title: Three-dimensional structure of the biotin carboxylase subunit of acetyl-CoA carboxylase.
abstract: Acetyl-CoA carboxylase is found in all animals, plants, and bacteria and catalyzes the first committed step in fatty acid synthesis. It is a multicomponent enzyme containing a biotin carboxylase activity, a biotin carboxyl carrier protein, and a carboxyltransferase functionality. Here we report the X-ray structure of the biotin carboxylase component from Escherichia coli determined to 2.4-A resolution. The structure 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 binding motif with five strands of parallel beta-sheet flanked on either side by alpha-helices. The "B-domain" extends from the main body of the subunit where it folds into two alpha-helical regions and three strands of beta-sheet. Following the excursion into the B-domain, the polypeptide chain folds back into the body of the protein where it forms an eight-stranded antiparallel beta-sheet. In addition to this major secondary structural element, the C-terminal domain also contains a smaller three-stranded antiparallel beta-sheet and seven alpha-helices. The active site of the enzyme has been identified tentatively by a difference Fourier map calculated between X-ray data from the native crystals and from crystals soaked in a Ag+/biotin complex. Those amino acid residues believed to form part of the active site pocket include His 209-Glu 211, His 236-Glu 241, Glu 276, Ile 287-Glu 296, and Arg 338.2+ represents the first X-ray model of a biotin-dependent carboxylase.
authors.affiliation: Institute for Enzyme Research, Graduate School, University of Wisconsin, Madison 53705.
authors.forename: G L
authors.initials: GL
authors.lastname: Waldrop
authors.forename: I
authors.initials: I
authors.lastname: Rayment
authors.forename: H M
authors.initials: HM
authors.lastname: Holden
mesh_terms: D000103:Acetyl-CoA Carboxylase / Q000737:chemistry*; D000596:Amino Acids / Q000737:chemistry; D001665:Binding Sites; D019731:Carbon-Nitrogen Ligases; D018360:Crystallography, X-Ray; D004926:Escherichia coli / Q000201:enzymology; D008025:Ligases / Q000737:chemistry*; D008958:Models, Molecular; D011487:Protein Conformation; D017433:Protein Structure, Secondary
chemical_list: D000596:Amino Acids; D008025:Ligases; D019731:Carbon-Nitrogen Ligases; C019791:biotin carboxylase; D000103:Acetyl-CoA Carboxylase