Early interest in biotin involved the so-called egg white injury factor. When it was confirmed that egg white injury was caused by a deficiency and not a toxicity, pursuit of the missing substance led eventually to the discovery of biotin. Research on the vitamin brought a new concept to nutrition, that of the 'antivitamin' or substances capable of negating the action of vitamins before their use as cofac-tors. In the case of biotin, the 'antivitamin' turned out to be the protein avidin, which bound biotin tenaciously and limited its intestinal absorption.
Reactivity Biotin can be thought of as another one-carbon cofactor, but for biotin this is CO2. Thus, biotin-requiring enzymes catalyze carboxyla-tion, decarboxylation, or transcarboxylation reactions. The active form of biotin is 'biocytin' (e-N-biotinyl L-lysine), which is formed by the covalent attachment of the biotin side chain to the e-amino group of a lysine residue on the apoen-zyme as catalyzed by a specific synthetase (Figure 5B). The condensation requires ATP and proceeds via a biotinyl-AMP intermediate with the apoenzyme catalyzing formation of the amide bond. The resulting unique structure combines the aliphatic chains or biotin and lysine permitting the ring structure of biotin to extend about 14 A from the enzyme's surface (Figure 5B).
The active site on the biotinyl group is one of the N in the 5-member ring (Figure 5C). An N-carboxyl derivative serves as a donor of CO2 to an appropriate substrate acceptor. The reaction occurs in two steps and requires an ATP-dependent formation of a carboxy biotinyl enzyme. If the enzyme is a carbox-ylase, there are two main substrate types: (1) acyl-CoA derivatives, which include acetyl-CoA,
Peptide chain I
Figure 5 Biotin and its coenzyme. (A) Biotin as a vitamin, (B) biotin attached to the e-amino group of lysine to form the coenzyme biocytin, and (C) the carboxy derivative of biotin prepared to donate CO2 to a substrate.
propionyl CoA; and (2) simple a-keto acids such as pyruvate. Each substrate must contain a carbonyl group adjacent to or conjugated with the carbon receiving the carboxyl group from carboxy biocytin. Perhaps the most familiar biotin carboxylase enzymes in mammalian systems are acetyl-CoA carboxylase in fatty acid biosynthesis, propionyl CoA carboxylase in odd-chain fatty acid catabolism, pyruvate carboxylase in gluconeogenesis, and ¡-methyl-crotonyl CoA carboxylase in leucine catabolism.
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