Bacterial sodium ion-coupled energetics

Abstract

For many bacteria Na⁺ bioenergetics is important as a link between exergonic and endergonic reactions in the membrane. This article focusses on two primary Na⁺ pumps in bacteria, the Na⁺-translocating oxaloacetate decarboxylase ofKlebsiella pneumoniae and the Na⁺-translocating F₁F₀ ATPase ofPropionigenium modestum. Oxaloacetate decarboxylase is an essential enzyme of the citrate fermentation pathway and has the additional function to conserve the free energy of decarboxylation by conversion into a Na⁺ gradient. Oxaloacetate decarboxylase is composed of three different subunits and the related methylmalonyl-CoA decarboxylase consists of five different subunits. The genes encoding these enzymes have been cloned and sequenced. Remarkable are large areas of complete sequence identity in the integral membrane-bound β-subunits including two conserved aspartates that may be important for Na⁺ translocation. The coupling ratio of the decarboxylase Na⁺ pumps depended on$\Delta \tilde \mu Na^ +$ and decreased from two to zero Na⁺ uptake per decarboxylation event as$\Delta \tilde \mu Na^ +$ increased from zero to the steady state level.

InP. modestum,$\Delta \tilde \mu Na^ +$ is generated in the course of succinate fermentation to propionate and CO₂. This$\Delta \tilde \mu Na^ +$ is used by a unique Na⁺-translocating F₁F₀ ATPase for ATP synthesis. The enzyme is related to H⁺-translocating F₁F₀ ATPases. The F₀ part is entirely responsible for the coupling of ion specificity. A hybrid ATPase formed by in vivo complementation of anEscherichia coli deletion mutant was completely functional as a Na⁺-ATP synthase conferring theE. coli strain the ability of Na⁺-dependent growth on succinate. The hybrid consisted of subunits a, c, b, δ and part of α fromP. modestum and of the remaining subunits fromE. coli. Studies on Na⁺ translocation through the F₀ part of theP. modestum ATPase revealed typical transporter-like properties. Sodium ions specifically protected the ATPase from the modification of glutamate-65 in subunit c by dicyclohexylcarbodiimide in a pH-dependent manner indicating that the Na⁺ binding site is at this highly conserved acidic amino acid residue of subunit c within the middle of the membrane.