The mechanism of action of yeast beta-hydroxy-beta-methylglutaryl-coenzyme A reductase has been investigated through kinetic studies on the oxidation of mevaldate by nicotinamide adeninine dinucleotide phosphate (NADP) in the presence of coenzyme A (CoA) and on the reduction of mevaldate by reduced NADP (NADPH) in the absence of presence of CoA or acetyl-CoA. NADP and mevalonate were also used as product inhibitors of the reduction of mevaldate. In the reduction of mevaldate to mevalonate, coenzyme A and acetyl-CoA decreased the Km for mevaldate 30- and 3-fold, respectively. Both compounds increased the Vmax 1.5-fold. These results suggest that CoA is an allosteric activator for the second reductive step and that it acts by enhancing the binding of mevaldate. The intersecting patterns obtained from initial velocities and the patterns produced by product inhibitions suggest the following features of the mechanism. The binding of substrates and release of products proceeds sequentially in both reductive steps, and is ordered throughout or random with respect to the binding of the beta-hydroxy-beta-methylglutaryl-coenzymeA and the first NADPH. The binding of NADPH enhances the binding of the beta-hydroxy-beta-methylglutaryl portion of the CoA ester and the binding of free mevaldate, whereas the binding of NADP leads to an increased affinity of the enzyme for the hemithioacetal (of mevaldate and CoA) and for mevalonate. Thus, the replacement of NADP by NADPH after the first reductive step promotes the conversion of the hemithioacetal to the free carbonyl form, which is then rapidly reduced. The products, CoA and mevalonic acid, of the second reductive step leave the enzyme before the release of the second NADP. This release of the last product is probably the rate-limiting step for the overall process.

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