Malic enzyme, product inhibition

PEP carboxylase occurs in yeast, bacteria, and higher plants, but not in animals. The enzyme is specifically inhibited by aspartate, which is produced by transamination of oxaloacetate. Thus, organisms utilizing this enzyme control aspartate production by regulation of PEP carboxylase. Malic enzyme is found in the cytosol or mitochondria of many animal and plant ceils and is an NADPIT-dependent enzyme. 

NADH as an end product. This implicates oxidized malic acid, either pyruvic or oxaloacetic acid, as another end product. By adding commercial preparations of L-lactic dehydrogenase or malic dehydrogenase to the reaction mixture, Morenzoni (90) concluded that the end product was pyruvic acid. Attempts were then made to show whether two enzymes—malate carboxy lyase and the classic malic enzyme, malate oxidoreductase (decarboxylating), were involved or if the two activities were on the same enzyme. The preponderance of evidence indicated that only one enzyme is involved. This evidence came from temperature inactivation studies, heavy-metal inhibition studies, and ratio measurements of the two activities of partially purified preparations of Schiitz and Radlers malo-lactic enzyme (76, 90). This is not the first case of a single enzyme having two different activities (91). 

PEP is carboxylated to oxaloacetate, transaminated to aspartate ar d shunted into the bundle sheath. Aspartate then is transaminated back to oxaloacetate in the mesophyll, reduced to malate by NADH and decarboxylated to pyruvate by NAD-linked malic enzyme in the bundle-sheath mitochondria. There is no net production of NAD(P)H so the chloroplasts have PSII and may be less effective at inhibiting photorespiration. Pyruvate is phosphorylated in the mesophyll back to PEP, using 2 ATP (Fig. 13.18). 

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