Pyruvate carboxylase biotin dependent

Biotin is involved in carboxylation and decarboxylation reactions. It is covalently bound to its enzyme. In the carboxylase reaction, C02 is first attached to biotin at the ureido nitrogen, opposite the side chain in an ATP-dependent reaction. The activated C02 is then transferred from carboxybiotin to the substrate. The four enzymes of the intermediary metabolism requiring biotin as a prosthetic group are pyruvate carboxylase (pyruvate oxaloacetate), propionyl-CoA-carboxylase (propionyl-CoA methylmalonyl-CoA), 3-methylcroto-nyl-CoA-carboxylase (metabolism of leucine), and actyl-CoA-carboxylase (acetyl-CoA malonyl-CoA) [1]. 

Vitamin H (biotin) is present in liver, egg yolk, and other foods it is also synthesized by the intestinal flora. In the body, biotin is covalently attached via a lysine side chain to enzymes that catalyze carboxylation reactions. Biotin-dependent carboxylases include pyruvate carboxylase (see p. 154) and acetyl-CoA carboxylase (see p. 162). CO2 binds, using up ATP, to one of the two N atoms of biotin, from which it is transferred to the acceptor (see p. 108). 

Carboxylation of pyruvate to oxaloacetate (OAA) by pyruvate carboxylase is a biotin-dependent reaction (see Figure 8.24). This reaction is important because it replenishes the citric acid cycle intermediates, and provides substrate for gluconeogenesis (see p. 116). 

When pyruvate with a chiral methyl group is carboxylated by pyruvate carboxylase the configuration at C-3 is retained. The carboxyl enters from the 2-si side, the same side from which the proton (marked H ) was removed to form the enolate anion (Eq. 14-12). Comparable stereochemistry has been established for other biotin-dependent enzymes.64 65... 

Thus pyruvate carboxylase generates oxaloacetate for gluconeogenesis but also must maintain oxaloacetate levels for citric acid cycle function. For the latter reason, the activity of pyruvate carboxylase depends absolutely on the presence of acetyl CoA the biotin prosthetic group of the enzyme cannot be carboxy-lated unless acetyl CoA is bound to the enzyme. This allosteric activation by acetyl CoA ensures that more oxaloacetate is made when excess acetyl CoA is present. In this role of maintaining the level of citric acid cycle intermediates, the pyruvate carboxylase reaction is said to be anaplerotic, that is filling up. ... 

Figure 12-2. Metabolic pathways involving the four biotin-dependent carboxylases. The solid rectangular blocks indicate the locations of the enzymes ACC, acetyl-CoA carboxylase PMCC, P-methylcrotonyl-CoA carboxylase PC, pyruvate carboxylase PCC, propionyl-CoA carboxylase. Isolated deficiencies of the first three carboxylases (mitochondrial) have been established (isolated ACC deficiency has not been confirmed). At least the activities of the three mitochondrial carboxylases can be secondarily deficient in the untreated multiple carboxylase deficiencies, biotin holocarboxylase synthetase deficiency and biotinidase deficiency. Lowercase characters indicate metabolites that are frequently found at elevated concentrations in urine of children with both multiple carboxylase deficiencies. The isolated deficiencies have elevations of those metabolites directly related to their respective enzyme deficiency.

In mammals and birds, there are four biotin-dependent carboxylases acetyl CoA carboxylase, pyruvate carboxylase, propionyl CoA carboxylase, and methylcrotonyl CoA carboxylase. Congenital deficiency of three of the four human biotin-dependent carboxylases has been reported. 

The activities of biotin-dependent carboxylases fall in deficiency, resulting in impaired gluconeogenesis, with accumulation of lactate, pyruvate, and alanine, and impaired lipogenesis, with accumulation of acetyl CoA, resulting in ketosis. There are also changes in the fatty acid composition of membrane lipids. A variety of abnormal organic acids are excreted by bothbiotin-deficient patients and experimental animals (as shown in Table 11.1). 

The first partial reaction of pyruvate carboxylase, the formation of carboxybiotin, depends on the presence of acetyl CoA. Biotin is not carboxylated unless acetyl CoA is bound to the enzyme. Acetyl CoA has no effect on the second partial reaction. The allosteric activation of pyruvate carboxylase by acetyl CoA is an important physiological control mechanism that will be discussed in Section 17.3.1. 

How is oxaloacetate replenished Mammals lack the enzymes for the net conversion of acetyl CoA into oxaloacetate or any other citric acid cycle intermediate. Rather, oxaloacetate is formed by the carboxylation of pyruvate, in a reaction catalyzed by the biotin-dependent enzyme pyruvate carboxylase. 

Biotin is a water-soluble vitamin. It is a cofactor for four ATP-dependent carboxylases acetyl-CoA carboxylase, pyruvate carboxylase, propionyl-CoA carboxylase, and p-methylcrotonyl-CoA carboxylase. Biotin occurs covalently bound to the enzymes via the terminal amino group of a lysine residue. With the normal and continual turnover of these enzymes in the body, the biotin is released, but then utilized again as a cofactor when the enzymes are re-synthesized. The structure of biotin is shown in Figure 9.32,... 

Other biotin-dependent enzymes include propionyl-CoA carboxylase and pyruvate carboxylase (Chapter 15). The latter, like acetyl-CoA carboxylase, is subject to allosteric regulation. Pyruvate carboxylase, a mitochondrial enzyme, is activated by acetyl-CoA and converts pyruvate to oxaloacetate which, in turn, is converted to glucose via... 

The answer is b. (Murray, pp 627-661. Scriver, pp 3897-3964. Sack, pp 121-138. Wilson, pp 287-320.) The vitamin biotin is the cofactor required by carboxylating enzymes such as acetyl CoA, pyruvate, and propionyl CoA carboxylases. The fixation of CO2 by these biotin-dependent enzymes occurs in two stages. In the first, bicarbonate ion reacts with adenosine triphosphate (ATP) and the biotin carrier protein moiety of the enzyme in the second, the active CO2 reacts with the substrate—e.g., acetyl CoA. 

PEPC catalyses carboxylation of phosphoenol pyruvate (PEP), employing bicarbonate as carboxyl donor (Cooper and Wood, 1971). As in all such enzymes employing bicarbonate in place of CO2, the enzyme must activate bicarbonate towards carboxyl transfer. In this respect, as in others, PEPC resembles biotin-dependent carboxylases. Two distinct mechanisms are postulated for PEPC one involving the intermediacy of carboxyphosphate, the other a cyclic transition state and pseudorotation at phosphorus. 

Pyruvate carboxylase (EC 6.4.1.1) a biotin-dependent ligase, in animals and plants, which catalyses the addition of CO2 to pyruvate Pyruvate + CO2 + ATP + H2O v Oxaloacetate + ADP + Pj. The enzyme is Mn -dependent, and it is practically inactive in the absence of its positive allosteric elector, ace-tyl-CoA. This reaction is an important early stage of Gluconeogenesis (see), and is an example of CO2 fixation in the animal organism. For the mode of attachment of the coenzyme, biotin, and the mechanism of CO2 transfer, see Biotin enzymes. The active form of Pc. is a tetramer, M, 600,000 (yeast), 650,000... 

Biotin serves as the prosthetic group of several enzymes that catalyse the transfer of carbon dioxide from one substrate to another. In animals there are three biotin-dependent enzymes of particular importance pyruvate carboxylase (carbohydrate synthesis from lactate), acetyl coenzyme A carboxylase (fatty acid synthesis) and propionyl coenzyme A carboxylase (the pathway of conversion of propionate to succinyl-CoA). The specific role of these enzymes in metabolism is discussed in Chapter 9. 

Bioassays also include methods that determine biotin indirectly through its biological function in the test animal. Thus, measurement of the activity of biotin-dependent pyruvate carboxylase in the blood has been utilized as an indicator of biotin content. 

In mammalian tissues, four biotin-dependent carboxylases are enzymes of intermediate metabolism (Samols et al. 1988). Pyruvate carboxylase (PC EC... 

Biotin functions to transfer carbon dioxide in a small number of carboxylation reactions. The reactive intermediate is 1-N-carboxybiocytin (Figure 11.25), formed from bicarbonate in an ATP-dependent reaction. A single holocarboxylase synthetase acts on the apoenzymes of acetyl CoA carboxylase (a key enzyme in fatty acid synthesis section 5.6.1), pyruvate carboxylase (a key enzyme in gluconeogenesis section 5.7), propionyl CoA carboxylase and methylcrotonyl CoA carboxylase to form the active holoenzymes from (inactive) apoenzymes and free biotin. 

The reverse of the last irreversible reaction in glycolysis is actually two successive enzyme-catalyzed reactions. First pyruvate is converted to oxaloacetate by pyruvate carboxylase, a biotin-dependent enzyme whose mechanism we looked at in Section 24.4. Oxaloacetate is Ihen converted to phosphoenolpyruvate. In this reaction, the 3-oxocarboxyhc acid is decarboxylated (Section 18.17) and the oxygen of the enolate ion attacks the y-phosphorus of GTP (see page 1194). 

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