On the mechanism of sodium ion translocation by methylmalonyl‐CoA decarboxylase from Veillonella alcalescens

Veillonella alcalescens during lactate degradation developed an Na + concentration gradient with 7–8 times higher external than internal Na + concentrations in the logarithmic growth phase. The gradient declined to a factor of 1.9 in the late stationary phase. Methylmalonyl‐CoA decarboxylase reconstituted into proteoliposomes performed an active electrogenic Na + transport, creating Ψ of 60 mV, pNa + of 50 mV, and of 110 mV. In the initial phase of the transport, the decarboxylase catalyzed the uptake of 2 Na + ions/malonyl‐CoA molecule decarboxylated. During further development of the electrochemical Na + gradient, this ratio gradually declined to zero, when decarboxylation continued without further increase of the internal Na + concentration. The rate of malonyl‐CoA decarboxylation declined initially during development of the membrane potential, but remained unchanged later on. Monensin abolished the Na + gradient and increased the malonyl‐CoA decarboxylation rate 2.8‐fold. On dissipating the membrane potential with valinomycin, the internal Na + concentration reached three times higher values than in its absence, and the decarboxylation rate increased 2.8‐fold. Methylmalonyl‐CoA decarboxylase catalyzed an exchange of internal and external Na + ions in addition to net Na + accumulation. The initial rate of Na + influx was double that of malonyl‐CoA decarboxylation. In the following, both rates decreased about twofold in parallel to values which remained constant during further development of the electrochemical Na + gradient. Thus, Na + influx and malonyl‐CoA decarboxylation follow a stoichiometry of approximately 2:1, independent of the magnitude of the electrochemical Na + gradient and are thus highly coupled events.