Biotin carboxylase, encoded by the accC gene, catalyzes the first step in fatty acid biosynthesis. It catalyzes the ATP-dependent transfer of carboxyl group from bicarbonate to biotin molecule of biotin carboxylase carrier protein (BCCP) . During second part of reaction transcarboxylase transfers the carboxyl group from carboxybiotinylated BCCP to acetyl-CoA to form malonyl-CoA and regenerate biotinylated BCCP. The formation of malonyl-CoA in turn controls the rate of fatty acid metabolism. Crystal structure of biotin carboxylase-ADP complex from Campylobacter jejuni was solved at 1.8 A resolution. The structure is of an alpha-beta fold and a dimer in the crystal. The structure is of an alpha-beta fold and protein molecules form dimer. Based on the structural peculiarities, the part of small domain (residues 133 - 202) swings back while interacting with adenosyl ligand to make compact patch. The binding region of biotin carboxylase is much conserved in different bacteria species. In accordance with what was found earlier in adenine-binding region Glu201-Leu204 and Ile157-Lys159 Leu204 is substituted by Ile204, ribose-binding region is formed by His209, Gln233, His236 and Ile437 is changed to Thr437, and His438 to Asn438. Phosphate groups of the ligand interact with glycine-rich group (residues 162-168) together with Lys116, Lys159 and Gln237 which interacts with the third phosphate group of the ligand.