Isolation and Characterization of Biotin Carboxylase from Pea Chloroplasts

Pea (Pisum sativum L.) leaf acetyl-coenzyme A carboxylase (ACCase) exists as two structurally different forms: a major, chloroplastic, dissociable form and a minor, multifunctional enzyme form located in the leaf epidermis. The dissociable form is able to carboxylate free D-biotin as an alternate substrate in place of the natural substrate, biotin carboxyl carrier protein. Here we report the purification of the biotin carboxylase component of the chloroplastic pea leaf ACCase. The purified enzyme, free from carboxyltransferase activity, is composed of two firmly bound polypeptides, one of which (38 kD) is biotinylated. In contrast to bacterial biotin carboxylase, which retains full activity upon removal of the biotin carboxyl carrier component, attempts to dissociate the two subunits of the plant complex led to a complete loss of biotin carboxylase activity. Steady-state kinetic studies of the biotin carboxylase reaction reveal that addition of all substrates on the enzyme is sequential and that no product release is possible until all three substrates (MgATP, D-biotin, bicarbonate) are bound to the enzyme and all chemical processes at the active site are completed. In agreement with this mechanism, bicarbonate-dependent ATP hydrolysis by the enzyme is found to be strictly dependent on the presence of exogenous D-biotin in the reaction medium.