Chemical mechanism of homoisocitrate dehydrogenase from Saccharomyces cerevisiae

Nitrogen nutrition effects on δ13C of plant respired CO2 are mostly caused by concurrent changes in organic acid utilisation and remobilisation

Nitrogen (N) nutrition impacts on primary carbon metabolism and can lead to changes in δ13C of respired CO2. However, uncertainty remains as to whether (1) the effect of N nutrition is observed in all species, (2) N source also impacts on respired CO2 in roots and (3) a metabolic model can be constructed to predict δ13C of respired CO2 under different N sources. Here, we carried out isotopic measurements of respired CO2 and various metabolites using two species (spinach, French bean) grown under different NH4 +:NO3 - ratios. Both species showed a similar pattern, with a progressive 13C-depletion in leaf-respired CO2 as the ammonium proportion increased, while δ13C in root-respired CO2 showed little change. Supervised multivariate analysis showed that δ13C of respired CO2 was mostly determined by organic acid (malate, citrate) metabolism, in both leaves and roots. We then took advantage of nonstationary, two-pool modelling that explained 73% of variance in δ13C in respired CO2. It demonstrates the critical role of the balance between the utilisation of respiratory intermediates and the remobilisation of stored organic acids, regardless of anaplerotic bicarbonate fixation by phosphoenolpyruvate carboxylase and the organ considered.

Allosteric Activation Shifts the Rate-Limiting Step in a Short-Form ATP Phosphoribosyltransferase

Short-form ATP phosphoribosyltransferase (ATPPRT) is a hetero-octameric allosteric enzyme comprising four catalytic subunits (HisG_S) and four regulatory subunits (HisZ). ATPPRT catalyzes the Mg²⁺-dependent condensation of ATP and 5-phospho-α-d-ribosyl-1-pyrophosphate (PRPP) to generate N¹-(5-phospho-β-d-ribosyl)-ATP (PRATP) and pyrophosphate, the first reaction of histidine biosynthesis. While HisG_S is catalytically active on its own, its activity is allosterically enhanced by HisZ in the absence of histidine. In the presence of histidine, HisZ mediates allosteric inhibition of ATPPRT. Here, initial velocity patterns, isothermal titration calorimetry, and differential scanning fluorimetry establish a distinct kinetic mechanism for ATPPRT where PRPP is the first substrate to bind. AMP is an inhibitor of HisG_S, but steady-state kinetics and ³¹P NMR spectroscopy demonstrate that ADP is an alternative substrate. Replacement of Mg²⁺ by Mn²⁺ enhances catalysis by HisG_S but not by the holoenzyme, suggesting different rate-limiting steps for nonactivated and activated enzyme forms. Density functional theory calculations posit an S_N2-like transition state stabilized by two equivalents of the metal ion. Natural bond orbital charge analysis points to Mn²⁺ increasing HisG_S reaction rate via more efficient charge stabilization at the transition state. High solvent viscosity increases HisG_S's catalytic rate, but decreases the hetero-octamer's, indicating that chemistry and product release are rate-limiting for HisG_S and ATPPRT, respectively. This is confirmed by pre-steady-state kinetics, with a burst in product formation observed with the hetero-octamer but not with HisG_S. These results are consistent with an activation mechanism whereby HisZ binding leads to a more active conformation of HisG_S, accelerating chemistry beyond the product release rate.

Evidence for an induced conformational change in the catalytic mechanism of homoisocitrate dehydrogenase for Saccharomyces cerevisiae: Characterization of the D271N mutant enzyme

Homoisocitrate dehydrogenase (HIcDH) catalyzes the NAD(+)-dependent oxidative decarboxylation of HIc to α-ketoadipate, the fourth step in the α-aminoadipate pathway responsible for the de novo synthesis of l-lysine in fungi. A mechanism has been proposed for the enzyme that makes use of a Lys-Tyr pair as acid-base catalysts, with Lys acting as a base to accept a proton from the α-hydroxyl of homoisocitrate, and Tyr acting as an acid to protonate the C3 of the enol of α-ketoadipate in the enolization reaction. Three conserved aspartate residues, D243, D267 and D271, coordinate Mg(2+), which is also coordinated to the α-carboxylate and α-hydroxyl of homoisocitrate. On the basis of kinetic isotope effects, it was proposed that a conformational change to close the active site and organize the active site for catalysis contributed to rate limitation of the overall reaction of the Saccharomyces cerevisiae HIcDH (Lin, Y., Volkman, J., Nicholas, K. M., Yamamoto, T., Eguchi, T., Nimmo, S. L., West, A. H., and Cook, P. F. (2008) Biochemistry47, 4169-4180.). In order to test this hypothesis, site-directed mutagenesis was used to change D271, a metal ion ligand and binding determinant for MgHIc, to N. The mutant enzyme was characterized using initial rate studies. A decrease of 520-fold was observed in V and V/KMgHIc, suggesting the same step(s) limit the reaction at limiting and saturating MgHIc concentrations. Solvent kinetic deuterium isotope effects (SKIE) and viscosity effects are consistent with a rate-limiting pre-catalytic conformational change at saturating reactant concentrations. In addition, at limiting MgHIc, an inverse (SKIE) of 0.7 coupled to a significant normal effect of viscosogen (2.1) indicates equilibrium binding of MgHIc prior to the rate-limiting conformational change. The maximum rate exhibits a small partial change at high pH suggesting a pH-dependent conformational change, while V/KMgHIc exhibits the same partial change observed in V, and a decrease at low pH with a pKa of 6 reflecting the requirement for the unprotonated form of MgHIc to bind to enzyme. However, neither parameter reflects the pH dependence of the chemical reaction. This pH independence of the chemical reaction over the range 5.5-9.5 is consistent with the much slower conformational change that would effectively perturb the observed pK values for catalytic groups to lower and higher pH. In other words, the pH dependence of the chemical reaction will only be observed when chemistry becomes slower than the rate of the conformational change. Data support the hypothesis of the existence of a pre-catalytic conformational change coupled to the binding of MgHIc.

Evolution of a transition state: role of Lys100 in the active site of isocitrate dehydrogenase

An active site lysine essential to catalysis in isocitrate dehydrogenase (IDH) is absent from related enzymes. As all family members catalyze the same oxidative β-decarboxylation at the (2R)-malate core common to their substrates, it seems odd that an amino acid essential to one is not found in all. Ordinarily, hydride transfer to a nicotinamide C4 neutralizes the positive charge at N1 directly. In IDH, the negatively charged C4-carboxylate of isocitrate stabilizes the ground state positive charge on the adjacent nicotinamide N1, opposing hydride transfer. The critical lysine is poised to stabilize-and perhaps even protonate-an oxyanion formed on the nicotinamide 3-carboxamide, thereby enabling the hydride to be transferred while the positive charge at N1 is maintained. IDH might catalyze the same overall reaction as other family members, but dehydrogenation proceeds through a distinct, though related, transition state. Partial activation of lysine mutants by K(+) and NH4 (+) represents a throwback to the primordial state of the first promiscuous substrate family member.

Opposite carbon isotope discrimination during dark respiration in leaves versus roots - a review

In general, leaves are (13) C-depleted compared with all other organs (e.g. roots, stem/trunk and fruits). Different hypotheses are formulated in the literature to explain this difference. One of these states that CO2 respired by leaves in the dark is (13) C-enriched compared with leaf organic matter, while it is (13) C-depleted in the case of root respiration. The opposite respiratory fractionation between leaves and roots was invoked as an explanation for the widespread between-organ isotopic differences. After summarizing the basics of photosynthetic and post-photosynthetic discrimination, we mainly review the recent findings on the isotopic composition of CO2 respired by leaves (autotrophic organs) and roots (heterotrophic organs) compared with respective plant material (i.e. apparent respiratory fractionation) as well as its metabolic origin. The potential impact of such fractionation on the isotopic signal of organic matter (OM) is discussed. Some perspectives for future studies are also proposed .

Lysine biosynthesis in microbes: relevance as drug target and prospects for β-lactam antibiotics production

Plants as well as pro- and eukaryotic microorganisms are able to synthesise lysine via de novo synthesis. While plants and bacteria, with some exceptions, rely on variations of the meso-diaminopimelate pathway for lysine biosynthesis, fungi exclusively use the α-aminoadipate pathway. Although bacteria and fungi are, in principle, both suitable as lysine producers, current industrial fermentations rely on the use of bacteria. In contrast, fungi are important producers of β-lactam antibiotics such as penicillins or cephalosporins. The synthesis of these antibiotics strictly depends on α-aminoadipate deriving from lysine biosynthesis. Interestingly, despite the resulting industrial importance of the fungal α-aminoadipate pathway, biochemical reactions leading to α-aminoadipate formation have only been studied on a limited number of fungal species. In this respect, just recently an essential isomerisation reaction required for the formation of α-aminoadipate has been elucidated in detail. This review summarises biochemical pathways leading to lysine production, discusses the suitability of interrupting lysine biosynthesis as target for new antibacterial and antifungal compounds and emphasises on biochemical reactions involved in the formation of α-aminoadipate in fungi as an essential intermediate for both, lysine and β-lactam antibiotics production.

Enzyme redesign guided by cancer-derived IDH1 mutations

Mutations in an enzyme can result in a neomorphic catalytic activity in cancers. We applied cancer-associated mutations from isocitrate dehydrogenases (IDHs) to homologous residues in the active sites of homoisocitrate dehydrogenases (HIDHs) to derive enzymes that catalyze the conversion of 2-oxoadipate to (R)-2-hydroxyadipate, a critical step for adipic acid production. Thus, we provide a prototypic example of how insights from cancer genome sequencing and functional studies can aid in enzyme redesign.

Evidence in support of lysine 77 and histidine 96 as acid-base catalytic residues in saccharopine dehydrogenase from Saccharomyces cerevisiae

Saccharopine dehydrogenase (SDH) catalyzes the final reaction in the α-aminoadipate pathway, the conversion of l-saccharopine to l-lysine (Lys) and α-ketoglutarate (α-kg) using NAD⁺ as an oxidant. The enzyme utilizes a general acid-base mechanism to conduct its reaction with a base proposed to accept a proton from the secondary amine of saccharopine in the oxidation step and a group proposed to activate water to hydrolyze the resulting imine. Crystal structures of an open apo form and a closed form of the enzyme with saccharopine and NADH bound have been determined at 2.0 and 2.2 Å resolution, respectively. In the ternary complex, a significant movement of domain I relative to domain II that closes the active site cleft between the two domains and brings H96 and K77 into the proximity of the substrate binding site is observed. The hydride transfer distance is 3.6 Å, and the side chains of H96 and K77 are properly positioned to act as acid-base catalysts. Preparation of the K77M and H96Q single-mutant and K77M/H96Q double-mutant enzymes provides data consistent with their role as the general acid-base catalysts in the SDH reaction. The side chain of K77 initially accepts a proton from the ε-amine of the substrate Lys and eventually donates it to the imino nitrogen as it is reduced to a secondary amine in the hydride transfer step, and H96 protonates the carbonyl oxygen as the carbinolamine is formed. The K77M, H976Q, and K77M/H96Q mutant enzymes give 145-, 28-, and 700-fold decreases in V/E(t) and >10³-fold increases in V₂/K(Lys)E(t) and V₂/K(α-kg)E(t) (the double mutation gives >10⁵-fold decreases in the second-order rate constants). In addition, the K77M mutant enzyme exhibits a primary deuterium kinetic isotope effect of 2.0 and an inverse solvent deuterium isotope effect of 0.77 on V₂/K(Lys). A value of 2.0 was also observed for (D)(V₂/K(Lys))(D₂O) when the primary deuterium kinetic isotope effect was repeated in D₂O, consistent with a rate-limiting hydride transfer step. A viscosity effect of 0.8 was observed on V₂/K(Lys), indicating the solvent deuterium isotope effect resulted from stabilization of an enzyme form prior to hydride transfer. A small normal solvent isotope effect is observed on V, which decreases slightly when repeated with NADD, consistent with a contribution from product release to rate limitation. In addition, V₂/K(Lys)E(t) is pH-independent, which is consistent with the loss of an acid-base catalyst and perturbation of the pK(a) of the second catalytic group to a higher pH, likely a result of a change in the overall charge of the active site. The primary deuterium kinetic isotope effect for H96Q, measured in H₂O or D₂O, is within error equal to 1. A solvent deuterium isotope effect of 2.4 is observed with NADH or NADD as the dinucleotide substrate. Data suggest rate-limiting imine formation, consistent with the proposed role of H96 in protonating the leaving hydroxyl as the imine is formed. The pH-rate profile for V₂/K(Lys)E(t) exhibits the pK(a) for K77, perturbed to a value of ∼9, which must be unprotonated to accept a proton from the ε-amine of the substrate Lys so that it can act as a nucleophile. Overall, data are consistent with a role for K77 acting as the base that accepts a proton from the ε-amine of the substrate lysine prior to nucleophilic attack on the α-oxo group of α-ketoglutarate, and finally donating a proton to the imine nitrogen as it is reduced to give saccharopine. In addition, data indicate a role for H96 acting as a general acid-base catalyst in the formation of the imine between the ε-amine of lysine and the α-oxo group of α-ketoglutarate.

(12)C/(13)C fractionations in plant primary metabolism

Natural (13)C abundance is now an unavoidable tool to study ecosystem and plant carbon economies. A growing number of studies take advantage of isotopic fractionation between carbon pools or (13)C abundance in respiratory CO(2) to examine the carbon source of respiration, plant biomass production or organic matter sequestration in soils. (12)C/(13)C isotope effects associated with plant metabolism are thus essential to understand natural isotopic signals. However, isotope effects of enzymes do not influence metabolites separately, but combine to yield a (12)C/(13)C isotopologue redistribution orchestrated by metabolic flux patterns. In this review, we summarise key metabolic isotope effects and integrate them into the corpus of plant primary carbon metabolism.

The 13C/12C isotopic signal of day-respired CO2 in variegated leaves of Pelargonium × hortorum

In leaves, although it is accepted that CO(2) evolved by dark respiration after illumination is naturally (13) C-enriched compared to organic matter or substrate sucrose, much uncertainty remains on whether day respiration produces (13) C-depleted or (13) C-enriched CO(2). Here, we applied equations described previously for mesocosm CO(2) exchange to investigate the carbon isotope composition of CO(2) respired by autotrophic and heterotrophic tissues of Pelargonium × hortorum leaves, taking advantage of leaf variegation. Day-respired CO(2) was slightly (13) C-depleted compared to organic matter both under 21% O(2) and 2% O(2). Furthermore, most, if not all CO(2) molecules evolved in the light came from carbon atoms that had been fixed previously before the experiments, in both variegated and green leaves. We conclude that the usual definition of day respiratory fractionation, that assumes carbon fixed by current net photosynthesis is the respiratory substrate, is not valid in Pelargonium leaves under our conditions. In variegated leaves, total organic matter was slightly (13) C-depleted in white areas and so were most primary metabolites. This small isotopic difference between white and green areas probably came from the small contribution of photosynthetic CO(2) refixation and the specific nitrogen metabolism in white leaf areas.

Kinetic isotope effects in asymmetric reactions

Kinetic isotope effects are exquisitely sensitive probes of transition structure. As such, kinetic isotope effects offer a uniquely useful probe for the symmetry-breaking process that is inherent to stereoselective reactions. In this Concept article, we explore the role of steric and electronic effects in stereocontrol, and we relate these concepts to recent studies carried out in our laboratory. We also explore the way in which kinetic isotope effects serve as useful points of contact with computational models of transition structures. Finally, we discuss future opportunities for kinetic isotope effects to play a role in asymmetric catalyst development.

Evaluation of lysine biosynthesis as an antifungal drug target: biochemical characterization of Aspergillus fumigatus homocitrate synthase and virulence studies

Aspergillus fumigatus is the main cause of severe invasive aspergillosis. To combat this life-threatening infection, only limited numbers of antifungals are available. The fungal alpha-aminoadipate pathway, which is essential for lysine biosynthesis, has been suggested as a potential antifungal drug target. Here we reanalyzed the role of this pathway for establishment of invasive aspergillosis in murine models. We selected the first pathway-specific enzyme, homocitrate synthase (HcsA), for biochemical characterization and for study of its role in virulence. A. fumigatus HcsA was specific for the substrates acetyl-coenzyme A (acetyl-CoA) and alpha-ketoglutarate, and its activity was independent of any metal ions. In contrast to the case for other homocitrate synthases, enzymatic activity was hardly affected by lysine and gene expression increased under conditions of lysine supplementation. An hcsA deletion mutant was lysine auxotrophic and unable to germinate on unhydrolyzed proteins given as a sole nutrient source. However, the addition of partially purified A. fumigatus proteases restored growth, confirming the importance of free lysine to complement auxotrophy. In contrast to lysine-auxotrophic mutants from other fungal species, the mutant grew on blood and serum, indicating the existence of high-affinity lysine uptake systems. In agreement, although the virulence of the mutant was strongly attenuated in murine models of bronchopulmonary aspergillosis, virulence was partially restored by lysine supplementation via the drinking water. Additionally, in contrast to the case for attenuated pulmonary infections, the mutant retained full virulence when injected intravenously. Therefore, we concluded that inhibition of fungal lysine biosynthesis, at least for disseminating invasive aspergillosis, does not appear to provide a suitable target for new antifungals.

Site-directed mutagenesis as a probe of the acid-base catalytic mechanism of homoisocitrate dehydrogenase from Saccharomyces cerevisiae

Homoisocitrate dehydrogenase (HIcDH) catalyzes the Mg2+- and K+-dependent oxidative decarboxylation of homoisocitrate to alpha-ketoadipate using NAD as the oxidant. A recent consideration of the structures of enzymes in the same family as HIcDH, including isopropylmalate and isocitrate dehydrogenases, suggests all of the family members utilize a Lys-Tyr pair to catalyze the acid-base chemistry of the reaction [Aktas, D. F., and Cook, P. F. (2009) Biochemistry 48, 3565-3577]. Multiple-sequence alignment indicates the active site Lys-Tyr pair consists of lysine 206 and tyrosine 150. Therefore, the K206M and Y150F mutants of HIcDH were prepared and characterized to test the potential roles of these residues as acid-base catalysts. The V/Et values of the K206M and Y150F mutant enzymes at pH 7.5 are decreased by approximately 2400- and approximately 680-fold, respectively, compared to that of wild-type HIcDH; the K(m) for HIc does not change significantly. V/Et and V/K(MgHIc)Et for the K206M mutant enzyme are pH-independent below pH 6 and decrease to a constant value above pH 7, while V/K(NAD)Et is independent over the pH range from 6.2 to 9.5. In the case of the Y150F mutant enzyme, V/Et and V/K(NAD)Et are pH-independent above pH 9.5 and decrease to a constant value below pH 8. This behavior can be compared to that of the wild-type enzyme, where V/Et decreases at high and low pH, giving pKa values of approximately 6.5 and approximately 9.5, respectively. Data were interpreted in terms of a group with a pKa of 6.5 that acts as a general base in the hydride transfer step and a group with a pKa of 9.5 that acts as a general acid to protonate C3 in the tautomerization reaction [Lin, Y., Volkman, J., Nicholas, K. M., Yamamoto, T., Eguchi, T., Nimmo, S. L., West, A. H., and Cook, P. F. (2008) Biochemistry 47, 4169-4180]. Solvent deuterium isotope effects on V and V/K(MgHIc) were near unity for the K206M mutant enzyme but approximately 2.2 for the Y150F mutant enzyme. The dramatic decreases in activity, the measured solvent deuterium isotope effects, and changes in the pH dependence of kinetic parameters compared to that of the wild type are consistent with K206 acting as a general base in the hydride transfer step of the wild-type enzyme but as a general acid in the Y150F mutant enzyme, replacing Y150 in the tautomerization reaction. In addition, Y150 acts as a general acid in the tautomerization reaction of the wild-type enzyme and replaces K206 as the general base in the hydride transfer step of the K206M mutant enzyme.

Chemical mechanism of saccharopine reductase from Saccharomyces cerevisiae

Saccharopine reductase (SR) [saccharopine dehydrogenase (l-glutamate forming), EC 1.5.1.10] catalyzes the condensation of l-alpha-aminoadipate-delta-semialdehyde (AASA) with l-glutamate to give an imine, which is reduced by NADPH to give saccharopine. An acid-base chemical mechanism has been proposed for SR on the basis of pH-rate profiles and solvent deuterium kinetic isotope effects. A finite solvent isotope effect is observed indicating that proton(s) are in flight in the rate-limiting step(s) and likely the same step is limiting under both limiting and saturating substrate concentrations. A concave upward proton inventory suggests that more than one proton is transferred in a single transition state, likely a conformation change required to open the site and release products. Two groups are involved in the acid-base chemistry of the reaction. One of these groups catalyzes the steps involved in forming the imine between the alpha-amine of glutamate and the aldehyde of AASA. The group, which has a pK(a) of about 8, is observed in the pH-rate profiles for V(1) and V(1)/K(Glu) and must be protonated for optimal activity. It is also observed in the V(2) and V(2)/K(Sacc) pH-rate profiles and is required unprotonated. The second group, which has a pK(a) of 5.6, accepts a proton from the alpha-amine of glutamate so that it can act as a nucleophile in forming a carbinolamine upon attack of the carbonyl of AASA.

A lysine-tyrosine pair carries out acid-base chemistry in the metal ion-dependent pyridine dinucleotide-linked beta-hydroxyacid oxidative decarboxylases

This work reviews published structural and kinetic data on the pyridine nucleotide-linked beta-hydroxyacid oxidative decarboxylases. The family of metal ion-dependent pyridine nucleotide-linked beta-hydroxyacid oxidative decarboxylases can be divided into two structural families with the malic enzyme, which has an (S)-hydroxyacid substrate, comprising one subfamily and isocitrate dehydrogenase, isopropylmalate dehydrogenase, homoisocitrate dehydrogenase, and tartrate dehydrogenase, which have an (R)-hydroxyacid substrate, comprising the second subclass. Multiple-sequence alignment of the members of the (R)-hydroxyacid family indicates a high degree of sequence identity with most of the active site residues conserved. The three-dimensional structures of the members of the (R)-hydroxyacid family with structures available superimpose on one another, and the active site structures of the enzymes have a similar overall geometry of residues in the substrate and metal ion binding sites. In addition, a number of residues in the malic enzyme active site are also conserved, and the arrangement of these residues has a similar geometry, although the (R)-hydroxyacid and (S)-hydroxyacid family sites are geometrically mirror images of one another. The active sites of the (R)-hydroxyacid family have a higher positive charge density when compared to those of the (S)-hydroxyacid family, largely due to the number of arginine residues in the vicinity of the substrate alpha-carboxylate and one fewer carboxylate ligand to the divalent metal ion. Data available for all of the enzymes in the family have been considered, and a general mechanism that makes use of a lysine (general base)-tyrosine (general acid) pair is proposed. Differences exist in the mechanism for generating the neutral form of lysine so that it can act as a base.

On the metabolic origin of the carbon isotope composition of CO2 evolved from darkened light-acclimated leaves in Ricinus communis

The (13)C isotopic signature (delta(13)C) of CO(2) respired from plants is widely used to assess carbon fluxes and ecosystem functioning. There is, however, a lack of knowledge of the metabolic basis of the delta(13)C value of respired CO(2). To elucidate the physiological mechanisms driving (12)C/(13)C fractionation during respiration, the delta(13)C of respired CO(2) from dark-acclimated leaves during the night, from darkened leaves during the light period, and from stems and roots of Ricinus communis was analysed. The delta(13)C of potential respiratory substrates, the respiratory quotient and the activities of phosphoenolpyruvatecarboxylase (PEPc) and key respiratory enzymes were also measured. It is shown here that the CO(2) evolved from darkened light-acclimated leaves during the light period is (13)C-enriched, and that this correlates with malate accumulation in the light and rapid malate decarboxylation just after the onset of darkness. Whilst CO(2) evolved from leaves was generally (13)C-enriched (but to a lesser extent during the night), CO(2) evolved from stems and roots was depleted compared with the putative respiratory substrates; the difference was mainly caused by intensive PEPc-catalysed CO(2) refixation in stems and roots. These results provide a physiological explanation for short-term variations of delta(13)C in CO(2), illustrating the effects of variations of metabolic fluxes through different biochemical pathways.

Potassium is an activator of homoisocitrate dehydrogenase from Saccharomyces cerevisiae

Potassium is an activator of the reaction catalyzed by homoisocitrate (HIc) dehydrogenase (HIcDH) from Saccharomyces cerevisiae with either the natural substrate, homoisocitrate, or the slow substrate isocitrate. On the basis of initial velocity studies, the selectivity of the activator site for monovalent ions was determined. Potassium is the best activator, and NH 4 (+) and Rb (+) are also activators of the reaction, while Cs (+), Li (+), and Na (+) are not. Chloride inhibits the reaction, while acetate is much less effective. Substitution of potassium acetate for KCl changes the kinetic mechanism of HIcDH from a steady state random to a fully ordered mechanism with the binding of MgHIc followed by K (+) and NAD. The change in mechanism likely reflects an apparent increase in the affinity of enzyme for MgHIc as a result of elimination of the inhibitory effect of Cl (-). The V/K NAD pH-rate profile in the absence of K (+) exhibits a >10-fold decrease in the affinity of enzyme for NAD upon deprotonation of an enzyme side chain with a p K a of about 5.5-6. On the other hand, the affinity for NAD is relatively constant at high pH in the presence of 200 mM KCl. Since the affinity of the dinucleotide decreases as the enzyme group is protonated and the effect is overcome by a monovalent cation, the enzyme residue may be a neutral acid, aspartate or glutamate. Data suggest that K (+) replaces the proton, and likely binds to the enzyme residue, the pyrophosphoryl moiety of NAD, or both. Viscosity and solvent deuterium isotope effects studies suggest the isomerization of E-MgHIc binary complex limits the rate in the absence of K (+).