Crystal Structures of a Poplar Thioredoxin Peroxidase that Exhibits the Structure of Glutathione Peroxidases: Insights into Redox-driven Conformational Changes

Glutathione peroxidases (GPXs) are a group of enzymes that regulate the levels of reactive oxygen species in cells and tissues, and protect them against oxidative damage. Contrary to most of their counterparts in animal cells, the higher plant GPX homologues identified so far possess cysteine instead of selenocysteine in their active site. Interestingly, the plant GPXs are not dependent on glutathione but rather on thioredoxin as their in vitro electron donor. We have determined the crystal structures of the reduced and oxidized form of Populus trichocarpaxdeltoides GPX5 (PtGPX5), using a selenomethionine derivative. PtGPX5 exhibits an overall structure similar to that of the known animal GPXs. PtGPX5 crystallized in the assumed physiological dimeric form, displaying a pseudo ten-stranded beta sheet core. Comparison of both redox structures indicates that a drastic conformational change is necessary to bring the two distant cysteine residues together to form an intramolecular disulfide bond. In addition, a computer model of a complex of PtGPX5 and its in vitro recycling partner thioredoxin h1 is proposed on the basis of the crystal packing of the oxidized form enzyme. A possible role of PtGPX5 as a heavy-metal sink is also discussed.

Invariant Thr244 is essential for the efficient acylation step of the non-phosphorylating glyceraldehyde-3-phosphate dehydrogenase from Streptococcus mutans

One of the most striking features of several X-ray structures of CoA-independent ALDHs (aldehyde dehydrogenases) in complex with NAD(P) is the conformational flexibility of the NMN moiety. However, the fact that the rate of the acylation step is high in GAPN (non-phosphorylating glyceraldehyde-3-phosphate dehydrogenase) from Streptococcus mutans implies an optimal positioning of the nicotinamide ring relative to the hemithioacetal intermediate within the ternary GAPN complex to allow an efficient and stereospecific hydride transfer. Substitutions of serine for invariant Thr244 and alanine for Lys178 result in a drastic decrease of the efficiency of hydride transfer which becomes rate-limiting. The crystal structure of the binary complex T244S GAPN-NADP shows that the absence of the beta-methyl group leads to a well-defined conformation of the NMN part, including the nicotinamide ring, clearly different from that depicted to be suitable for an efficient hydride transfer in the wild-type. The approximately 0.6-unit increase in pK(app) of the catalytic Cys302 observed in the ternary complex for both mutated GAPNs is likely to be due to a slight difference in positioning of the nicotinamide ring relative to Cys302 with respect to the wild-type ternary complex. Taken together, the data support a critical role of the Thr244 beta-methyl group, held in position through a hydrogen-bond interaction between the Thr244 beta-hydroxy group and the epsilon-amino group of Lys178, in permitting the nicotinamide ring to adopt a conformation suitable for an efficient hydride transfer during the acylation step for all the members of the CoA-independent ALDH family.

The first crystal structure of a thioacylenzyme intermediate in the ALDH family: new coenzyme conformation and relevance to catalysis

Crystal structures of several members of the nonphosphorylating CoA-independent aldehyde dehydrogenase (ALDH) family have shown that the peculiar binding mode of the cofactor to the Rossmann fold results in a conformational flexibility for the nicotinamide moiety of the cofactor. This has been hypothesized to constitute an essential feature of the catalytic mechanism because the conformation of the cofactor required for the acylation step is not appropriate for the deacylation step. In the present study, the structure of a reaction intermediate of the E268A-glyceraldehyde 3-phosphate dehydrogenase (GAPN) from Streptococcus mutans, obtained by soaking the crystals of the enzyme/NADP complex with the natural substrate, is reported. The substrate is bound covalently in the four monomers and presents the geometric characteristics expected for a thioacylenzyme intermediate. Control experiments assessed that reduction of the coenzyme has occurred within the crystal. The structure reveals that reduction of the cofactor upon acylation leads to an extensive motion of the nicotinamide moiety with a flip of the reduced pyridinium ring away from the active site without significant changes of the protein structure. This event positions the reduced nicotinamide moiety in a pocket that likely constitutes the exit door for NADPH. Arguments are provided that the structure reported here constitutes a reasonable picture of the first thioacylenzyme intermediate characterized thus far in the ALDH family and that the position of the reduced nicotinamide moiety observed in GAPN is the one suitable for the deacylation step within all of the nonphosphorylating CoA-independent ALDH family.

Crystal structure and solution NMR dynamics of a D (type II) peroxiredoxin glutaredoxin and thioredoxin dependent: a new insight into the peroxiredoxin oligomerism

Peroxiredoxins (Prxs) constitute a family of thiol peroxidases that reduce hydrogen peroxide, peroxinitrite, and hydroperoxides using a strictly conserved cysteine. Very abundant in all organisms, Prxs are produced as diverse isoforms characterized by different catalytic mechanisms and various thiol-containing reducing agents. The oligomeric state of Prxs and the link with their functionality is a subject of intensive research. We present here a combined X-ray and nuclear magnetic resonance (NMR) study of a plant Prx that belongs to the D-Prx (type II) subfamily. The Populus trichocarpa Prx is the first Prx shown to be regenerated in vitro by both the glutaredoxin and thioredoxin systems. The crystal structure and solution NMR provide evidence that the reduced protein is a specific noncovalent homodimer both in the crystal and in solution. The dimer interface is roughly perpendicular to the plane of the central beta sheet and differs from the interface of A- and B-Prx dimers, where proteins associate in the plane parallel to the beta sheet. The homodimer interface involves residues strongly conserved in the D (type II) Prxs, suggesting that all Prxs of this family can homodimerize. The study provides a new insight into the Prx oligomerism and the basis for protein-protein and enzyme-substrate interaction studies by NMR.

New antitumour cyclic astin analogues: synthesis, conformation and bioactivity

Astins, antitumour cyclic pentapeptides, were isolated from the Aster tataricus. Their chemical structures, consist of a 16-membered ring system containing a unique beta,gamma-dichlorinated proline [Pro(Cl)2], other non-coded amino acid residues and a cis conformation in one of the peptide bonds. The astin backbone conformation, along with the cis peptide bond in which the beta,gamma-dichlorinated proline residue is involved, was considered to play an important role in their antineoplastic activities on sarcoma 180A and P388 lymphocytic leukaemia in mice, but the scope and potential applications of this activity remain unclear. With the aim at improving our knowledge of the conformational properties influencing the bioactivity in this class of compounds, new astin-related cyclopeptides were synthesized differing from the natural products by the presence of some non-proteinogenic amino acid residues: Aib, Abu, -(S)beta3-hPhe and a peptide bond surrogate (-SO2-NH-). The analogues prepared c(-Pro-Thr-Aib-beta3-Phe-Abu-), c[Pro-Thr-Aib-(S)beta3-hPhe-Abu], c[Pro-Abu-Ser-(S)beta3-hPhe psi(CH2-SO2-NH)-Abu] and c[Pro-Thr-Aib-(S)beta3-hPhe psi(CH2-SO2-NH)-Abu] were synthesized by classical methods in solution and tested for their antitumour effect. These molecules were studied by crystal-state x-ray diffraction analysis and/or solution NMR and MD techniques.

Active site mutagenesis and phospholipid hydroperoxide reductase activity of poplar type II peroxiredoxin

The nature of the active site and the substrate specificity of poplar type II peroxiredoxin, an enzyme which preferentially uses glutaredoxin as an electron donor, were investigated in this study. The type II peroxiredoxin is able to use phospholipid hydroperoxide nearly as efficiently as hydrogen peroxide. Two of the hyper-conserved amino acid residues in peroxiredoxins have been altered, by site-directed mutagenesis, generating the mutants T48V and R129Q. The two mutant proteins are inactive with hydrogen peroxide or tertiary butyl hydroperoxide as substrates. On the other hand, the mutant enzymes catalyse the degradation of cumene hydroperoxide with low efficiency. This suggests that the thiol-dependent regeneration process of the catalytic cysteine is not affected by the mutations and that all substrates are not accommodated identically in the active site.

Crystallization and preliminary X-ray studies of the glutaredoxin from poplar in complex with glutathione

A monocysteinic mutant of poplar glutaredoxin (C30S) has been overproduced and purified. The protein has been crystallized in complex with glutathione using the hanging-drop vapour-diffusion technique in the presence of PEG 4000 as a precipitating agent. A native data set was collected at 1.55 A resolution. The crystals belong to space group P2(1)2(1)2(1), with unit-cell parameters a = 45.7, b = 49.1, c = 104.8 A. Isomorphous crystals of a selenomethionine derivative were grown under the same conditions. Three data sets were collected at 1.73 A using the FIP synchrotron beamline at the ESRF. The positions of the Se atoms were determined and model rebuilding and refinement are in progress.

Alpha-aminoxy acids as building blocks for the oxime and hydroxylamine pseudopeptide links. Application to the synthesis of human elastase inhibitors

The aminoxy acids NH2-O-C(alpha)HR-CO2H are much more easily obtained in the enantiomerically pure form than the analogous hydrazino acids NH2-NH-C(alpha)HR-CO2H, and it has been shown that the isosteric amidoxy psi[CO-NH-O] and hydrazide psi[CO-NH-NH] amide surrogates Induce two quite similar gamma-like folded structures. An aminoxy acid can also be N-coupled to a peptide aldehyde to give the aldoxime psi[CH = N-O] link or to a peptide ketone to form the ketoxime psi[CR= N-O] link. The former can be further reduced into the hydroxylamine psi[CH2-NH-O] link which gives rise to reduced amidoxy peptides. The structural properties Induced by these amide surrogates were studied, using IR and NMR spectroscopy, paying particular attention to the Z/E-isomerism of the oxime link. In order to investigate their inhibitory potency, the three amide surrogates were introduced in the Pro3-Val4 and Val4-Ala5 position of Z-Ala1-Ala2-Pro3-Val4-Ala5-Ala6-NHiPr, a substrate which is cleaved in the Val4-Ala5 position by human leukocyte elastase (HLE). The [Val4psi[CO-NH-O]Ala5] analogue was still a substrate, while the [Pro3psi[CO-NH-O]Val4] and [Val4psi[CH = N-O]Ala5] pseudopeptides acted as HLE competitive inhibitors.

Crystal structure of two ternary complexes of phosphorylating glyceraldehyde-3-phosphate dehydrogenase from Bacillus stearothermophilus with NAD and D-glyceraldehyde 3-phosphate

The crystal structure of the phosphorylating glyceraldehyde-3-phosphate dehydrogenase (GAPDH) from Bacillus stearothermophilus was solved in complex with its cofactor, NAD, and its physiological substrate, D-glyceraldehyde 3-phosphate (D-G3P). To isolate a stable ternary complex, the nucleophilic residue of the active site, Cys(149), was substituted with alanine or serine. The C149A and C149S GAPDH ternary complexes were obtained by soaking the crystals of the corresponding binary complexes (enzyme.NAD) in a solution containing G3P. The structures of the two binary and the two ternary complexes are presented. The D-G3P adopts the same conformation in the two ternary complexes. It is bound in a non-covalent way, in the free aldehyde form, its C-3 phosphate group being positioned in the P(s) site and not in the P(i) site. Its C-1 carbonyl oxygen points toward the essential His(176), which supports the role proposed for this residue along the two steps of the catalytic pathway. Arguments are provided that the structures reported here are representative of a productive enzyme.NAD.D-G3P complex in the ground state (Michaelis complex).

Thioredoxins and related proteins in photosynthetic organisms: molecular basis for thiol dependent regulation

Thioredoxins are small molecular weight disulfide oxidoreductases specialized in the reduction of disulfide bonds on other proteins. Generally, the enzymes which are selectively and reversibly reduced by these proteins oscillate between an oxidized and inactive conformation and a reduced and active conformation. Thioredoxin constitutes the archetype of a family of protein disulfide oxidoreductases which comprises glutaredoxin and protein disulfide isomerase. Thioredoxin and glutaredoxin serve many roles in the cell, including the redox regulation of target enzymes and transcription factors. They can also serve as hydrogen donors to peroxiredoxins, recently discovered heme free peroxidases, the function of which is to get rid of hydroperoxides in the cell. This review describes the molecular basis for the functioning and interaction between these enzymes in photosynthetic organisms.

Crystallization and preliminary X-ray data of a bifunctional peroxiredoxin from poplar

Two variants (wild type and V152C mutant) of a bifunctional poplar peroxiredoxin have been overexpressed in Escherichia coli cells. The two recombinant enzymes were purified and crystallized using the hanging-drop vapour-diffusion technique. Data sets were collected to 1.62 and 2.48 A resolution using X-ray synchrotron-source radiation from two crystal forms of wild-type peroxiredoxin which belonged to the monoclinic space group P2(1) (with unit-cell parameters a = 59.26, b = 68.80, c = 75.71 A, beta = 93.45 degrees ) and to the orthorhombic space group P2(1)2(1)2 (with unit-cell parameters a = 64.70, b = 130.73, c = 35.59 A), respectively. Data were also collected to 2.17 A resolution using a home X-ray source from a V152C peroxiredoxin crystal which belongs to the triclinic space group (P1), with unit-cell parameters a = 36.65, b = 41.53, c = 58.06 A, alpha = 70.52, beta = 93.45, gamma = 64.31 degrees. Phases have been obtained using molecular replacement with the structure of human peroxiredoxin V (PDB code 1hd2) as a search model. Refinement of the structures is in progress.

Crystallization and preliminary X-ray study of an N-terminal fragment of rat liver ribosomal P2 protein

Ribosomal P proteins have been shown to be involved in the binding of elongation factors and participate in factor-dependent GTP hydrolysis. The P proteins form the pentamer (P1/P2)(2)-P0 constituting the lateral flexible stalk of the 60S ribosomal subunit. The highly soluble domain (1-65) of rat liver P2 has been overexpressed in Escherichia coli as an N-terminal poly-His-tagged protein and crystallized. To reduce nucleation and improve crystal morphology and diffraction power, the crystals were grown in a gel matrix and an oil barrier was added between the reservoir and the drop to reduce the rate of vapour diffusion. This dramatically reduced the nucleation in the drops and yielded diffraction-quality crystals. Data were collected to 2.4 A resolution at beamline ID 14-1, ESRF. The crystals belong to the orthorhombic space group P2(1)2(1)2, with unit-cell parameters a = 37.7, b = 96.7, c = 135.0 A.

Crystal structure of the W35A mutant thioredoxin h from Chlamydomonas reinhardtii: the substitution of the conserved active site Trp leads to modifications in the environment of the two catalytic cysteines

The conformational analysis of W35A thioredoxin h from the eukaryotic green alga Chlamydomonas reinhardtii in the solid state has been carried out by x-ray diffraction, with the aim to clarify the role of Trp in the catalysis. Comparative analysis of W35A mutant with wild-type (WT) thioredoxin shows that, even if the structural motif of thioredoxin is not perturbed, the substitution of Trp35 by an Ala leads to significant changes in protein conformation near the active site. This rearrangement increases its solvent exposure and explains the change of the pKa values of the catalytic cysteines. The substitution of the Trp residue also influences the crystal packing as well as the recognition ability of thioredoxin. The solid state analysis suggests that the Trp residue has a structural function both to force the active site in the bioactive conformation, and to mediate the protein-protein recognition.

Crystal structure of the wild-type and D30A mutant thioredoxin h of Chlamydomonas reinhardtii and implications for the catalytic mechanism

Thioredoxins are ubiquitous proteins which catalyse the reduction of disulphide bridges on target proteins. The catalytic mechanism proceeds via a mixed disulphide intermediate whose breakdown should be enhanced by the involvement of a conserved buried residue, Asp-30, as a base catalyst towards residue Cys-39. We report here the crystal structure of wild-type and D30A mutant thioredoxin h from Chlamydomonas reinhardtii, which constitutes the first crystal structure of a cytosolic thioredoxin isolated from a eukaryotic plant organism. The role of residue Asp-30 in catalysis has been revisited since the distance between the carboxylate OD1 of Asp-30 and the sulphur SG of Cys-39 is too great to support the hypothesis of direct proton transfer. A careful analysis of all available crystal structures reveals that the relative positioning of residues Asp-30 and Cys-39 as well as hydrophobic contacts in the vicinity of residue Asp-30 do not allow a conformational change sufficient to bring the two residues close enough for a direct proton transfer. This suggests that protonation/deprotonation of Cys-39 should be mediated by a water molecule. Molecular-dynamics simulations, carried out either in vacuo or in water, as well as proton-inventory experiments, support this hypothesis. The results are discussed with respect to biochemical and structural data.

Biological activities and structural properties of the atypical bacteriocins mesenterocin 52b and leucocin b-ta33a

The antibacterial spectra and modes of action of synthetic peptides corresponding to mesenterocin 52B and leucocin B-TA33a greatly differ despite their high sequence homology. Circular dichroism experiments establish the capacity of each of these two peptides to partly fold into an amphiphilic helix that might be crucial for their adsorption at lipophilic-hydrophilic interfaces.

Comparative enzymatic properties of GapB-encoded erythrose-4-phosphate dehydrogenase of Escherichia coli and phosphorylating glyceraldehyde-3-phosphate dehydrogenase

GapB-encoded protein of Escherichia coli and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) share more than 40% amino acid identity. Most of the amino acids involved in the binding of cofactor and substrates to GAPDH are conserved in GapB-encoded protein. This enzyme shows an efficient non-phosphorylating erythrose-4-phosphate dehydrogenase activity (Zhao, G., Pease, A. J., Bharani, N., and Winkler, M. E. (1995) J. Bacteriol. 177, 2804-2812) but a low phosphorylating glyceraldehyde-3-phosphate dehydrogenase activity, whereas GAPDH shows a high efficient phosphorylating glyceraldehyde-3-phosphate dehydrogenase activity and a low phosphorylating erythrose-4-phosphate dehydrogenase activity. To identify the structural factors responsible for these differences, comparative kinetic and binding studies have been carried out on both GapB-encoded protein of Escherichia coli and GAPDH of Bacillus stearothermophilus. The KD constant of GapB-encoded protein for NAD is 800-fold higher than that of GAPDH. The chemical mechanism of erythrose 4-phosphate oxidation by GapB-encoded protein is shown to proceed through a two-step mechanism involving covalent intermediates with Cys-149, with rates associated to the acylation and deacylation processes of 280 s-1 and 20 s-1, respectively. No isotopic solvent effect is observed suggesting that the rate-limiting step is not hydrolysis. The rate of oxidation of glyceraldehyde 3-phosphate is 0.12 s-1 and is hydride transfer limiting, at least 2000-fold less efficient compared with that of erythrose 4-phosphate. Thus, it can be concluded that it is only the structure of the substrates that prevails in forming a ternary complex enzyme-NAD-thiohemiacetal productive (or not) for hydride transfer in the acylation step. This conclusion is reinforced by the fact that the rate of oxidation for erythrose 4-phosphate by GAPDH is 0.1 s-1 and is limited by the acylation step, whereas glyceraldehyde 3-phosphate acylation is efficient and is not rate-determining (>/=800 s-1). Substituting Asn for His-176 on GapB-encoded protein, a residue postulated to facilitate hydride transfer as a base catalyst, decreases 40-fold the kcat of glyceraldehyde 3-phosphate oxidation. This suggests that the non-efficient positioning of the C-1 atom of glyceraldehyde 3-phosphate relative to the pyridinium of the cofactor within the ternary complex is responsible for the low catalytic efficiency. No phosphorylating activity on erythrose 4-phosphate with GapB-encoded protein is observed although the Pi site is operative as proven by the oxidative phosphorylation of glyceraldehyde 3-phosphate. Thus the binding of inorganic phosphate to the Pi site likely is not productive for attacking efficiently the thioacyl intermediate formed with erythrose 4-phosphate, whereas a water molecule is an efficient nucleophile for the hydrolysis of the thioacyl intermediate. Compared with glyceraldehyde-3-phosphate dehydrogenase activity, this corresponds to an activation of the deacylation step by >/=4.5 kcal.mol-1. Altogether these results suggest subtle structural differences between the active sites of GAPDH and GapB-encoded protein that could be revealed and/or modulated by the structure of the substrate bound. This also indicates that a protein engineering approach could be used to convert a phosphorylating aldehyde dehydrogenase into an efficient non-phosphorylating one and vice versa.

Comparison of the structures of wild-type and a N313T mutant of Escherichia coli glyceraldehyde 3-phosphate dehydrogenases: implication for NAD binding and cooperativity

The crystal structure of wild-type and N313T mutant glyceraldehyde 3-phosphate dehydrogenases from Escherichia coli was determined in the presence of NAD at 1.8 angstrom and 2.17 angstrom, respectively. The structure of the monomer and of the tetramer are similar to those observed for other GAPDHs. An exhaustive analysis of the hydrophobic clusters and the hydrogen bond networks explain the high degree of sequence conservation in GAPDHs. The structural effect of the N313T mutation is a change in the (phi,psi) angles of nearby residues Asn236 and Val237, while the structure around the mutated residue remains unchanged. A detailed comparison of the wild-type and N313T mutant E. coli GAPDH with the apo and holo forms of Bacillus stearothermophilus GAPDH is carried out in relation to the apo --> holo transition. An unbiased set of about 60 residues, whose C(alpha) atoms remain in the same relative position in the different forms of the tetramer, is defined as the tetramer "core" which acts as a fixed scaffold around which structural rearrangements occur during the apo --> holo transition. This core essentially includes beta-strands from the beta-sheets forming the O-P and Q-R interfaces, in particular strand beta1 which bears catalytic residue His176. During the apo --> holo transition, dimer O-P rotates around the molecular P-axis by about +1 degrees, and dimer O-R by about -1 degrees. Further rotations of the NAD binding domain relative to the catalytic domain are discussed in relation to the molecular symmetry. The possible effect on NAD binding cooperativity of mutations around the tetramer core is exemplified by residue 252. The presence of a conserved hydrophilic patch embedded in the hydrophobic O-P interface is highlighted. A mechanism for substrate binding, different from those currently proposed, is described where the hydroxyl group of the substrate C(2) atom is hydrogen bonded to Cys149N.

Circular permutation within the coenzyme binding domain of the tetrameric glyceraldehyde-3-phosphate dehydrogenase from Bacillus stearothermophilus

A circularly permuted (cp) variant of the phosphorylating NAD-dependent glyceraldehyde-3-phosphate dehydrogenase (GAPDH) from Bacillus stearothermophilus has been constructed with N- and C-termini created within the coenzyme binding domain. The cp variant has a kcat value equal to 40% of the wild-type value, whereas Km and KD values for NAD show a threefold decrease compared to wild type. These results indicate that the folding process and the conformational changes that accompany NAD binding during the catalytic event occur efficiently in the permuted variant and that NAD binding is tighter. Reversible denaturation experiments show that the stability of the variant is only reduced by 0.7 kcal/mol compared to the wild-type enzyme. These experiments confirm and extend results obtained recently on other permuted proteins. For multimeric proteins, such as GAPDH, which harbor subunits with two structural domains, the natural location of the N- and C-termini is not a prerequisite for optimal folding and biological activity.

Crystallization and preliminary X-ray diffraction studies of Escherichia coli glyceraldehyde-3-phosphate dehydrogenase

Phosphorylating glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a key enzyme in glycolysis. Single crystals of NAD-dependent GAPDH from Escherichia coli have been obtained by vapour diffusion at room temperature using trisodium citrate as precipitant. In almost the same crystallization conditions, two kinds of crystals were found to be suitable for X-ray diffraction. The crystals with only one half of a tetramer in the asymmetric unit were chosen for high- resolution analysis. They belonged to space group C222(1), with cell dimensions a = 79.1, b = 189.6 and c = 122.2 A. These crystals diffracted to 1.8 A resolution.

Characterization of the two anion-recognition sites of glyceraldehyde-3-phosphate dehydrogenase from Bacillus stearothermophilus by site-directed mutagenesis and chemical modification

The active site of the glycolytic glyceraldehyde-3-phosphate dehydrogenase (GAPDH) contains two anion recognition sites which have been attributed to the phosphate binding of the substrates, namely, glyceraldehyde 3-phosphate (Ps site) and inorganic phosphate (Pi site) [Moras et al. (1975) J. Biol. Chem. 250, 9137-9162]. In order to probe the role of both sites during the catalytic event, Arg 195 from the Pi site and Arg 231 from the Ps site of the Bacillus stearothermophilus enzyme have been changed to Leu and Gly, respectively, by site-directed mutagenesis. A comparative study of the chemical reactivity of the mutants and wild type toward 2,3-butanedione revealed a similarly high reactivity only for the R195L mutant and wild type, suggesting that only Arg 231 is chemically reactive toward 2,3-butanedione and that its reactivity is not influenced by the presence of the residue Arg 195, which is only 4 A distant. The kinetic consequences of the mutations were also analyzed for the consecutive steps in the forward catalytic reaction. The replacement of Arg 195 by Leu leads to a marked decrease of the rate of the first steps of the reaction which lead to the acylenzyme formation, in particular, the rate of enzyme-substrate association, while these steps occur at a similar or higher rate when Arg 231 is replaced by Gly. Furthermore, the mutations R195L and R231G also result in a 550-fold and 16,400-fold decrease in the second-order rate constant of phosphorolysis. This step becomes rate-determining for the R195L mutant.(ABSTRACT TRUNCATED AT 250 WORDS)