Catalytic domain, repeat 2, of phosphatidylserine synthases from gram-negative bacteria
Catalytic domain, repeat 2, of phosphatidylserine synthases (PSSs) from gram-negative bacteria. There are two subclasses of PSS enzymes in bacteria: subclass I of gram-negative bacteria and subclass II of gram-positive bacteria. It is common that PSSs in gram-positive bacteria and yeast are tight membrane-associated enzymes. By contrast, the gram-negative bacterial PSSs, such as Escherichia coli PSS, are commonly bound to the ribosomes. They are peripheral membrane proteins that can interact with the surface of the inner membrane by binding to the lipid substrate (CDP-diacylglycerol) and the lipid product (phosphatidylserine). The prototypical member of this subfamily is Escherichia coli PSS (also called CDP-diacylglycerol-L-serine O-phosphatidyltransferase, EC 2.7.8.8), which catalyzes the exchange reactions between CMP and CDP-diacylglycerol, and between serine and phosphatidylserine. The phosphatidylserine is then decarboxylated by phosphatidylserine decarboxylase to yield phosphatidylethanolamine, the major phospholipid in Escherichia coli. It also catalyzes the hydrolysis of CDP-diacylglycerol to form phosphatidic acid with the release of CMP. PSS may utilize a ping-pong mechanism involving a phosphatidyl-enzyme intermediate, which is distinct from those of gram-positive bacterial phosphatidylserine synthases. Moreover, all members in this subfamily have two HKD motifs (H-x-K-x(4)-D, where x represents any amino acid residue) that characterizes the phospholipase D (PLD) superfamily. The two motifs constitute an active site for the formation of a covalent substrate-enzyme intermediate.
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