Grissom C B, Willeford K O, Wedding R T
Biochemistry. 1987 May 5;26(9):2594-6. doi: 10.1021/bi00383a027.
The 13C primary kinetic isotope effect on the decarboxylation of malate by nicotinamide adenine dinucleotide malic enzyme from Crassula argentea is 1.0199 +/- 0.0006 with proteo L-malate-2-H and 1.0162 +/- 0.0003 with malate-2-d. The primary deuterium isotope effect is 1.45 +/- 0.10 on V/K and 1.93 +/- 0.13 on Vmax. This indicates a stepwise conversion of malate to pyruvate and CO2 with hydride transfer preceding decarboxylation, thereby suggesting a discrete oxaloacetate intermediate. This is in agreement with the stepwise nature of the chemical mechanism of other malic enzymes despite the Crassula enzyme's inability to reduce or decarboxylate oxaloacetate. Differences in morphology and allosteric regulation between enzymes suggest specialization of the Crassula malic enzyme for the physiology of crassulacean acid metabolism while maintaining the catalytic events found in malic enzymes from animal sources.
景天科植物银波锦中烟酰胺腺嘌呤二核苷酸苹果酸酶催化苹果酸脱羧反应的(^{13}C)一级动力学同位素效应，以蛋白质型(L - 苹果酸 - 2 - H)为底物时为(1.0199 \pm 0.0006)，以苹果酸 - 2 - d为底物时为(1.0162 \pm 0.0003)。一级氘同位素效应在(V/K)上为(1.45 \pm 0.10)，在(V_{max})上为(1.93 \pm 0.13)。这表明苹果酸逐步转化为丙酮酸和二氧化碳，脱羧反应之前有氢化物转移，从而暗示存在离散的草酰乙酸中间体。尽管景天科植物的该酶无法还原或脱羧草酰乙酸，但这与其他苹果酸酶化学机制的逐步性质是一致的。不同酶之间在形态和别构调节上的差异表明，景天科植物苹果酸酶专门适应景天酸代谢的生理过程，同时保持了动物源苹果酸酶中的催化过程。
