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 (+).