Vitamin B7 (biotin) biosynthesis pathway (Zea mays)

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Biotin-dependent enzymes catalyze carboxylation, decarboxylation, or transcarboxylation reactions in the synthesis of fatty acids, isoleucine, and valine, and in gluconeogenesis. In plants, five biotin-containing proteins have been characterized so far, only four of them catalysts: two structurally distinct acetyl-CoA carboxylases, 3-methylcrotonyl-CoA carboxylase and geranoyl-CoA carboxylase. The fifth is a noncatalytic biotin protein that accumulates in seeds and is thought to play a role in storing biotin (Nikolau et al., 2003). Biotin is synthesized from pimeloyl-CoA in four steps, of which the first is peroxisomal; the other three steps are almost surely mitochondrial. The ligase that attaches biotin to carboxylases is present in cytosol, plastids, and mitochondria. Seed biotin-binding proteins are probably biotinylated by a special enzyme, not by the ligases that act on biotin-dependent enzymes (Job et al., 2001). Seed biotin-binding proteins may act as biotin reserves; if so, a biotinidase is required to cleave the biotin-lysine bond (Alban et al., 2000)