Purification, characterisation and reconstitution of glutaconyl‐CoA decarboxylase, a biotin‐dependent sodium pump from anaerobic bacteria

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Abstract
Glutaconyl‐CoA decarboxylase from Acidaminococcus fermentans is a biotin enzyme, which is integrated into membranes. It is activated by Triton X‐100 and inhibited by avidin. The results obtained by a combination of both agents indicate that biotin and the substrate‐binding site are located on the same side of the membrane. The decarboxylase was solubilized with Triton X‐100 and purified by affinity chromatography on monomeric avidin‐Sepharose. The enzyme is composed of three types of polypeptides: the group of α chains (Mr 120000–140000) containing the biotin, the β chain (60000) and an apparently hydrophobic γ chain (35000). Sodium ions specifically protected the latter chain from tryptic digestion. It was supposed, therefore, that this chain might function as the Na+ channel. The β and γ chains but not the α chain could be labelled by N‐ethyl‐[14C]maleimide. Similar decarboxylases but with much smaller biotin peptides (Mr 15000–20000) were isolated from Peptococcus aerogenes and Clostridium symbiosum. The decarboxylases from all three organisms could be reconstituted to active sodium pumps by incubation with phospholipid vesicles and octylglucoside followed by dilution. The Na+ uptake catalysed by the enzyme from A. fermentans was completely inhibited by monensin and activated twofold by valinomycin/K+ indicating an electrogenic Na+ pump. The coupling between Na+ transport and decarboxylation was not tight. During the reaction the ratio decreased from initially 1 to 0.2. The three organisms mentioned above and Clostridium tetanomorphum without glutaconyl‐CoA decarboxylase are able to ferment glutamate and require 10 mM Na+ for rapid growth. There is no correlation between the concentration of monensin necessary to inhibit growth and the presence of decarboxylase in these organisms.