Malate dehydrogenase cytoplasmic

Succinate dehydrogenase is the enzyme that catalyzes the oxidation of succinate to fumarate and is also part of the respiratory chain 544 Malate dehydrogenase is one of several enzymes in the TCA cycle present in both the cytoplasm and mitochondria 544 Citrate is a multifunctional compound predominantly synthesized and released by astrocytes 544... 

Hill, E., Tsernoglou, D., Webb, L., Banaszak, L. Polypeptide conformation of cytoplasmic malate dehydrogenase from an electron density map at 3.0 A resolution. J. Mol. Biol. 72, 577-591 (1972). 

The tricarboxylic acid cycle not only takes up acetyl CoA from fatty acid degradation, but also supplies the material for the biosynthesis of fatty acids and isoprenoids. Acetyl CoA, which is formed in the matrix space of mitochondria by pyruvate dehydrogenase (see p. 134), is not capable of passing through the inner mitochondrial membrane. The acetyl residue is therefore condensed with oxaloacetate by mitochondrial citrate synthase to form citrate. This then leaves the mitochondria by antiport with malate (right see p. 212). In the cytoplasm, it is cleaved again by ATP-dependent citrate lyase [4] into acetyl-CoA and oxaloacetate. The oxaloacetate formed is reduced by a cytoplasmic malate dehydrogenase to malate [2], which then returns to the mitochondrion via the antiport already mentioned. Alternatively, the malate can be oxidized by malic enzyme" [5], with decarboxylation, to pyruvate. The NADPH+H formed in this process is also used for fatty acid biosynthesis. 

In the malate shuttle (left)—which operates in the heart, liver, and kidneys, for example-oxaloacetic acid is reduced to malate by malate dehydrogenase (MDH, [2a]) with the help of NADH+HT In antiport for 2-oxogluta-rate, malate is transferred to the matrix, where the mitochondrial isoenzyme for MDH [2b] regenerates oxaloacetic acid and NADH+HT The latter is reoxidized by complex I of the respiratory chain, while oxaloacetic acid, for which a transporter is not available in the inner membrane, is first transaminated to aspartate by aspartate aminotransferase (AST, [3a]). Aspartate leaves the matrix again, and in the cytoplasm once again supplies oxalo-acetate for step [2a] and glutamate for return transport into the matrix [3b]. On balance, only NADH+H"" is moved from the cytoplasm into the matrix ATP is not needed for this. 

The ratio [NAD+]/[NADH] appears to be maintained at a relatively constant value and in equilibrium with a series of different reduced and oxidized substrate pairs. Thus, it was observed that in the cytoplasm of rat liver cells, the dehydrogenations catalyzed by lactate dehydrogenase, sn-glycerol 3-phosphate dehydrogenase, and malate dehydrogenase are all at equilibrium with the same ratio of [NAD+]/[NADH].166 In one experiment rat livers were removed and frozen in less than 8 s by "freeze-clamping" (Section L,2) and the concentrations of different components of the cytoplasm determined167 the ratio [NAD+] / [NADH] was found to be 634, while the ratio of [lactate]/[pyruvate] was 14.2. From these values an... 

Fig. 1.6 Reactions of the malate-aspartate shuttle showing the transport of reducing equivalents from cytoplasm to mitochondria. a-KG, a-Ketoglutarate Asp, aspartate Glu, glutamate OAA, oxaloacetate aspartate aminotransferase (1) and malate dehydrogenase (2)...

Oxaloacetate, the product of the first step in gluconeogenesis, must leave the mitochondrion and enter the cytosol where the subsequent enzyme steps take place. Since the inner mitochondrial membrane is impermeable to oxaloacetate, it is converted to malate by mitochondrial malate dehydrogenase. This leaves the mitochondrion and is converted back to oxaloacetate in the cytosol by cytoplasmic malate dehydrogenase. 

A similar shuttle, the malate-aspartate shuttle, operates in heart and liver (Fig. 6). Oxaloacetate in the cytosol is converted to malate by cytoplasmic malate dehydrogenase, reoxidizing NADH to NAD+ in the process. The malate enters the mitochondrion via a malate-a-ketoglutarate carrier in the inner mitochondrial membrane. In the matrix the malate is reoxidized to oxaloacetate by NAD+ to form NADH. Oxaloacetate does not easily cross the inner mitochondrial membrane and so is transaminated to form aspartate which then exits from the mitochondrion... 

Dahlhoff, E., and G.N. Somero (1993a). Kinetic and structural adaptations of cytoplasmic malate dehydrogenases of eastern Pacific abalones (genus Haliotis) from different thermal habitats biochemical correlates of biogeographical patterning. J. Exp. Biol. 185 137-150. 

The next reaction occurs in the cytoplasm. Malate is transported to the cytoplasm by a dicarboxylate carrier which is specific for malate, succinate, and fumarate and which requires the entry of P, or one of these dicarboxylate anions. Cytoplasmic malate is then converted to oxaloacetate by cytoplasmic malate dehydrogenase ... 

The enzymes are, respectively (1) mitochondrial malate dehydrogenase (2) cytoplasmic malate dehydrogenase (3) phosphoenolpyruvate carboxykinase (4) pyruvate kinase and pyruvate dehydrogenase. The acetyl-CoA could then condense with oxaloacetate (produced from a second molecule of aspartate) to yield citrate. Aspartate could, therefore, continue to supply acctyl-CoA, which would continue to fuel the citric acid cycle. 

Pyruvate carboxylase is a mitochondrial enzyme, vhereas the other enzymes of gluconeogenesis are cytoplasmic. Oxaloacetate, the product of the pyruvate carboxylase reaction, is reduced to malate inside the mitochondrion for transport to the cytosol. The reduction is accomplished by an NADH-linked malate dehydrogenase. When malate has been transported across the mitochondrial membrane, it is reoxidized to oxaloacetate by an NAD+-linked malate dehydrogenase in the cytosol (Figure 16.28). 

A. D. Chapman, A. Cortes, T.R. Daffom, A.R. Clarke, and R.L. Brady. 1999. Structural basis of substrate specificity in malate dehydrogenases Crystal structure of a ternary complex of porcine cytoplasmic malate dehydrogenase, alpha-ketomalonate and tetrahydoNAD J. Mol. Biol. 285 703-712. (PubMed)... 

A single polypeptide chain can in theory exist in an infinite number of different conformations. However, one specific conformation generally appears to be the most stable for any given sequence of amino acids, and this conformation is assumed by the chain as it is synthesized within the cell. Thus, the primary structure of the polypeptide chain also determines its three-dimensional secondary and tertiary structures. It is conceivable that in some cases there may be several alternative conformations ("conforraers ) of a single chain that are of nearly equal stabilities and therefore these alternative forms may coexist. This possibility was first suggested to account for the heterogeneity noted in preparations of the cytoplasmic and mitochondrial isoenzymes of malate dehydrogenase and has also been proposed as an explanation of the multiple electrophoretic zones of erythrocyte acid phosphatase. However, no multiple enzyme forms have been shown unequivocally to be due to conformational isomerism. 

The malate-aspartate shuttle is quantitatively more significant in all vertebrate tissues. It is unidirectional, requiring cytoplasmic and mitochondrial malate dehydrogenases and aspartate aminotransferases as well as two membrane-bound carrier systems (Figure 14-22). In this process, the reducing equivalents of NADH are... 

Pyruvate carboxylase is a mitochondrial enzyme, whereas the other enzymes of gluconeogenesis are present primarily in the cytoplasm. Oxaloacetate, the product of the pyruvate carboxylase reaction, must thus be transported to the cytoplasm to complete the pathway. Oxaloacetate is transported from a mitochondrion in the form of malate oxaloacetate is reduced to malate inside the mitochondrion by an NADH-linked malate dehydrogenase. After malate has been transported across the mitochondrial membrane, it is reoxidized to oxaloacetate by an NAD -linked malate dehydrogenase in the cytoplasm (Figure 16.26). The formation of oxaloacetate from malate also provides NADH for use in subsequent steps in gluconeogenesis. Finally, oxaloacetate is simultaneously decarboxylated and phospho-ry lated by phosphoenolpyruvate carboxy kinase to generate phosphoenol pyruvate. The phosphoryl donor is GTP. The GO2 that was added to pyruvate by pyruvate carboxylase comes off in this step. 

PEP carboxykinase is found within the mitochondria of some species and in the cytoplasm of others. In humans this enzymatic activity is found in both compartments. Because the inner mitochondrial membrane is impermeable to OAA, cells that lack mitochondrial PEP carboxykinase transfer OAA into the cytoplasm by using, for example, the malate shuttle. In this process, OAA is converted into malate by mitochondrial malate dehydrogenase. After the transport of malate across mitochondrial membrane, the reverse reaction is catalyzed by cytoplasmic malate dehydrogenase. 

When citrate, a citric acid cycle intermediate, moves from the mitochondrial matrix into the cytoplasm, it is cleaved to form acetyl-CoA and oxaloacetate by citrate lyase. The citrate lyase reaction is driven by ATP hydrolysis. Most of the oxaloacetate is reduced to malate by malate dehydrogenase. Malate may then be oxidized to pyruvate and CO, by malic enzyme. The NADPH produced in this reaction is used in cytoplasmic biosynthetic processes, such as fatty acid synthesis. Pyruvate enters the mitochondria, where it may be converted to oxaloacetate or acetyl-CoA. Malate may also reenter the mitochondria, where it is reoxidized to form oxaloacetate. 

The reversible inhibition of mitochondrial malate dehydrogenase by oxaloacetate (>0.25 mM) has been studied in some detail (95). It is noncompetitive toward NADH. A dead-end abortive complex, which would cause uncompetitive inhibition, and a nonproductive binary complex of enzyme and oxaloacetate, to account for the competitive element of the inhibition, were postulated. The mechanism in Eq. (15) involving an active complex EB is an alternative interpretation for these results and for the less pronounced oxaloacetate inhibition observed with the cytoplasmic enzyme (55). Cytoplasmic malate dehydrogenases from several tissues and species are less susceptible to oxaloacetate inhibition than the mitochondrial enzymes (96,97) and, in the case of the chicken heart enzymes, more susceptible to malate inhibition (97). Activation by high concentrations of malate has been observed with the mitochondrial enzyme from bovine heart (98,99). 

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