Function of Escherichia coli biotin carboxylase requires catalytic activity of both subunits of the homodimer

Biotin carboxylase catalyzes the ATP-dependent carboxylation of biotin and is one component of the multienzyme. complex acetyl-CoA carboxylase that catalyzes the first committed step in fatty acid synthesis. The Escherichia coli biotin carboxylase is readily isolated from the other components of the acetyl-CoA carboxylase complex such that enzymatic activity is retained. The three-dimensional structure of biotin carboxylase, determined by x-ray crystallography, demonstrated that the enzyme is a homodimer consisting of two active sites in which each subunit contains a complete active site. To understand how each subunit contributes to the overall function of biotin carboxylase, we made hybrid molecules in which one subunit had a wild-type active site, and the other subunit contained an active site mutation known to significantly affect the activity of the enzyme. One of the two genes encoded a poly-histidine tag at its N terminus, whereas the other gene had an N-terminal FLAG epitope tag. The two genes were assembled into a mini-operon that was induced to give high level expression of both enzymes. "Hybrid" dimers composed of one subunit with a wild-type active site and a second subunit having a mutant, active site were obtained by sequential chromatographic steps on columns of immobilized nickel chelate and anti-FLAG affinity matrices. In vitro kinetic studies of biotin carboxylase dimers in which both subunits were wild type revealed that the presence of the N-terminal tags did not alter the activity of the enzyme. However, kinetic assays of hy brid dimer biotin carboxylase molecules in which one subunit had an active site mutation (R292A, N290A, K238Q or E288K) and the other subunit had a wild-type active site resulted in 39-, 28-, 94-, and 285-fold decreases in the activity of these enzymes, respectively. The dominant negative effects of these mutant subunits were also detected in vivo by monitoring the rate of fatty acid biosynthesis by [C-14]acetate labeling of cellular lipids. Expression of the mutant biotin carboxylase genes from an inducible arabinose promoter resulted in a significantly reduced rate of fatty acid synthesis relative to the same strain that expressed the wild type gene. Thus, both the in vitro and in vivo data indicate that both subunits of biotin carboxylase are required for activity and that the two subunits must be in communication during enzyme function.