Catalytic domain, repeat 2, of phospholipase D from Streptomyces Sp. Strain PMF and similar proteins
Catalytic domain, repeat 2, of phospholipases D (PLD, EC 3.1.4.4) from Streptomyces Sp. Strain PMF (PMFPLD) and similar proteins, which are generally extracellular and bear N-terminal signal sequences. PMFPLD hydrolyzes the terminal phosphodiester bond of phospholipids with the formation of phosphatidic acid and alcohols. Phosphatidic acid is an essential compound involved in signal transduction. It also catalyzes a transphosphatidylation of phospholipids to acceptor alcohols, by which various phospholipids can be synthesized. In contrast to eukaryotic PLDs, PMFPLD has a compact structure, which consists of two catalytic domains, but lacks the regulatory domains. Each catalytic domain contains one copy of the HKD motif (H-x-K-x(4)-D, where x represents any amino acid residue) that characterizes the PLD superfamily. Two HKD motifs from two domains form a single active site. Like other PLD enzymes, PMFPLD may utilize a common two-step ping-pong catalytic mechanism involving an enzyme-substrate intermediate to cleave phosphodiester bonds. The two histidine residues from the two HKD motifs play key roles in the catalysis. Upon substrate binding, a histidine residue from one HKD motif could function as the nucleophile, attacking the phosphodiester bond to create a covalent phosphohistidine intermediate, while the other histidine residue from the second HKD motif could serve as a general acid, stabilizing the leaving group. A calcium-dependent PLD from Streptomyce chromofuscus is excluded from this family, since it displays very little sequence homology with other Streptomyces PLDs. Moreover, it does not contain the conserved HKD motif and hydrolyzes the phospholipids via a different mechanism.
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