Holocarboxylase synthetase biotin metabolism

Figure 11.1. Metabolism of biotin. Holocarboxylase synthetase (biotin protein ligase), EC 6.3.4.10 and biotinidase (biotinamide amidohydrolase), EC 3.5.1.12. Relative molecular mass (Mr) biotin, 244.3 and biocytin, 372.5.

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.

MetabolicaUy, biotin is of central importance in lipogenesis, gluconeogen-esis, and the catabolism of branched-chain (and other) amino acids. There are two well-characterized biotin-responsive inborn errors of metabolism, which are fatal if untreated holocarboxylase synthetase deficiency and biotinidase deficiency. In addition, biotin induces a number of enzymes, including glu-cokinase and other key enzymes of glycolysis. Biotinylation of histones may be important in regulation of the cell cycle. 

Biotin deficiency may be caused by inborn errors on other proteins involved in biotin homeostasis biotidinase, the sodium-dependent multivitamin transporter and holocarboxylase synthetase (Zempleni et al. 2008). A congenital deficiency of either of these proteins may create impairments in essential metabolisms, causing clinical signs with various intensities. 

Biotin is central to the metabolism of carbohydrates, amino acids, and lipids as biotin is the prosthetic group of the carboxylases. In addition to this metabolic function, biotin influences transcription in organisms ranging from bacteria to humans. Biotin exerts complex effects on cell cycle and gene transcription through epigenetic mechanisms. Nuclear biotin holocarboxylase synthetase seems to interact with other chromatin proteins to form a multiprotein gene repressor complex. 

Holocarboxylase synthetase (EC 6.3.4.10) (HCS or biotin ligase) catalyzes biotin incorporation into various carboxylases which are essential for housekeeping cellular metabolism, as for example ACCase that realizes the first committed step in fatty acid biosynthesis. These carboxylases are synthesized at first as inactive apoproteins which are then modified into active holocarboxylases by post-translational addition of D-biotin to a specific Lysine residue in the apoprotein. This covalent reaction occurs via an amide linkage between the biotin carboxyl group and the e-amino group of a unique Lysine residue present in a specific sequence of all biotin-dependent carboxylases. This covalent attachment occurs in two steps (1) and (2) as follows ... 

Holocarboxylase synthetase deficiency [3, 4] is the classic infantile form of multiple carboxylase deficiency. Untreated it is uniformly fatal, while early diagnosis and treatment with biotin usually lead to the disappearance of all of the manifestations of the disease. Life-threatening illness is associated with massive ketosis and metabolic acidosis. A bright red cutaneous eruption may cover the body, and there is alopecia totalis. Immune function, both T and B cell, may be defective. 

Disorders in the metabolism of 3-methylcrotonyl-CoA are due to deficient activity in vivo of the 3-methylcrotonyl-CoA carboxylase enzyme system. As stated in the introduction to this section, this is a biotin-dependent enzyme system in common with other mitochondrial carboxylase enzymes, in which the D-biotin is attached to the carboxylase apoenzyme to form the active holocarboxylase by holocarboxylase synthetase. The biotin-dependent enzyme systems have been extensively reviewed elsewhere (Moss and Lane, 1971 Wood and Barden, 1977 Lynen, 1979), but some comment on the mechanism of enzyme activation and action is warranted here. Most of the available information on these aspects has been obtained from work with micro-organisms, and the mammalian enzyme systems have been relatively little studied. However, the mechanism and metabolic pathways involved appear to be similar for both micro-organisms and animals. The reaction is dependent on ATP and magnesium ions and initially depends on the coupling... 

Organic acids in human metabolic diseases of the biotin to the apoenzyme by holocarboxylase synthetase ... 

It is apparent that several abnormalities in this system may lead to deficient metabolism of 3-methylcrotonyl-CoA and hence an associated abnormal organic aciduria. Defects of the apocarboxylase affecting either active site could produce an isolated 3-methylcrotonyl-CoA carboxylase deficiency. Deficient activity of holocarboxylase synthetase would lead to multiple carboxylase deficiency and a similar disorder would be expected if a defect occurred in biotin uptake by the cell or transport into the mitochondria. It would be surprising, however, if the patients with the latter disorders would be responsive in vivo to biotin therapy and the molecular basis for the response in other cases and hence of the exact nature of the underlying primary defects remains to be elucidated by further study. 

---