The interaction of glycine, aspartic acid, and lysine by the protonated macrocyclic ligand 6,19-bis(2-hydroxypropyl)-3,6,9,16,19,22-hexaaza-tricyclo-[22.2.2.2(11,14)]triaconta-11,13,24,26,27,29-hexaene

A new hexaaza macrocyclic ligand (L) bearing two 2-hydroxypropyl pendants, 6,19-bis(2-hydroxypropyl)-3,6,9,16,19,22-hexaaza-tricyclo-[22.2.2.2(11,14)]triaconta-11,13,24,26,27,29-hexaene has been synthesized and characterized. The macrocyclic ligand was isolated as a colorless crystal, monoclinic, P2(1)/n, with a=10.757(2), b=14.214(3), c=13.746(3) A, beta=101.40(3) degrees, V=2060.3(7) A3, Z=2, R1=0.0695, and wR2=0.1538 [I>2sigma(I)]. Potentiometric studies of the macrocyclic ligand and three types of amino acids, glycine (equal numbers of carboxylate and amino groups), aspartic acid (more carboxylate groups than amino group), and lysine (more amino groups than carboxylate group) have been performed. The stability constants for the new macrocycle and binary complexes of the amino acid with the macrocyclic ligand are reported. Binary complexes are formed in aqueous solution as a result of hydrogen bonding interaction and electrostatic attraction between the host and the guest. The binding Schemes for the recognition of amino acids are suggested. From the results, it seems that this new macrocyclic ligand is able to bind three different amino acids with selectivity in aqueous solution, and the strength of binding is of the order lysine < glycine < aspartic acid.     

Inactivation of human fibroblast collagenase by chloroacetyl N-hydroxypeptide derivatives.

When human fibroblast collagenase was incubated with ClCH2CO-(N-OH)Leu-Ala-Gly-NH2 (2-5 mM) in Tris buffer, pH 7.4 at 25 degrees C, a slow, time-dependent inhibition of the enzyme was observed. Dialysis against a buffer to remove free inhibitor did not reactivate the enzyme. A reversible competitive inhibitor, phthaloyl-GlyP-Ile-Trp-NHBzl (50 microM) partially protected the enzyme from inactivation by the compound. From the concentration dependent rates of inactivation Ki = 0.5 +/- 0.1 mM and k3, the rate constant for inactivation = 3.4 +/- 0.3 x 10(-3) min-1 were determined. The inactivation followed the pH optimum (6.5-7.0) for the enzyme activity, suggesting direct involvement of the same active site residue(s). The reaction mode of the inhibitor may be analogous to that of the inactivation of Pseudomonas aeruginosa elastase [Nishino, N. and Powers, J. (1980) J. Biol. Chem., 255, 3482] in which the catalytic glutamate carboxyl was alkylated by the inhibitor after its binding to enzyme through the hydroxamic Zn2+ ligand. All carboxyl groups in the inactivated collagenase were modified with 0.1 M ethyl dimethylaminopropyl carbodiimide/0.5 M glycinamide in 4 M guanidine at pH 5. The inactivator-affected carboxyl group was then regenerated with 1 M imidazole at pH 8.9, 37 degrees C for 12 h and the protein was radiolabeled with 3H-glycine methyl ester and carbodiimide to incorporate 0.9 residue glycine per mol enzyme.     

A transition state analog for phosphate diester cleavage catalyzed by a small enzyme-like metal ion complex

The values of K(i) for methylphosphate dianion (MP(2-)) inhibition of the cleavage of 2-hydroxypropyl-4-nitrophenyl phosphate (HpPNP) catalyzed by 1,3-bis(1,4,7-triazacyclonon-1-yl)-2-hydroxypropane (Zn(2)(1)(H(2)O)) approach a small limiting value of 6 microM at pH相似文献   

Heparin-binding properties of lactoferrin and lysozyme.

1. Binding of biotin-heparin to immobilized lactoferrin and lysozyme was optimum at pH 6.0, 100 mM NaCl. Complex interactions between NaCl and CaCl2 concentrations were observed for heparin binding to both proteins. 2. The metal ions Cu2+, Zn2+, Fe2+ and Fe3+ inhibited heparin binding, with half-maximal inhibition of binding to lactoferrin occurring between 600 microM and 1 mM and for lysozyme between 500 and 800 microM. 3. Binding of biotin-heparin to both proteins was inhibited to varying degrees by heparin, heparan sulfate, chondroitin sulfate A, dextran sulfate and DNA.     

Synthesis and pH-sensitive luminescence of bis-terpyridyl iridium(III) complexes incorporating pendent pyridyl groups

A series of iridium(III) bis-terpyridine complexes have been prepared which incorporate pendent pyridyl groups at the 4′-positions of one or both of the terpyridine (tpy) ligands. These include: three mutually isomeric homoleptic complexes, in which the nitrogen atom of the pendent pyridyl is para, meta or ortho to the C-C bond to the terpyridine; their heteroleptic analogues in which the second ligand is 4′-tolyl-terpyridine (ttpy); analogous complexes of the new ligand, 4′-(2,6-dimethylpyrid-4-yl)-terpyridine; and related complexes incorporating an additional phenyl ring interposed between the terpyridine and the pendent pyridyl group. All of the complexes are luminescent in air-equilibrated aqueous solution at room temperature. The homoleptic complexes display structured emission resembling that of unsubstituted [Ir(tpy)₂]³⁺, with luminescence lifetimes of around 1 μs under these conditions. The heteroleptic analogues give broader, red-shifted emission spectra, similar to that of [Ir(ttpy)₂]³⁺, indicating that emission in these complexes arises primarily from a lower-energy excited state associated with the 4′-tolyl-terpyridine ligand. A further red-shift for the complexes incorporating the additional phenyl ring suggests that the emissive state involves the more conjugated phenylpyridyl-appended ligand in these cases. The luminescence of all of the heteroleptic complexes investigated, except the meta-substituted system, is sensitive to the protonation state of the pendent pyridyl group, and the structure of the ligand can have a significant influence on both the magnitude of the response and the pH region over which it occurs.     

Selective recognition of pyrimidine-pyrimidine DNA mismatches by distance-constrained macrocyclic bis-intercalators

Binding of three macrocyclic bis-intercalators, derivatives of acridine and naphthalene, and two acyclic model compounds to mismatch-containing and matched duplex oligodeoxynucleotides was analyzed by thermal denaturation experiments, electrospray ionization mass spectrometry studies (ESI-MS) and fluorescent intercalator displacement (FID) titrations. The macrocyclic bis-intercalators bind to duplexes containing mismatched thymine bases with high selectivity over the fully matched ones, whereas the acyclic model compounds are much less selective and strongly bind to the matched DNA. Moreover, the results from thermal denaturation experiments are in very good agreement with the binding affinities obtained by ESI-MS and FID measurements. The FID results also demonstrate that the macrocyclic naphthalene derivative BisNP preferentially binds to pyrimidine–pyrimidine mismatches compared to all other possible base mismatches. This ligand also efficiently competes with a DNA enzyme (M.TaqI) for binding to a duplex with a TT-mismatch, as shown by competitive fluorescence titrations. Altogether, our results demonstrate that macrocyclic distance-constrained bis-intercalators are efficient and selective mismatch-binding ligands that can interfere with mismatch-binding enzymes.     

Active site histidine in pig liver aminolevulic acid dehydratase modified by diethylpyrocarbonate and protected by Zn2+ ions

1. The effect of diethylpyrocarbonate (DEP) (0.1-0.35 mM) on the purified pig liver amino-levulic acid dehydratase (ALA-D) containing 0.3 g-atoms Zn/subunit, under different pHs (6.0-7.5), temperature (0-18 degrees C) and time (0-60 min) was studied. 2. Three histidyl residues/subunit were modified by DEP (0.2 mM, pH 6.8), but activity was completely lost after the first one had reacted, indicating the presence of one histidine residue essential for ALA-D catalysis. Reactivation by treatment with hydroxylamine (0.7 mM, pH 7.0) confirmed that only histidine and no other nucleophile amino acids were directly involved in DEP inhibition. 3. Zn ions (0.5 mM) and the substrate ALA (5-10 mM) protected against DEP inactivation, protection was dependent on pH. 4. Sn, Se, Hg, Cd, Mn, Co and Pb (0.01-0.1 mM) did not significantly protect ALA-D against inactivation. 5. It is concluded that the substrate and Zn binding sites and the essential histidyl residues are in close proximity in the active center. It is proposed that in the catalytic synthesis of porphobilinogen from ALA, histidine groups have the specific role of transporting protons from the aqueous media to a hydrophobic active site.     

Mercurial-promoted Zn2+ release from Escherichia coli aspartate transcarbamoylase

The release of Zn2+ from aspartate transcarbamoylase (ATCase; c6r6) upon challenge by p-hydroxymercuriphenylsulfonate (PMPS) has been studied using the sensitive, high-affinity metallochromic indicator 4-(2-pyridylazo)resorcinol at pH 7.0. When the--SH group of each catalytic (c) chain is protected, 1 Zn2+ is released for every 4 eq of PMPS added to ATCase during titration of the 24--SH groups of regulatory (r) chains. Moreover, the release of Zn2+ is a linear function of PMPS added, indicating that the rate-limiting step in Zn2+ release is mercurial attack on the 1st of the 4 r--SH groups bonded tetrahedrally to Zn2+ in an r chain near c:r contacts. Dissociation of ATCase is linked to Zn2+ release and mercaptide formation; e.g. upon addition of 4 eq of PMPS to ATCase in 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (Hepes) buffer, 1/6th of ATCase is dissociated to c3 and r2 subunits at approximately 83% of the rate of Zn2+ release, with no accumulation of the c6r4 intermediate as is observed in KPO4 buffer. Adding less than or equal to 4 PMPS/ATCase, the release of Zn2+ is first-order in [PMPS] and is virtually independent of [ATCase] with an activation energy of 18 kcal/mol. With large excesses of PMPS, stopped-flow traces show a lag period followed by pseudo first-order release of Zn2+ from ATCase and the reaction order in [PMPS] = approximately 1.3. Under these conditions, PMPS has a chaotropic effect on ATCase; the activation energy for Zn2+ release is much lower than that obtained with limiting PMPS and is increased by the presence of phosphate or active-site ligand from 6.6 to approximately 12 kcal/mol. A reasonable explanation of the observed kinetic data is that the organomercurial reagent binds reversibly to nitrogenous side chain groups in an ATCase molecule prior to the rate-limiting reaction with a sulfhydryl group.     

Kinetic analysis of carboxy ester hydrolysis by a dinuclear Zn(II) complex

A dinuclear Zn(II) complex with hexaaza macrocyclic ligand bearing two 2-hydroxypropyl pendants, 3,6,9,16,19,22-hexaaza-6,19-bis(2-hydroxypropyl)-tricyclo [22,2,2,2(11,14)]triaconta-11,13,24,26,27,29-hexane (L) was synthesized and studied as a catalyst of the cleavage of 4-nitrophenyl acetate (NA). X-ray diffraction analysis of [Zn(2)LCl(2)]Cl(2)(.)6H(2)O revealed that Zn(II) adopts a trigonal-bipyramidal geometry. The complexation constants of L with Zn(II) have been determined at 298 K by means of potentiometric titration. [Zn(2)H(-2)L](2+) is the dominant species in aqueous solution around pH 8. The Zn(2)L-promoted hydrolysis of NA showed a second-order rate constant of 0.33 M(-1)s(-1) at pH 9.0, and the main promoter species are concluded to be the deprotonated species [Zn(2)H(-2)L](2+).     

Chemical mechanism of the adenosine cyclic 3',5'-monophosphate dependent protein kinase from pH studies

The pH dependence of kinetic parameters and inhibitor dissociation constants for the adenosine cyclic 3',5'-monophosphate dependent protein kinase reaction has been determined. Data are consistent with a mechanism in which reactants selectively bind to enzyme with the catalytic base unprotonated and an enzyme group required protonated for peptide (Leu-Arg-Arg-Ala-Ser-Leu-Gly) binding. Binding of the peptide apparently locks both of the above enzyme residues in their correct protonation state. MgATP preferentially binds fully ionized and requires an enzyme residue (probably lysine) to be protonated. The maximum velocity and V/KMgATP are pH independent. The V/K for Ser-peptide is bell-shaped with pK values of 6.2 and 8.5 estimated. The pH dependence of 1/Ki for Leu-Arg-Arg-Ala-Ala-Leu-Gly is also bell-shaped, giving pK values identical with those obtained for V/KSer-peptide, while the Ki for MgAMP-PCP increases from a constant value of 650 microM above pH 8 to a constant value of 4 mM below pH 5.5. The Ki for uncomplexed Mg2+ obtained from the Mg2+ dependence of V and V/KMgATP is apparently pH independent.     

Modification of acetylcholinesterase with the fluorescent thiol reagent S-mercuric-N-dansylcysteine

11s acetylcholinesterase (EC 3.1.1.7) from Torpedo californica electroplax, purified by a combination of affinity and gel chromatography was found to react stoichiometrically with S-mercuric-N-dansylcysteine. Approximately four mols of reagent per mol of enzyme were incorporated when the modification was carried out in 1.0 mM Tris-C1, pH 7.5, either in the presence or absence of 0.1 M NaCl. Prior incubation of the enzyme with 1.0 x 10(-4) M Zn2+ allowed the incorporation of about six mols of reagent per mol of enzyme. Binding of the reagent produced shifts in the emission and excitation wavelength maxima which were similar for all reaction conditions; however the enhancement of fluorescence intensity which accompanied binding of reagent was dependent on the ionic composition of the reaction medium. The modified enzyme remained active towards the active site titrant 7-(dimethylcarbamoyloxy)-N-methylquinolinium and retained its sensitivity towards inactivation by Zn2+. The results suggest that acetylcholinesterase as prepared contains several accessible thiol groups, and that the bound reagent may prove to be a useful probe of ligand-induced conformational changes in acetylcholinesterase.     

Synthesis of {15-benzyloxy-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),13,15-triene}nickel(II) perchlorate and its analogs, and their catalytic behavior in reductive debromination of 1-bromo-4-tert-butylbenzene

New nickel(II) complexes with macrocyclic ligands bearing benzyloxy [(5), (9)], 2-methylbenzyloxy (7), 3-methylbenzyloxy (8), and hydroxy (6) groups on the pyridine ring have been synthesized. Structures of the hydroxy substituted macrocyclic ligand (L-OH·3HCl·H₂O), and the benzyloxy substituted ligand (L-OBn·3HCl) and its nickel(II) complex (5), as well as an analogous Ni(II) complex (8), have been revealed by X-ray crystallography. Their catalytic capabilities in the reductive debromination of 1-bromo-4-tert-butylbenzene have been elucidated, which has revealed that the pyridine ring can be a suitable position for the introduction of functional groups while maintaining the catalytic capabilities of the nickel(II) complexes.     

Synthesis, spectroscopic, structural and complexation studies of a new tetra-naphthylmethylene pendant-armed macrocyclic ligand

A novel emissive tetra-naphthylmethylene pendant-armed macrocyclic ligand and a series of complexes with monovalent and divalent metal ions have been synthesized. Solid compounds have been isolated as mononuclear (Co(II), Cu(II) and Zn(II)) or dinuclear (Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Ag(I)), complexes, depending on the counterions used. The chemical and photophysical properties of the free ligand, the protonation behavior and its metal complexes have been investigated in solution. UV-Vis spectroscopy has revealed a 1:1 binding stoichiometry for Cu(II), Zn(II), Cd(II), Ni(II) and Co(II), and 2:1 molar ratio for Ag(I). In chloroform, the free ligand presents two emission bands related to the monomer naphthalene emission and a red-shifted band attibutable to an exciplex due to a charge transfer from the nitrogen lone electron pair to the excited chromophore. Upon protonation of the free amines or due to metal complexation, the exciplex band disappears. The crystal structure of [Ag₂L(NO₃)₂] is also reported. The structure reveals that both metal ions are into the macrocyclic cavity in a distorted square plane {AgN₃O} environment. Each Ag(I) atom interacts with two neighbouring amine nitrogen atoms, one pyridine nitrogen and one oxygen atom from a monodentate nitrate ion.     

Sweetening ruthenium and osmium: organometallic arene complexes containing aspartame

The novel organometallic sandwich complexes [(eta(6)-p-cymene)Ru(eta(6)-aspartame)](OTf)(2) (1) (OTf = trifluoromethanesulfonate) and [(eta(6)-p-cymene)Os(eta(6)-aspartame)](OTf)(2) (2) incorporating the artificial sweetener aspartame have been synthesised and characterised. A number of properties of aspartame were found to be altered on binding to either metal. The pK (a) values of both the carboxyl and the amino groups of aspartame are lowered by between 0.35 and 0.57 pH units, causing partial deprotonation of the amino group at pH 7.4 (physiological pH). The rate of degradation of aspartame to 3,6-dioxo-5-phenylmethylpiperazine acetic acid (diketopiperazine) increased over threefold from 0.12 to 0.36 h(-1) for 1, and to 0.43 h(-1) for 2. Furthermore, the reduction potential of the ligand shifted from -1.133 to -0.619 V for 2. For the ruthenium complex 1 the process occurred in two steps, the first (at -0.38 V) within a biologically accessible range. This facilitates reactions with biological reductants such as ascorbate. Binding to and activation of the sweet taste receptor was not observed for these metal complexes up to concentrations of 1 mM. The factors which affect the ability of metal-bound aspartame to interact with the receptor site are discussed.     

Characterization of an inhibitory metal binding site in carboxypeptidase A

The specificity of metal ion inhibition of bovine carboxypeptidase A ([(CPD)Zn]) catalysis is examined under stopped-flow conditions with use of the fluorescent peptide substrate Dns-Gly-Ala-Phe. The enzyme is inhibited competitively by Zn(II), Pb(II), and Cd(II) with apparent KI values of 2.4 x 10(-5), 4.8 x 10(-5), and 1.1 x 10(-2) M in 0.5 M NaCl at pH 7.5 and 25 degrees C. The kcat/Km value, 7.3 x 10(6) M-1 s-1, is affected less than 10% at 1 x 10(-4) M Mn(II) or Cu(II) and at 1 x 10(-2) M Co(II), Ni(II), Hg(II), or Pt(IV). Zn(II) and Pb(II) are mutually exclusive inhibitors. Previous studies of the pH dependence of Zn(II) inhibition [Larsen, K. S., & Auld, D. S. (1989) Biochemistry 28, 9620] indicated that [(CPD)Zn] is selectively inhibited by a zinc monohydroxide complex, ZnOH+, and that ionization of a ligand, LH, in the enzyme's inhibitory site (pKLH 5.8) is obligatory for its binding. The present study allows further definition of this inhibitory zinc site. The ionizable ligand (LH) is assigned to Glu-270, since specific chemical modification of this residue decreases the binding affinity of [(CPD)Zn] for Zn(II) and Pb(II) by more than 60- and 200-fold, respectively. A bridging interaction between the Glu-270-coordinated metal hydroxide and the catalytic metal ion is implicated from the ability of Zn(II) and Pb(II) to induce a perturbation in the electronic absorption spectrum of cobalt carboxypeptidase A ([(CPD)Co]).(ABSTRACT TRUNCATED AT 250 WORDS)     

A lead-dependent DNAzyme with a two-step mechanism

A detailed biochemical and mechanistic study of in vitro selected variants of 8-17 DNAzymes is presented. Even though the 8-17 DNAzyme motif has been obtained through in vitro selection under three different conditions involving 10 mM Mg(2+) (called 8-17), 0.5 mM Mg(2+)/50 mM histidine (called Mg5), or 100 microM Zn(2+) (called 17E), all variants are shown to be the most active with Pb(2+) (8-17: k(obs) approximately 0.5 min(-1); Mg5: k(obs) approximately 2 min(-1); 17E: k(obs) approximately 1 min(-1) with 200 microM Pb(2+) at pH 5.0). For the 17E variant of the 8-17 DNAzyme, the single-turnover rate constants followed the order of Pb(2+) > Zn(2+) > Mn(2+) approximately Co(2+) > Ni(2+) > Mg(2+) approximately Ca(2+) > Sr(2+) approximately Ba(2+). The catalytic rate is half-maximal at 13.5 microM Pb(2+), 0.97 mM Zn(2+), or 10.5 mM Mg(2+), suggesting that the metal-binding affinity of the DNAzymes is in the order of Pb(2+) > Zn(2+) > Mg(2+). The Pb(2+)-dependent activity increases linearly with pH and the slope of the plot of log k(obs) versus pH is approximately 1, suggesting a single deprotonation in the rate-limiting step of the reaction. Sequence variations of the DNAzyme confirm the importance of the G*T wobble pair, the two loops and the intervening stem in maintaining the active conformation of the system. While Mg(2+) and Zn(2+) catalyze only a transesterification reaction with formation of a product containing a 2',3'-cyclic phosphate, Pb(2+) catalyzes a transesterification reaction followed by hydrolysis of the 2',3'-cyclic phosphate. Although this two-step mechanism has shown to be operative in protein ribonucleases and in the leadzyme RNAzyme, it is now demonstrated for the first time that this DNAzyme may also use the same mechanism. Therefore, the two-step mechanism is observed in metalloenzymes of all classes, and this 8-17 DNAzyme provides a simple, stable, and cost-effective model system for understanding the structure of Pb(2+)-binding sites and their roles in the two-step mechanism.     

Effects of active site modification and reversible dissociation on the secondary structure of triosephosphate isomerase.

Binding of ligands to the catalytic center of mammalian triosephosphate isomerase (TPI) induces a conformational change(s) that enhances the specific deamidation of Asn71 at the subunit interface. Deamidation initiates dissociation and degradation of the enzyme in vivo and in vitro. We have utilized circular dichroism spectroscopy to examine the conformational changes in the enzyme upon ligand binding and subunit dissociation/reassociation. Native TPI from rabbit, chicken, and yeast exhibit similar spectra at pH 7.5, but are substantially different at pH 9.5. Covalent reaction of the active site Glu 165 with the substrate analogue 3-chloroacetol phosphate results in a conformational change (decrease in beta-sheet) which is similar in TPI from all three species. Reversible dissociation of the dimeric enzyme in guanidine followed by dialysis, although permitting full recovery of catalytic activity, results in refolded dimers with decreased alpha-helix. These conformational changes induced by ligand binding, pH, or reversible dissociation explain, in part, the differences in the chemical and physical properties of the enzyme from the three species at alkaline pH, the increased lability of the dissociated/reassociated enzyme, and corroborate 31P NMR data on substrate-induced conformational changes. These studies also support the concept of molecular wear and tear whereby ligand binding at the catalytic center induces conformational changes that increase the probability of covalent modification and ultimate degradation of the protein.     

Molecular design of an acid–base cooperative catalyst for RNA cleavage based on a dizinc complex

The effects of donor groups of dizinc complexes, formed from a 2:1 mixture of Zn(II) and a dinucleating ligand, on adenylyl(3'-5')adenosine (ApA) cleavage have been studied. Two dinucleating ligands were used: one had two 2-pyridylmethyl and two 2-hydroxyethyl moieties on the 1,3-diamino-2-propanol linker moiety (2), and the other had two 2-pyridylmethyl and two carboxymethyl moieties on the 1,3-diamino-2-propanol linker moiety (3(2-)). The dizinc complex with2 [(Zn(2+))(2)-2] showed higher activities toward ApA cleavage than the dizinc complex using an analogous dinucleating ligand having four 2-pyridylmethyl donor moieties [(Zn(2+))(2)-1] at pH 5-8. The former showed a bell-shaped pH-rate constant profile, whereas the latter showed a sigmoidal pattern. The differences in the pH-rate constant profile are attributable to the various distributions of the monohydroxo-dizinc species, i.e. dideprotonated species, which are responsible for ApA cleavage. The monohydroxo species of (Zn(2+))(2)-2 has two acidic protons, which are not present in the corresponding monohydroxo species of (Zn(2+))(2)-1. The existence of both intracomplex acid (ROH or H(2)O) and base catalysts (RO(-) or OH(-)) in (Zn(2+))(2)-2 can explain its higher activity toward ApA cleavage than that of (Zn(2+))(2)-1. In contrast, (Zn(2+))(2)-3(2-) showed lower activity toward ApA cleavage at pH 7.0, which can be ascribed to the absence of the monohydroxo-dizinc species under these conditions.     

Catalytic mechanism of Zn2+-dependent polyol dehydrogenases: kinetic comparison of sheep liver sorbitol dehydrogenase with wild-type and Glu154-->Cys forms of yeast xylitol dehydrogenase

Co-ordination of catalytic Zn2+ in sorbitol/xylitol dehydrogenases of the medium-chain dehydrogenase/reductase superfamily involves direct or water-mediated interactions from a glutamic acid residue, which substitutes a homologous cysteine ligand in alcohol dehydrogenases of the yeast and liver type. Glu154 of xylitol dehydrogenase from the yeast Galactocandida mastotermitis (termed GmXDH) was mutated to a cysteine residue (E154C) to revert this replacement. In spite of their variable Zn2+ content (0.10-0.40 atom/subunit), purified preparations of E154C exhibited a constant catalytic Zn2+ centre activity (kcat) of 1.19+/-0.03 s(-1) and did not require exogenous Zn2+ for activity or stability. E154C retained 0.019+/-0.003% and 0.74+/-0.03% of wild-type catalytic efficiency (kcat/K(sorbitol)=7800+/-700 M(-1) x s(-1)) and kcat (=161+/-4 s(-1)) for NAD+-dependent oxidation of sorbitol at 25 degrees C respectively. The pH profile of kcat/K(sorbitol) for E154C decreased below an apparent pK of 9.1+/-0.3, reflecting a shift in pK by about +1.7-1.9 pH units compared with the corresponding pH profiles for GmXDH and sheep liver sorbitol dehydrogenase (termed slSDH). The difference in pK for profiles determined in 1H2O and 2H2O solvent was similar and unusually small for all three enzymes (approximately +0.2 log units), suggesting that the observed pK in the binary enzyme-NAD+ complexes could be due to Zn2+-bound water. Under conditions eliminating their different pH-dependences, wild-type and mutant GmXDH displayed similar primary and solvent deuterium kinetic isotope effects of 1.7+/-0.2 (E154C, 1.7+/-0.1) and 1.9+/-0.3 (E154C, 2.4+/-0.2) on kcat/K(sorbitol) respectively. Transient kinetic studies of NAD+ reduction and proton release during sorbitol oxidation by slSDH at pH 8.2 show that two protons are lost with a rate constant of 687+/-12 s(-1) in the pre-steady state, which features a turnover of 0.9+/-0.1 enzyme equivalents as NADH was produced with a rate constant of 409+/-3 s(-1). The results support an auxiliary participation of Glu154 in catalysis, and possible mechanisms of proton transfer in sorbitol/xylitol dehydrogenases are discussed.