Sodium ion-translocating decarboxylases

Abstract

The review is concerned with three Na +-dependent biotin-containing decarboxylases, which catalyse the substitution of CO 2 by H + with retention of configuration (Δ G°′=−30 kJ/mol): oxaloacetate decarboxylase from enterobacteria, methylmalonyl-CoA decarboxylase from Veillonella parvula and Propiogenium modestum, and glutaconyl-CoA decarboxylase from Acidaminococcus fermentans. The enzymes represent complexes of four functional domains or subunits, a carboxytransferase, a mobile alanine- and proline-rich biotin carrier, a 9–11 membrane-spanning helix-containing Na +-dependent carboxybiotin decarboxylase and a membrane anchor. In the first catalytic step the carboxyl group of the substrate is converted to a kinetically activated carboxylate in N-carboxybiotin. After swing-over to the decarboxylase, an electrochemical Na + gradient is generated; the free energy of the decarboxylation is used to translocate 1–2 Na + from the inside to the outside, whereas the proton comes from the outside. At high [Na +], however, the decarboxylases appear to catalyse a mere Na +/Na + exchange. This finding has implications for the life of P. modestum in sea water, which relies on the synthesis of ATP via ΔμNa + generated by decarboxylation. In many sequenced genomes from Bacteria and Archaea homologues of the carboxybiotin decarboxylase from A. fermentans with up to 80% sequence identity have been detected.