The rhizobia are gram-negative bacteria of the genera Rhizobium, Bradyrhizobium, Mesorhizobium, Sinorhizobium and Azorhizobium and are able to fix atmospheric nitrogen in symbiotic association with a compatible plant host. Carbon dioxide is essential for the growth of rhizobia (Lowe, Evans 1962), and the fact that biotin (vitamin H) participates in many CO2 fixing reactions explains why this vitamin has such a profound effect on the growth and metabolism of these organisms (West, Wilson 1940; Allen, Allen 1950; Encarnación et al. 1995; Streit, Phillips 1996).

Biotin performs an essential metabolic role in all organisms as the prosthetic group of the biotin-dependent carboxylases (BDCs), in which it functions in the activation and transfer of CO2. The generalized two-step reaction catalyzed by BDCs is:

(i) ENZYME-biotin + HC03" + Mg-ATP -> ENZYME-biotin-C02" + ADP + P¡ + H+

(ii) ENZYME-biotin-C02" + acceptor-H -> ENZYME-biotin + acceptor-C02"

BDCs contain three functional regions which participate in the carboxylation reaction. These functional regions are designated the biotin carboxylase (BC), biotin carboxyl carrier protein (BCCP) and carboxyltransferase (CT) domains or subunits. In some BDCs all of these functional regions are present as domains on a single protein, while in others they are distributed between two or more subunits of the enzyme.

The biotin prosthetic group is attached by the enzyme biotin protein ligase to a specific lysine residue in the BCCP. In the first-half reaction shown above, the BC region of the enzyme catalyzes the ATP-dependent carboxylation of the biotin attached to the BCCP. In the second step, the activated CO2 is transferred from the biotin to the acceptor substrate in a reaction catalyzed by the CT region.

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