Biotin-dependent carboxylases are widely distributed in nature and have important functions in many cellular processes. These enzymes share a conserved biotin carboxylase (BC) component, which catalyzes the carboxylation of biotin. The crystal structure of biotin carboxylase in complex with its substrate and product provides valuable insights into the mechanism of this enzyme. The structure reveals the binding mode of biotin and the conformational changes that occur upon substrate binding. This information can be used to design inhibitors of biotin carboxylase and to understand the role of this enzyme in various cellular processes.