Acetyl-CoA carboxylase catalyzes the committed step in
fatty acid synthesis in all plants, animals, and bacteria. The Escherichia coli form is a
multifunctional enzyme consisting of three separate
proteins:
biotin carboxylase,
carboxyltransferase, and the
biotin carboxyl carrier protein. The
biotin carboxylase component, which catalyzes the
ATP-dependent carboxylation of
biotin using
bicarbonate as the carboxylate source, has a homologous functionally identical subunit in the mammalian
biotin-dependent
enzymes propionyl-CoA carboxylase and 3-methylcrotonyl-CoA carboxylase. In humans, mutations in either of these
enzymes result in the metabolic deficiency
propionic acidemia or methylcrotonylglycinuria. The lack of a system for structure-function studies of these two
biotin-dependent carboxylases has prevented a detailed analysis of the disease-causing mutations. However, structural data are available for E. coli
biotin carboxylase as is a system for its overexpression and purification. Thus, we have constructed three site-directed mutants of
biotin carboxylase that are homologous to three missense mutations found in
propionic acidemia or methylcrotonylglycinuria patients. The mutants M169K, R338Q, and R338S of E. coli
biotin carboxylase were selected for study to mimic the disease-causing mutations M204K and R374Q of
propionyl-CoA carboxylase and R385S of 3-methylcrotonyl-CoA carboxylase. These three mutants were subjected to a rigorous kinetic analysis to determine the function of the residues in the catalytic mechanism of
biotin carboxylase as well as to establish a molecular basis for the two diseases. The results of the kinetic studies have revealed the first evidence for negative cooperativity with respect to
bicarbonate and suggest that Arg-338 serves to orient the
carboxyphosphate intermediate for optimal carboxylation of
biotin.