Fermenting bacteria and archaea developed specific enzyme complexes for energy conservation that completely differ from those of the respiratory chain. Glutaconyl-CoA decarboxylase (Gcd), a family member of Na+-translocating biotin-containing decarboxylases couples the exergonic decarboxylation of glutaconyl-CoA to crotonyl-CoA with the formation of an electrochemical Na+-gradient across the membrane. According to the current mechanistic proposal the carboxylate of glutaconyl-CoA is first transferred onto biotin implicating a large conformational change. Subsequently, the decarboxylation of carboxybiotin drives Na+translocation, which was demonstrated for all Na+-translocating biotin-containing decarboxylases using inverted membrane vesicles or purified enzymes after incorporation into artificial liposomes.
The oligomeric composition of the Gcd complex and therefore also the molecular mass are unknown. For molecular and low-resolution structural studies we initiated native gel, multi-angle light scattering, Laser-induced liquid bead ion desorption mass spectrometric and preliminary electron microscopic experiments.