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

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Abstract
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.