Bacterial sodium ion-coupled energetics |
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Authors: | P. Dimroth |
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Affiliation: | (1) Mikrobiologisches Institut, Eidgenössische Technische Hochschule, ETH-Zentrum, Schmelzbergstr. 7, CH-8092 Zürich, Switzerland |
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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 -subunits including two conserved aspartates that may be important for Na+ translocation. The coupling ratio of the decarboxylase Na+ pumps depended on and decreased from two to zero Na+ uptake per decarboxylation event as increased from zero to the steady state level.InP. modestum, is generated in the course of succinate fermentation to propionate and CO2. This 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, and part of 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. |
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Keywords: | citrate fermentation F1F0 ATPase ion translocation mechanism Klebsiella pneumoniae Propionigenium modestum sodium translocating decarboxylases |
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