Movement of the biotin carboxylase B-domain as a result of ATP binding

Acetyl-CoA carboxylase catalyzes the first committed step in fatty acid syn thesis. In Escherichia coli, the enzyme is composed of three distinct prote in components: biotin carboxylase, biotin carboxyl carrier protein, and car boxyltransferase. The biotin carboxylase component has served for many year s as a paradigm for mechanistic studies devoted toward understanding more c omplicated biotin-dependent carboxylases. The three-dimensional x-ray struc ture of an unliganded form of E. coli biotin carboxylase was originally sol ved in 1994 to 2.4-Angstrom resolution. This study revealed the architectur e of the enzyme and demonstrated that the protein belongs to the ATP-grasp superfamily. Here we describe the three-dimensional structure of the E, col i biotin carboxylase complexed with ATP and determined to 2.5-Angstrom reso lution. The major conformational change that occurs upon nucleotide binding is a rotation of approximately 45 degrees of one domain relative to the ot her domains thereby closing off the active site pocket. Key residues involv ed in binding the nucleotide to the protein include Lys-116, His-236, and G lu-201. The backbone amide groups of Gly-165 and Gly-166 participate in hyd rogen bonding interactions with the phosphoryl oxygens of the nucleotide. A comparison of this closed form of biotin carboxylase with carbamoyl-phosph ate synthetase is presented.