous significant analyses of phosphodiester cleavage mechansim, comparatively little is known about the nucleophiles in these reactions. To determine whether hydroxide acts as a nucleophile or a general base in the hydrolysis of thymidine-5'-p-nitrophenyl phosphate,we determined solvent deuterium isotope effects (D2Ok), ionic strength effects, and 18O isotope effects on the solvent nucleophile (18knuc). The D2Ok for hydroxide-catalyzed phosphodiester hydrolysis is slightly inverse (0.9 +/- 0.1), suggesting that a proton transfer does not occur in the transition state. A significant alpha effect is observed with hydroperoxide, demonstrating that oxyanions can act as nucleophiles in the reaction. Additionally, the ionic strength dependencies of hydroxide and hydroperoxide catalysis are indistinguishable, suggesting that they act by the same mechanism. Finally, the 18knuc for the hydroxide-catalyzed reaction is 1.068 +/- 0.007, well in excess of the equilibrium 18O isotope effect between water and hydroxide (1.040 +/- 0.003). Together, the data are most consistent with direct nucleophilic attack by hydroxide. From the observed 18knuc and the known equilibrium component, the kinetic component of the isotope effect was calculated to be 1.027 +/- 0.010. This large kinetic component suggests that little bond order to the nucleophile occurs in the transition state.     

Millisecond dynamics in glutaredoxin during catalytic turnover is dependent on substrate binding and absent in the resting states

Conformational dynamics is important for enzyme function. Which motions of enzymes determine catalytic efficiency and whether the same motions are important for all enzymes, however, are not well understood. Here we address conformational dynamics in glutaredoxin during catalytic turnover with a combination of NMR magnetization transfer, R(2) relaxation dispersion, and ligand titration experiments. Glutaredoxins catalyze a glutathione exchange reaction, forming a stable glutathinoylated enzyme intermediate. The equilibrium between the reduced state and the glutathionylated state was biochemically tuned to exchange on the millisecond time scale. The conformational changes of the protein backbone during catalysis were followed by (15)N nuclear spin relaxation dispersion experiments. A conformational transition that is well described by a two-state process with an exchange rate corresponding to the glutathione exchange rate was observed for 23 residues. Binding of reduced glutathione resulted in competitive inhibition of the reduced enzyme having kinetics similar to that of the reaction. This observation couples the motions observed during catalysis directly to substrate binding. Backbone motions on the time scale of catalytic turnover were not observed for the enzyme in the resting states, implying that alternative conformers do not accumulate to significant concentrations. These results infer that the turnover rate in glutaredoxin is governed by formation of a productive enzyme-substrate encounter complex, and that catalysis proceeds by an induced fit mechanism rather than by conformer selection driven by intrinsic conformational dynamics.     

Tunneling and classical paths for proton transfer in an enzyme reaction dominated by tunneling: oxidation of tryptamine by aromatic amine dehydrogenase

Proton tunneling dominates the oxidative deamination of tryptamine catalyzed by the enzyme aromatic amine dehydrogenase. For reaction with the fast substrate tryptamine, a H/D kinetic isotope effect (KIE) of 55 +/- 6 has been reported-one of the largest observed in an enzyme reaction. We present here a computational analysis of this proton-transfer reaction, applying combined quantum mechanics/molecular mechanics (QM/MM) methods (PM3-SRP//PM3/CHARMM22). In particular, we extend our previous computational study (Masgrau et al. Science 2006, 312, 237) by using improved energy corrections, high-level QM/MM methods, and an ensemble of paths to estimate the tunneling contributions. We have carried out QM/MM molecular dynamics simulations and variational transition state theory calculations with small-curvature tunneling corrections. The results provide detailed insight into the processes involved in the reaction. Transfer to the O2 oxygen of the catalytic base, Asp128beta, is found to be the favored reaction both thermodynamically and kinetically, even though O1 is closer in the reactant complex. Comparison of quantum and classical models of proton transfer allows estimation of the contribution of hydrogen tunneling in lowering the barrier to reaction in the enzyme. A reduction of the activation free energy due to tunneling of 3.1 kcal mol-1 is found, which represents a rate enhancement due to tunneling by 2 orders of magnitude. The calculated KIE of 30 is significantly elevated over the semiclassical limit, in agreement with the experimental observations; a semiclassical value of 6 is obtained when tunneling is omitted. A polarization of the C-H bond to be broken is observed due to the close proximity of the catalytic aspartate and the (formally) positively charged imine nitrogen. A comparison is also made with the related quinoprotein methylamine dehydrogenase (MADH)-the much lower KIE of 11 that we obtain for the MADH/methylamine system is found to arise from a more endothermic potential energy surface for the MADH reaction.     

The Host/Guest Type of Excited‐State Proton Transfer; a General Review

Contemporary progress regarding guest/host types of excited‐state double proton transfer has been reviewed, among which are the biprotonic transfer within doubly H‐bonded host/guest complexes, the transfer through a solvent bridge relay, the intramolecular double proton transfer and solvation dynamics coupled proton transfer. Of particular emphases are the photophysical and photochemical properties of excited‐state double proton transfer (ESDPT) in 7‐azaindole and its corresponding analogues. From the chemical aspect, two types of ESDPT reaction, namely the catalytic and non‐catalytic types of ESDPT, have been classified and reviewed separately. For the case of static host/guest hydrogen‐bonded complexes both hydrogen‐bonding strength and configuration (i.e. geometry) play key roles in accounting for the reaction dynamics. In addition to the dynamical concern, excited‐state thermodynamics are of importance to fine‐tune the proton transfer reaction in the non‐catalytic host/guest type of ESDPT. The mechanisms of protic solvent assisted ESDPT, depending on host molecules and proton‐transfer models, have been reviewed where the plausible resolution is deduced. Particular attention has been given to the excited‐state proton transfer dynamics in pure water, aiming at its future perspective in biological applications. Finally, the differentiation in mechanism between solvent diffusive reorganization and solvent relaxation to affect the host/guest ESPT dynamics is made and discussed in de tail.     

Proton delivery to ferryl heme in a heme peroxidase: enzymatic use of the Grotthuss mechanism

We test the hypothesized pathway by which protons are passed from the substrate, ascorbate, to the ferryl oxygen in the heme enzyme ascorbate peroxidase (APX). The role of amino acid side chains and bound solvent is demonstrated. We investigated solvent kinetic isotope effects (SKIE) for the wild-type enzyme and several site-directed replacements of the key residues which form the proposed proton path. Kinetic constants for H(2)O(2)-dependent enzyme oxidation to Compound I, k(1), and subsequent reduction of Compound II, k(3), were determined in steady-state assays by variation of both H(2)O(2) and ascorbate concentrations. A high value of the SKIE for wild type APX ((D)k(3) = 4.9) as well as a clear nonlinear dependence on the deuterium composition of the solvent in proton inventory experiments suggest the simultaneous participation of several protons in the transition state for proton transfer. The full SKIE and the proton inventory data were modeled by applying Gross-Butler-Swain-Kresge theory to a proton path inferred from the known structure of APX. The model has been tested by constructing and determining the X-ray structures of the R38K and R38A variants and accounts for their observed SKIEs. This work confirms APX uses two arginine residues in the proton path. Thus, Arg38 and Arg172 have dual roles, both in the formation of the ferryl species and binding of ascorbate respectively and to facilitate proton transfer between the two.     

Mechanism of sulfide oxidations by peroxymonocarbonate

A detailed mechanism for the oxidation of aryl sulfides by peroxymonocarbonate ion in cosolvent/water media is described. Kinetic studies were performed to characterize the transition state, including a Hammett correlation and variation of solvent composition. The results are consistent with a charge-separated transition state relative to the reactants, with an increase of positive charge on the sulfur following nucleophilic attack of the sulfide at the electrophilic oxygen of peroxymonocarbonate. In addition, an average solvent isotope effect of 1.5 +/- 0.2 for most aryl sulfide oxidations is consistent with proton transfer in the transition state of the rate-determining step. Activation parameters for oxidation of ethyl phenyl sulfide in tert-butyl alcohol/water are reported. From the pH dependence of oxidation rates and (13)C NMR equilibrium experiments, the estimated pK(a) of peroxymonocarbonate was found to be approximately 10.6.     

Proton transfer at helium temperatures during dioxygen activation by heme monooxygenases

In the first measurement of enzymatic proton transfer at liquid helium temperatures, we examine protonation of the peroxo-ferriheme state of heme oxygenase (HO) produced by in situ radiolytic cryoreduction of oxy-HO in H2O and D2O solvents at ca. 4 K and above, and compare these findings with analogous measurements for oxy-P450cam and for oxy-Mb. Proton transfer in HO occurs at helium temperatures in both solvents; it occurs in P450cam at approximately 50 K and higher; in Mb it does not occur until T > 170 K. For Mb, this transfer at 180 K is biphasic, and the majority phase shows a solvent kinetic isotope effect of 3.8. We discuss these results in the context of the picture of environmentally coupled tunneling, which links proton transfer to two classes of protein motions: environmental reorganization (lambda in Marcus-like equations) and protein fluctuations ("active dynamics"; gating) which modulate the distance of proton transfer.