Formate, active from pyruvate

Since formation of citraconic anhydride from pyruvic acid is one of "acid to acid type" transformations, such as reactions from isobutyric acid to methacrylic acid and from lactic acid to pyruvic acid, the required catalysts must be acidic [11). If the catalysts are basic, it may be impossible to obtained acidic products, because basic catalysts activate selectively acidic molecules and, as a result, they show a very high activity for the decomposition of acidic products [11]. 

Biotin (5) is the coenzyme of the carboxylases. Like pyridoxal phosphate, it has an amide-type bond via the carboxyl group with a lysine residue of the carboxylase. This bond is catalyzed by a specific enzyme. Using ATP, biotin reacts with hydrogen carbonate (HCOa ) to form N-carboxybiotin. From this activated form, carbon dioxide (CO2) is then transferred to other molecules, into which a carboxyl group is introduced in this way. Examples of biotindependent reactions of this type include the formation of oxaloacetic acid from pyruvate (see p. 154) and the synthesis of malonyl-CoA from acetyl-CoA (see p. 162). 

Higher Alcohols. The most abundant, volatile minor products of alcoholic fermentation are the higher alcohols or fusel alcohols. The most important are isoamyl (3-methyl-l-butanol), d-active amyl (2-methyl-l-butanol), isobutyl (2-methy 1-1-propanol), and n-propyl (1-propanol) alcohols. It is now recognized (4, 5, 6, 7) that these higher alcohols are formed by decarboxylation of particular a-keto acids to yield the corresponding aldehydes, and these in turn are reduced to the alcohols in a manner analogous to the formation of ethyl alcohol from pyruvic acid. 

L-Lactate dehydrogenase (l-LDH, EC 1.1.1.27) catalyzes the reduction of pyruvate to (S)-lactate with a simultaneous oxidation of NADH. l-LDH is found in all higher organisms. There are two kinds of l-LDHs enzymes from one group are activated by fructose 1,6-diphosphate while the other group stays independent [71]. l-LDH is highly selective for pyruvate, short-chain 2-keto acids and phenylpyruvic acid [80]. All bacterial NAD+-dependent LDHs form lactate from pyruvate in vivo, and there is no evidence at all that they catalyze the other direction as well. The equilibrium constant lies far on the direction of lactate formation, and thus the reaction catalyzed by bacterial LDHs can be considered almost irreversible. LDHs from some lacto-bacilli like Lactobacillus fermentum or L. cellobiosus show no or just poor reaction with lactate [71], whereas mammalian LDHs can be considered as reversible [71]. Well characterized l-LDHs are summarized in Table 2. 

Pyruvate Carboxylase. This enzyme has manganese firmly in its structure and acts together with phosphoenol pyruvate (PEP) carboxykinase, an enzyme that is activated by manganese ions. These enzymes are required to catalyze the formation of PEP from pyruvate, a key reaction in the hepatic synthesis of glucose. 

Figure 17.21 PATHWAY INTEGRATION Pathways active during exercise after a nights rest. The rate of the citric acid cycle increases during exercise, requiring the replenishment of oxaloacetate and acetyl CoA. Oxaloacetate is replenished by its formation from pyruvate. Acetyl CoA may be produced from the metabolism of both pyruvate and fatty acids.

A diet high in D-fructose produces increased concentrations of pyruvate, malate, and acetyl coenzyme A (AcSCoA).294 295 D-Fructose enhances formation of AcSCoA from pyruvate by stimulating pyruvate oxidation.296 As D-fructose causes a fall in the hepatic ATP concentration, pyruvate dehydrogenase (EC 1.2.4.1) is activated, and it produces an increase in AcSCoA formation from pyruvate.297 Thus, dietary sucrose and D-fructose produce higher hepatic fatty acid synthesis than does dietary D-glucose, because of their stimulation of AcSCoA formation. 

The enzymatic synthesis reaction of l-DOPA is carried out in a batchwise system with cells of E. herbicola with high TPL activity. Since pyruvate, one of the substrates, was unstable in the reaction mixture at a high temperature, low temperature was favored for the synthesis of l-DOPA. The reaction was carried out at 16 °C for 48 h in a reaction mixture containing various amounts of sodium pyruvate, 5 g of ammonium acetate, 0.6 g of pyrocatechol, 0.2 g of sodium sulfite, 0.1 g of EDTA, and cells harvested from 100 ml of broth, in a total volume of 100 ml. The pH was adjusted to 8.0 by the addition of ammonia. At 2-h intervals, sodium pyruvate and pyrocatechol were added to the reaction mixture to maintain the initial concentrations. The maximum synthesis of l-DOPA was obtained when the concentration of sodium pyruvate was kept at 0.5%. The substrates, pyrocatechol and pyruvate, were added at intervals to prevent the denaturation of TPL and to prevent byproduct formation. The addition of sodium sulfite is effective in maintaining the reactor in a reductive state and in preventing the oxidation of the l-DOPA produced. l-DOPA is insoluble in the reaction medium, so it appears as a crystalline precipitate during the reaction, at final amounts reaching 110 g/1 [19-21]. 

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