CO 2 -Fixing Enzymes in a Marine Psychrophile

Abstract

A psychrophilic marine Pseudomonas was found to contain phosphoenolpyruvate (PEP) carboxylase and an adenosine triphosphate-linked PEP carboxykinase. Some properties of these CO 2 -fixing enzymes were compared with those homologous enzymes from the terrestrial mesophile Enterobacter cloacae . The PEP carboxylases from both organisms were activated by acetyl-coenzyme A (CoA) and inhibited by l -aspartate. The enzyme from Pseudomonas was less dependent on the presence of the activator, but maximal activation was attained at acetyl-CoA concentrations much lower (50 μ m ) than those required to saturate the enzyme from E. cloacae . In both cases the main effect of acetyl-CoA was to decrease the K m value for PEP. The activity of PEP carboxylase from Pseudomonas was only slightly inhibited by NaCl, KCl, or NH 4 Cl up to 100 m m , whereas the enzyme from E. cloacae was inhibited by about 70% under similar experimental conditions. Both PEP carboxylase and PEP carboxykinase from Pseudomonas showed considerably lower thermal stability than their counterparts from E. cloacae . Our results suggest that the CO 2 -fixing enzymes from a marine Pseudomonas and E. cloacae are similar in nature and regulation, but they differ in properties related to the peculiar conditions of the marine environment.