초록
<P>The pressure to optimize enzymatic rate accelerations has driven the evolution of the induced-fit mechanism for enzyme catalysts where the binding interactions of nonreacting phosphodianion or adenosyl substrate pieces drive enzyme conformational changes to form protein substrate cages that are activated for catalysis. We report the results of experiments to test the hypothesis that utilization of the binding energy of the adenosine 5′-diphosphate ribose (ADP-ribose) fragment of the NAD cofactor to drive a protein conformational change activates <I>Candida boidinii</I> formate dehydrogenase (<I>Cb</I>FDH) for catalysis of hydride transfer from formate to NAD<SUP>+</SUP>. The ADP-ribose fragment provides a >14 kcal/mol stabilization of the transition state for <I>Cb</I>FDH-catalyzed hydride transfer from formate to NAD<SUP>+</SUP>. This is larger than the ca. 6 kcal/mol stabilization of the ground-state Michaelis complex between <I>Cb</I>FDH and NAD<SUP>+</SUP> (<I>K</I><SUB>NAD</SUB> = 0.032 mM). The ADP, AMP, and ribose 5′-phosphate fragments of NAD<SUP>+</SUP> activate <I>Cb</I>FDH for catalysis of hydride transfer from formate to nicotinamide riboside (NR). At a 1.0 M standard state, these activators stabilize the hydride transfer transition states by ≈5.5 (ADP), 5.5 (AMP), and 4.4 (ribose 5′-phosphate) kcal/mol. We propose that activation by these cofactor fragments is partly or entirely due to the ion-pair interaction between the guanidino side chain cation of R174 and the activator phosphate anion. This substitutes for the interaction between the α-adenosyl pyrophosphate anion of the whole NAD<SUP>+</SUP> cofactor that holds <I>Cb</I>FDH in the catalytically active closed conformation.</P><BR>[FIG OMISSION]</BR>