Carboxylase biotin-dependent

The biotin-dependent carboxylases that couple the decarboxylation of malonate to acetate in Malonomonas rubra to the transport of Na across the cytoplasmic membrane... 

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). 

Among the enzymes that occur in biological systems and which utilize C02 as a source of carbon, some contain metal atoms as the active center, where C02 is converted (as in RuBisCO or biotin-dependent carboxylases). However, some of these enzymes do not require metal, or, if so, it does not interact directly with C02. 

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.

Inherited isolated deficiencies of the three mitochondrial biotin-dependent carboxylases were described during the 1970s (Fig. 12-2). Children with each of the isolated deficiencies exhibit neurological symptoms during infancy or early childhood associated with metabolic compromise caused by the accumulation of abnormal metabolites resulting from the respective enzyme block. Each isolated deficiency is due to a structural abnormality in the respective mitochondrial enzyme, whereas the activities of... 

Although there have been reports of inherited isolated deficiencies of the three mitochondrial biotin-dependent carboxylases, there have not been any confirmed reports of inherited isolated acetyl-CoA carboxylase deficiency. What is a possible explanation for why isolated acetyl-CoA carboxylase deficiency has not been observed ... 

Wolf B, Feldman GL The biotin-dependent carboxylase deficiencies. Am J Hum Genet 34 699-716,1982. 

The biotin-dependent carboxylases catalyze a two-step reaction ... 

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 affected infants have a normal plasma concentration of biotin and excrete normal amounts of biotin in the urine. Skin fibroblasts have extremely low activities of aU four biotin-dependent carboxylases when they are cultured in media containing approximately physiological concentrations of biotin. But, culture with considerably higher concentrations of biotin results in normal activity of aU four carboxylases. The defect is in the affinity of holocarboxylase synthetase for biotin (its is 20- to 70-fold higher than normal). 

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). 

There is accumulation of the apoenzymes of biotin-dependent carboxylases in deficiency. Response to repletion is rapid, as a result of activation of the apoenzymes activation of biotin-dependent apoenzymes in vitro may provide an index of status (Section 11.4). 

The major functions of pantothenic acid are in CoA (Section 12.2.1) and as the prosthetic group for AGP in fatty acid synthesis (Section 12.2.3). In addition to its role in fatty acid oxidation, CoA is the major carrier of acyl groups for a wide variety of acyl transfer reactions. It is noteworthy that a wide variety of metabolic diseases in which there is defective metabolism of an acyl CoA derivative (e.g., the biotin-dependent carboxylase deficiencies Sections 11.2.2.1 and 11.2.3.1), CoA is spared by formation and excretion of acyl carnitine derivatives, possibly to such an extent that the capacity to synthesize carnitine is exceeded, resulting in functional carnitine deficiency (Section 14.1.2). 

WolfB and Feldman GL (1982) The biotin-dependent carboxylase AeBciencies. American Journal of Human Genetics 34, 699-716. 

Propionyl-CoA is an intermediary product in the metabo-hsm of four essential amino acids (isoleucine, valine, threonine, and methionine), the aliphatic side-chain of cholesterol, pyrimidines (uracd and thymine), and the final product of the [3-oxidation of odd-chain fatty acids. Under normal circumstances, propionyl-CoA first is converted by a biotin-dependent carboxylase to methylmalonyi-CoA, then to succinyl-CoA by an adenosylcobalamin-dependent mutase, leading to oxidation in the tricarboxylic acid cycle. Primary or secondary defects of these two enzymes were among the first organic acidurias to be discovered, and their natural history has been characterized perhaps better than any other inborn error of organic acid metabolism. 

Forming carbon-carbon bonds (biotin-dependent carboxylases)... 

Biotin (see Scheme 40) is the prosthetic group associated with the biotin-dependent carboxylases and transcarboxylases. The role of biotin is as a carboxyl group carrier. At the first active site, biotin is carboxylated at the N(l) position to form A-carboxybiotin, which acts as CO2 transport. At the second subsite, the carboxyl group is transferred from A-carboxybiotin to substrate. The mechanism of neither half-reaction has been fully defined (for recent discussions see Blonski et al., 1987 Thatcher et ai, 1986). 

The first half-reaction of biotin-dependent carboxylases. The reaction (13) catalysed at the first subsite of these carboxylases utilizes biotin, ATP and bicarbonate to yield inorganic phosphate. Pi, ADP and A-carboxybiotin. 

The evidence in support of the carboxyphosphate mechanism for biotin-dependent carboxylases has been summarized by Hansen and Knowles (1985). This may be reviewed critically as follows ... 

Carbamoyl phosphate, a putative carboxyphosphate analogue, is a substrate for the ATPase reaction of biotin-dependent carboxylases (Pokalis et al., 1974 Ashman and Keech, 1975). However, it has been argued that carbamoyl phosphate is better viewed as an analogue of 0-phosphobiotin (Kluger et al., 1979). 

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. 

Acetyl-CoA carboxylase (ACC EC 6.4.1.2) is a biotin-dependent carboxylase that produces malonyl-CoA from bicarbonate as a source of carboxyl group and ATP as a source of energy. The reaction catalyzes the conversion of acetyl-CoA into malonyl-CoA through the incorporation of a carboxyl group into the acetyl radical of the acetyl-CoA. This transcarboxylation reaction is performed following the three-step process followed by all biotin-dependent transcarboxylases (Scheme 9.1)... 

The E. coil carboxyltransferase component is a complex of two non identical subunits. a 35 IcDa a-subunit encoded by accA and a 33 kDa p-subunit encoded by occD. The deduced amino acid sequences of a- and -subunits show marked sequence simtlart-ties to the COOH-terminal and the NH2 terminaI moieties, respectively, of the tat pro-pionyl CoA carboxylase, a biotin-dependent carboxylase that catalyzes a similar carboxyltransferase reaction (71). 

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