Bacterial sodium ion-coupled energetics.
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 of Klebsiella pneumoniae and the Na(+)-translocating F1Fo ATPase of Propionigenium 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 beta-subunits including two conserved aspartates that may be important for Na+ translocation. The coupling ratio of the decarboxylase Na+ pumps depended on delta  muNa+ and decreased from two to zero Na+ uptake per decarboxylation event as delta  mu Na+ increased from zero to the steady state level. In P. modestum, delta  mu Na+ is generated in the course of succinate fermentation to propionate and CO2. This delta  mu Na+ is used by a unique Na(+)-translocating F1Fo ATPase for ATP synthesis. The enzyme is related to H(+)-translocating F1Fo ATPases. The Fo part is entirely responsible for the coupling of ion specificity. A hybrid ATPase formed by in vivo complementation of an Escherichia coli deletion mutant was completely functional as a Na(+)-ATP synthase conferring the E. coli strain the ability of Na(+)-dependent growth on succinate. The hybrid consisted of subunits a, c, b, delta and part of alpha from P. modestum and of the remaining subunits from E. coli. Studies on Na+ translocation through the Fo part of the P. 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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Mikrobiologisches Institut, Eidgenössische Technische Hochschule, ETH-Zentrum, Zürich, Switzerland.
D000255:Adenosine Triphosphate / Q000096:biosynthesis; D000595:Amino Acid Sequence; D001419:Bacteria / Q000235:genetics / Q000378:metabolism*; D001421:Bacteria, Anaerobic / Q000378:metabolism; D001665:Binding Sites; D002262:Carboxy-Lyases / Q000235:genetics / Q000378:metabolism; D002951:Citrates / Q000378:metabolism; D019343:Citric Acid; D004734:Energy Metabolism; D005285:Fermentation; D007711:Klebsiella pneumoniae / Q000378:metabolism; D043586:Methylmalonyl-CoA Decarboxylase; D008954:Models, Biological; D008969:Molecular Sequence Data; D006180:Proton-Translocating ATPases / Q000378:metabolism; D012964:Sodium / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism
D002951:Citrates; D019343:Citric Acid; D000255:Adenosine Triphosphate; D012964:Sodium; D006180:Proton-Translocating ATPases; D002262:Carboxy-Lyases; C021916:oxaloacetate decarboxylase; D000254:Sodium-Potassium-Exchanging ATPase; D043586:Methylmalonyl-CoA Decarboxylase