3-DIMENSIONAL STRUCTURE OF THE BIOTIN CARBOXYLASE SUBUNIT OF ACETYL-COA CARBOXYLASE

Acetyl-CoA carboxylase is found in all animals, plants, and bacteria a nd catalyzes the first committed step in fatty acid synthesis. It is a multicomponent enzyme containing a biotin carboxylase activity, a bio tin carboxyl carrier protein, and a carboxyltransferase functionality. Here we report the X-ray structure of the biotin carboxylase componen t from Escherichia coil determined to 2.4-Angstrom resolution. The str ucture was solved by a combination of multiple isomorphous replacement and electron density modification procedures. The overall fold of the molecule may be described in terms of three structural domains. The N -terminal region, formed by Met 1-Ile 103, adopts a dinucleotide bindi ng motif with five strands of parallel beta-sheet flanked on either si de by alpha-helices. The ''B-domain'' extends from the main body of th e subunit where it folds into two alpha-helical regions and three stra nds of beta-sheet. Following the excursion into the B-domain, the poly peptide chain folds back into the body of the protein where it forms a n eight-stranded antiparallel beta-sheet. In addition to this major se condary structural element, the C-terminal domain also contains a smal ler three-stranded antiparallel beta-sheet and seven alpha-helices. Th e active site of the enzyme has been identified tentatively by a diffe rence Fourier map calculated between X-ray data from the native crysta ls and from crystals soaked in a Ag+/biotin complex. Those amino acid residues believed to form part of the active site pocket include His 2 09-Glu 211, His 236-Glu 241, Glu 276, Ile 287-Glu 296, and Arg 338. Th e structure presented here represents the first X-ray model of a bioti n-dependent carboxylase.