Nicotinamide adenine dinucleotide malate dehydrogenase

A reversible covalent modification that plants use extensively is the reduction of cystine disulfide bridges to sulf-hydryls. Many of the enzymes of photosynthetic carbohydrate synthesis are activated in this way (table 9.3). Some of the enzymes of carbohydrate breakdown are inactivated by the same mechanism. The reductant is a small protein called thioredoxin, which undergoes a complementary oxidation of cysteine residues to cystine (fig. 9.5). Thioredoxin itself is reduced by electron-transfer reactions driven by sunlight, which serves as a signal to switch carbohydrate metabolism from carbohydrate breakdown to synthesis. In one of the regulated enzymes, phosphoribulokinase, one of the freed cysteines probably forms part of the catalytic active site. In nicotinamide-adenine dinucleotide phosphate (NADP)-malate dehydrogenase and fructose-1,6-bis-... 

Nicotinamide adenine dinucleotide, NAD+, is a coenzyme commonly involved in biological oxidation-reduction reactions (Figure 13.3). Now consider the final reaction of the citric acid cycle, an energy-harvesting pathway essential to life. In this reaction, catalyzed by malate dehydrogenase, malate is oxidized to produce oxaloacetate ... 

Cosubstrates are loosely bound coenzymes that are reqnired in stoichiometric amonnts by enzymes. The molecnle nicotinamide adenine dinucleotide (NAD) acts as a cosubstrate in the oxidation-rednction reaction that is catalyzed by malate dehydrogenase, one of the enzymes of the citric acid cycle. 

Dehydrogenase, malate (decarboxylating) (nicotinamide adenine dinucleotide (phosphate)) 922d, 4249, 4565... 

Dehydrogenase, malate (nicotinamide adenine dinucleotide phosphate) XXII-2 ... 

Figure 8.2 Malate Dehydrogenase (MDH). (a) Reversible reaction catalyzed by MDH where NADH is nicotinamide adenine dinucleotide, reduced form (b) ribbon display structure of MDH (porcine heart) (side view) (pdb 4mdh). The homo-dimeric protein consists of two polypeptides chains (yellow and red), with nicotinamide adenine dinudeotide (NAD+) in both independent catalytic sites illustrated in a ball and stick (blue) representation (c) chemical illustration of the tricarboxylic acid cycle (TCA) to demonstrate the importance of MDH catalysis in cycle closure. Enzyme abbreviations are PDH, pyruvate dehyrogensase CS, citrate synthase.

An alternate route utilises malate nicotinamide adenine dinucleotide phosphate (NADP) dehydrogenase (decarboxylating) to form malate, and then conversion to oxalacetate within the citric acid cycle to citrate. The relative importance of these two routes probably depends upon the subcellular distribution of the relevant enzymes in the tissue or subcellular organelle under study. This is referred to in detail in a later section. It should also be noted that pyruvate can be incorporated into the citric acid cycle either as oxalacetate or via acetyl CoA into citrate. This alternative applies only to the glycolytic pathway fatty acid oxidation, which is an alternate pathway of energy production, terminates with acetyl CoA which can only enter the citric acid cycle as citrate. 

Johnson, H.S., Hatch, M. D. Properties and regulation of leaf nicotinamide-adenine dinucleotide phosphate-malate dehydrogenase and malic enzyme in plants with the C4-dicarboxylic acid pathway of photosynthesis. Biochem. J. 779,173-280 (1970)... 

The reductive steps in fatty-acid synthesis require reduced nicotinamide adenine dinucleotide phosphate (NADPH). Some NADPH is produced during recycling of the oxaloacetate formed during the cytoplasmic hydrolysis of citrate, described above. Oxaloacetate is first converted to malate (via cytoplasmic malate dehydrogenase). Malate is then... 

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