Biochemistry - The Chemical Reactions of Living Cells, Volume 2 - D. Metzler 1980

Coenzymes – specialized natural reagents
Pyridine nucleotide coenzymes and dehydrogenases
Conformational changes accompanying the catalytic action of dehydrogenases

In the crystalline state, pig Heart malate dehydrogenase binds only one NAD⁺ molecule per dimer, whereas the binding of the second NAD⁺ molecule is significantly less favorable [79]. Similar anticooperativity has been observed for glyceraldehyde-3-phosphate dehydrogenase and alkaline phosphatase (Ch. 7, Sec. D, 1). However, there is no universal consensus regarding these experimental findings [80, 88a], and readers should approach them with considerable caution. It is tempting to speculate that the anticooperativity of coenzyme binding reflects cooperative interactions between subunits during catalysis. Suppose that only one conformation (A) binds the reduced substrate and NAD⁺, while the other conformation (B) binds NADH and the oxidized substrate. If the reduced substrate and NAD⁺ are present in excess and the oxidized substrate is efficiently removed from the system by subsequent oxidation, the following events can occur within the AB hybrid dimer. The subunit in conformation A can bind the substrates, react, and transition to conformation B. Concurrently, due to strong A–B interaction, the subunit originally in conformation B can revert to conformation A, thereby becoming ready to initiate a new catalytic cycle. Because conformation A has a low affinity for NADH, such Conformational Changes in the enzyme molecule would facilitate the release of the reduced coenzyme [81]. (It is well established that NADH dissociation is often the rate-limiting step in Reactions Catalyzed by dehydrogenases.) The hypothesis of such an alternating cycle, or "flip-flop" mechanism, was first proposed by Harada and Wolfe [81a]. This idea is appealing because it rationalizes the functional advantage of the dimeric form in many Enzymes that do not exhibit obvious allosteric properties. Lazdunski [82] suggested that flip-flop mechanisms are widespread among dimeric enzymes¹⁾, although it remains unclear whether they actually exist.