Malate synthese

Because of the fairly conclusive evidence that malate synthesized by dark CO2 fixation is not in equilibrium with malate synthesized in the mitochondria, it seems reasonable to conclude that the soluble malate dehydrogenase functions to a large extent in the dark CO2 fixation pathway. We also would predict, that insofar as P-enolpyruvate carboxylase-mediated synthesis of malate takes place in chloroplasts, the NADP malate dehydrogenase would be the coupling enzyme. 

Brandon (1967) postulated that diurnally changing thermoperiods must be responsible for the control of CAM. This hypothesis is based on in vitro experiments which showed that the malate-synthesizing enzymes (PEP carboxylase plus MDH) reach their optimum at 35° C, the thermal optimum of malate enzyme which may initiate malic acid consumption during CAM, was not reached even at 53° C. Thus, with in vitro combinations of PEP-C, MDH, and malate enzyme, net malate synthesis was observed only at temperatures clearly below 20° C. Above that temperature, depletion of malate dominated. Brandon (1967) concluded from these results that functioning of CAM predominates at low night and high day temperatures. 

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... 

Biotin is a growth factor for many bacteria, protozoa, plants, and probably all higher animals. In the absence of biotin, oxalacetate decarboxylation, oxalosuccinate carboxylation, a-ketoglutarate decarboxylation, malate decarboxylation, acetoacetate synthesis, citrulline synthesis, and purine and pyrimidine syntheses, are greatly depressed or absent in cells (Mil, Tl). All of these reactions require either the removal or fixation of carbon dioxide. Together with coenzyme A, biotin participates in carboxylations such as those in fatty acid and sterol syntheses. Active C02 is thought to be a carbonic acid derivative of biotin involved in these carboxylations (L10, W10). Biotin has also been involved in... 

Various constituents in plant foods can impede Ca absorption. Plant-based diets can be high in oxalate and phytate, which are recognized as inhibitors of Ca absorption. In fact, Ca absorption is considered to be inversely proportional to oxalic acid content of the food (Weaver et al, 1999). Phytic acid poses Ca absorption problems for those species imable to endogenously synthesize phytase (e.g., humans, birds, and pigs). The Ca in CCM is chelated with the citrate and malate anions, which may make CCM less reactive than other sources of Ca toward food components known to interact with Ca " cations. For example, Lihono et al (1997a) reported data suggesting that the Ca in CCM may be less likely to complex with phytates than other Ca salts. Qn this basis, CCM may be more appropriate for the fortification of soy or other phytic acid-containing products. 

While the glycerol 3-phosphate shuttle appears to be less efficient than the malate-aspartate shuttle because fewer ATP molecules are synthesized (see Chapter 7), its advantage is that it enables the cell to transport electrons in the presence of high amounts of NADH. 

Six ATPs will be synthesized if the aspartate-malate shuttle is used to transfer NADH generated through glycolysis to NADH in the mitochondrial matrix four molecules of ATP will be made if the glycerol phosphate shuttle delivers the electrons to ubiquinone in the inner mitochondrial membrane. 

Ideally, chemists should aspire to design and synthesize ILs with minimal toxicity and which readily biodegrade. The examples of ILs that come closest to this ideal are based on biological cations, such as choline and betaine, and anions that are commonly used in the food industry, such as tartrate, citrate, malate, and saccharinate. 

Synthesis of L-Malate in Wine Making The tartness of some wines is due to high concentrations of L-malate. Write a sequence of reactions showing how yeast cells synthesize L-malate from glucose under anaerobic conditions in the presence of dissolved C02 (HCO3 ). Note that the overall reaction for this fermentation cannot involve the consumption of nicotinamide coenzymes or citric acid cycle intermediates. 

Skeletal muscle and brain use a different NADH shuttle, the glycerol 3-phosphate shuttle (Fig. 19-28). It differs from the malate-aspartate shuttle in that it delivers the reducing equivalents from NADH to ubiquinone and thus into Complex III, not Complex I (Fig. 19-8), providing only enough energy to synthesize 1.5 ATP molecules per pair of electrons. 

Enzymes synthesizing succinate, fumarate, malate, and oxaloacetate... 

Some efficient syntheses have been reported such as those of p-hydroxy esters from tris(methylthio)methyllithium and epoxides. The case of a malate derivative from a commercially available chiral epoxide is described [286]. 

R and S isomers of HDT]acetic acid were synthesized by chemical and enzymatic methods that yield products of known stereochemistry.1819 The two isomers were then distinguished by using the following ingenious enzymatic assays. The acetic acid was first converted to acetyl-coenzyme A (by a reaction of the carboxyl group—and not the methyl—of acetic acid). The acetyl-coenzyme A was then condensed with glyoxylate to form malate in an essentially irreversible reaction catalyzed by malate synthase (equation 8.27). The crucial feature of this reaction is that it is subject to a normal kinetic isotope effect, so that more H than D... 

Nitrate respiration can support the synthesis of ATP, while proton pumping has been quantified for several physiological substrates. Stoichiometries of about 4H+/NO, and 2H+/N03" have been found for L-malate and formate, and succinate, D-lactate and glycerol respectively. There is evidence that about one mole of ATP is synthesized by oxidative phosphorylation per mole of nitrate reduced.1440... 

Note that, under aerobic conditions, the two NADH molecules that are synthesized are reoxidized via the electron transport chain generating ATP. Given the cytoplasmic location of these NADH molecules, each is reoxidized via the glycerol 3-phosphate shuttle (see Topic L2) and produces approximately two ATPs during oxidative phosphorylation or via the malate-aspartate shuttle (see Topic L2) and produces approximately three ATPs during oxidative phosphorylation. 

The formation of acetyl-CoA from pyruvate in animals is via the pyruvate dehydrogenase complex, which catalyzes the irreversible decarboxylation reaction. Carbohydrate is synthesized from oxaloacetate, which in turn is synthesized from pyruvate via pyruvate carboxylase. Since the pyruvate dehydrogenase reaction is irreversible, acetyl-CoA cannot be converted to pyruvate, and hence animals cannot realize a net gain of carbohydrate from acetyl-CoA. Because plants have a glyoxylate cycle and animals do not, plants synthesize one molecule of succinate and one molecule of malate from two molecules of acetyl-CoA and one of oxaloacetate. The malate is converted to oxaloacetate, which reacts with another molecule of acetyl-CoA and thereby continues the reactions of the glyoxylate cycle. The succinate is also converted to oxaloacetate via the enzymes of the citric acid cycle. Thus, one molecule of oxaloacetate is diverted to carbohydrate synthesis and, therefore, plants are able to achieve net synthesis of carbohydrate from acetyl-CoA. 

While the syntheses described here should produce chiral acetate samples, methods are needed to prove the existence of chirality and to determine its extent. The first method was devised independently by the Comforth and Arigoni groups and is still widely used [122,128,129]. It depends on the use of two enzymes, malate synthase and fumarase. Thus, overall, the sample of acetic acid is first converted to its CoA derivative (either with acetate kinase and phosphotransacetylase or chemically with ClCOOC2H5 and Co ASH) and then to malate and fumarate ... 

Pyrophosphate is rapidly hydrolyzed, and so the equivalent of four molecules of ATP are consumed in these reactions to synthesize one molecule of urea. The synthesis of fumarate by the urea cycle is important because it links the urea cycle and the citric acid cycle (Figure 23.17). Fumarate is hydrated to malate, which is in turn oxidized to oxaloacetate. Oxaloacetate has several possible fates (1) transamination to aspartate, (2) conversion into glucose by the gluconeogenic pathway, (3) condensation with acetyl CoA to form citrate, or (4) conversion into pyruvate. 

Fatty acid synthesis and degradation. Fatty acids are synthesized in the cytosol by the addition of two-carbon units to a growing chain on an acyl carrier protein. Malonyl CoA, the activated intermediate, is formed by the carboxylation of acetyl CoA. Acetyl groups are carried from mitochondria to the cytosol as citrate by the citrate-malate shuttle. In the cytosol, citrate is cleaved to yield acetyl CoA. In addition to transporting acetyl CoA, citrate in the cytosol stimulates acetyl CoA carboxylase, the enzyme catalyzing the committed step. When ATP and acetyl CoA are abundant, the level of citrate increases, which accelerates the rate of fatty acid synthesis (Figure 30.8). 

B. Malate dehydrogenase requires NAD+, which is synthesized from niacin. 

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