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Malate is acted on by malate dehydrogenase to become oxaloacetate, producing a molecule of NADH. After that, oxaloacetate will be recycled to aspartate, as transaminases prefer these keto acids over the others. This recycling maintains the flow of nitrogen into the cell. Relationship of oxaloacetic acid, malic acid, and aspartic acid
Malate dehydrogenase (EC 1.1.1.37) (MDH) is an enzyme that reversibly catalyzes the oxidation of malate to oxaloacetate using the reduction of NAD + to NADH. This reaction is part of many metabolic pathways , including the citric acid cycle .
After malate reaches the mitochondrial matrix, it is converted by mitochondrial malate dehydrogenase into oxaloacetate, during which NAD + is reduced with two electrons to form NADH. Oxaloacetate is then transformed into aspartate (since oxaloacetate cannot be transported into the cytosol) by mitochondrial aspartate aminotransferase.
Oxaloacetate + 2 H + + 2 e − → Malate-0.17 [10] While under standard conditions malate cannot reduce the more electronegative NAD +:NADH couple, in the cell the concentration of oxaloacetate is kept low enough that Malate dehydrogenase can reduce NAD + to NADH during the citric acid cycle. Fumarate + 2 H + + 2 e − → Succinate +0.03 [9]
In enzymology, a malate dehydrogenase (oxaloacetate-decarboxylating) (EC 1.1.1.38) is an enzyme that catalyzes the chemical reaction below (S)-malate + NAD + pyruvate + CO 2 + NADH. Thus, the two substrates of this enzyme are (S)-malate and NAD +, whereas its 3 products are pyruvate, CO 2, and NADH.
Phosphoenolpyruvate (PEP), a glycolysis pathway intermediate, is carboxylated by the enzyme PEP carboxylase to form oxaloacetate. [9] This is followed by the conversion of oxaloacetate to malate by the enzyme malate dehydrogenase. Fumarate hydratase then catalyses the dehydration of malate to produce fumarate. [10]
Malate, in the mitochondrial matrix, can be used to make pyruvate (catalyzed by malic enzyme) or oxaloacetic acid, both of which can enter the citric acid cycle. Glutamine can also be used to produce oxaloacetate during anaplerotic reactions in various cell types through "glutaminolysis", which is also seen in many c-Myc transformed cells. [3]
Both malate and oxaloacetate can be converted into phosphoenolpyruvate, which is the product of phosphoenolpyruvate carboxykinase, the first enzyme in gluconeogenesis. The net result of the glyoxylate cycle is therefore the production of glucose from fatty acids.