Use of competitive dead-end inhibitors to determine the chemical mechanism of action of yeast alcohol dehydrogenase.

In this work, we have postulated a comprehensive and unified chemical mechanism of action for yeast alcohol dehydrogenase (EC 1.1.1.1, constitutive, cytoplasmic), isolated from Saccharomyces cerevisiae. The chemical mechanism of yeast enzyme is based on the integrity of the proton relay system: His-51....NAD+....Thr-48....R.CH2OH(H2O)....Zn++, stretching from His-51 on the surface of enzyme to the active site zinc atom in the substrate-binding site of enzyme. Further, it is based on extensive studies of steady-state kinetic properties of enzyme which were published recently. In this study, we have reported the pH-dependence of dissociation constants for several competitive dead-end inhibitors of yeast enzyme froin their binary complexes with enzyme, or their ternary complexes with enzyme and NAD+ or NADH; inhibitors include: pyrazole, acetamide, sodium azide, 2-fluoroethanol, and 2,2,2-trifluorethanol. The unified mechanism describes the structures of four dissociation forms of apoenzyme, two forms of the binary complex E.NAD+, three forms of the ternary complex E.NAD+.alcohol, two forms of the ternary complex E.NADH.aldehyde and three binary complexes E.NADH. Appropriate pKa values have been ascribed to protonation forms of most of the above mentioned complexes of yeast enzyme with coenzymes and substrates.