Catalytic domain, repeat 2, of vertebrate phospholipases, PLD1 and PLD2, and similar proteins
Catalytic domain, repeat 2, of phospholipase D (PLD, EC 3.1.4.4) found in yeast, plants, and vertebrates, and their bacterial homologs. PLDs are involved in signal transduction, vesicle formation, protein transport, and mitosis by participating in phospholipid metabolism. They hydrolyze the terminal phosphodiester bond of phospholipids resulting in the formation of phosphatidic acid and alcohols. Phosphatidic acid is an essential compound involved in signal transduction. PLDs also catalyze the transphosphatidylation of phospholipids to acceptor alcohols, by which various phospholipids can be synthesized. Both prokaryotic and eukaryotic PLDs have two HKD motifs (H-x-K-x(4)-D, where x represents any amino acid residue) that characterizes the phospholipase D (PLD) superfamily. PLDs are active as bi-lobed monomers. Each monomer contains two domains, each of which carries one copy of the HKD motif. Two HKD motifs from two domains form a single active site. PLDs 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.
Comment:Based on similarity with Streptomyces sp. phospholipase D, which functions as a bi-lobed monomer with two catalytic domains. Each domain carries one copy of the conserved HKD motif and two domains form a single active site.
Comment:The HKD signature motif (expanded to H-x-K-x(4)-D-x(6)-G-S-x-N, where x represents any amino acid residue) characterizes the PLD superfamily.
Comment:Most residues in the HKD motif are part of the active site.
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