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Abstract:
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Arginine decarboxylase is a key enzyme involved in the polyamine pathway of organisms . Pyruvoyl -dependent arginine decarboxylases are expressed in the form of proenzymes that self -cleave to form N -terminal [beta] and C -terminal [alpha] subunits generating an active pyruvoyl group at the [alpha] terminus . We have identified an archaeal homolog of a pyruvoyl -dependent arginine decarboxylase in Chlamydophila pneumoniae that could play a role in the persistence of the organism in the host . The recombinant enzyme showed highest activity at pH 3 .4 , which is the lowest optimum pH ever reported for a pyruvoyl dependent arginine decarboxylase . The proton -consuming decarboxylation raises intracellular pH , and thereby plays a role in acid -resistance . It could inhibit the pro -inflammatory nitric oxide synthase resulting in asymptomatic infection . A variant protein Thr⁵²Ser at the predicted cleavage site showed less pro -enzyme cleavage and activity compared to the wild -type . The homologs of arginine decarboxylase and flanking arginine -agmatine antiporter were also found in different biovariants of Chlamydia trachomatis . In the invasive L2 strain of C . trachomatis , the presence of a nonsense codon in the gene encoding arginine decarboxylase enzyme prevented the expression of an active enzyme . The variant protein with tryptophan replacing nonsense codon restored arginine decarboxylase activity . The non -invasive D strain of C . trachomatis had an intact arginine decarboxylase gene , but it was recombinantly expressed as a proenzyme that was uncleaved . The arginine -agmatine antiporters from both the strains were active and transported tritiated arginine into their cells . The polyamine pathway of the crenarchaeon Sulfolobus solfataricus uses arginine to make putrescine , but the organism lacks homologs of arginine decarboxylase . However , it has two paralogs of pyruvoyl dependent S -adenosylmethionine decarboxylase − SSO0536 and SSO0585 . These enzymes were recombinantly expressed as pro -enzymes that self -cleaved into [beta] and [alpha] subunits . Even with a 47 % amino acid sequence identity , the SSO0536 protein exhibited significant arginine decarboxylase activity whereas SSO0585 protein had significant S -adenosylmethionine decarboxylase activity . This is the first report of an S -adenosylmethionine decarboxylase enzyme showing alternative decarboxylase activity . The chimeric protein with the [alpha] -subunit of SSO0585 and [beta] -subunit of SSO0536 had arginine decarboxylase activity , suggesting that the residues responsible for substrate recognition are located in the amino terminus . |