Dehydrogenases simple ordered mechanism

The initial rate equation is again of the form of Eq. (1) with the kinetic coefficients as in Table I, which shows that the mechanism differs from the simple ordered mechanism in three important respects. First, the isomerization steps are potentially rate-limiting evidence for such a rate-limiting step not attributable to product dissociation or the hydride-transfer step (fc) has been put forward for pig heart lactate dehydrogenase 25). Second, Eqs. (5) and (6) no longer apply in each case the function of kinetic coefficients will be smaller than the individual velocity constant (Table I). Third, because < ab/ a< b is smaller than it may also be smaller than the maximum specific rate of the reverse reaction that is, one of the maximum rate relations in Eq. (7) need not hold 26). This mechanism was in fact first suggested to account for anomalous maximum rate relations obtained with dehydrogenases for which there was other evidence for an ordered mechanism 27-29). 

The preferred pathway mechanism might be regarded at present as a rather general one for dehydrogenases, in place of the simple ordered mechanism. 

Thus, for a simple Ordered Bi Bi mechanism, the above ratios are always greater than unity. Values close to unity indicate that the dissociation of the product Q or a substrate A (usually the coenzymes with dehydrogenases) determines the maximum rate, that is, a Theorell-Chance mechanism. The values much greater than unity are inconsistent with a simple ordered mechanism, and suggest an isomerization of the enzyme-substrate complexes. 

LADHee and that the activity disappeared after carboxymethylation of a cysteine residue at the active site of LADH s [145]. In a recent study by Okuda and Okuda it was demonstrated that the -hydroxysteroid dehydrogenase activity in human liver was associated with a major isoenzyme of liver alcohol dehydrogenase (/82, 2) that the activity was inhibited by a chelating agent for Zn, which resides in the active site of the enzyme [146], Kinetic studies with the highly purified isoenzyme showed that neither a Theorell-Chance mechanism nor a simple ordered BiBi mechanism applied to the reaction. Evidence was obtained that the reaction was asymmetric in both directions. It has been established by Fukuba that the 4A-hydro-gen in NADH is involved [147]. 

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