Catalytic domain, repeat 2, of vertebrate phospholipase D1
Catalytic domain, repeat 2, of vertebrate phospholipase D1 (PLD1). PLDs play a pivotal role in transmembrane signaling and cellular regulation. 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. Vertebrate PLD1 is a membrane associated phosphatidylinositol 4,5-bisphosphate (PIP2)-dependent enzyme that selectively hydrolyzes phosphatidylcholine (PC). Protein cofactors and calcium might be required for its activation. Most vertebrate PLDs have adjacent Phox (PX) and the Pleckstrin homology (PH) domains at their N-terminus, which have been shown to mediate membrane targeting of the protein and are closely linked to polyphosphoinositide signaling. Like other members of the PLD superfamily, the monomer of vertebrate PLDs consists of two catalytic domains, each of which contains one copy of the conserved HKD motif (H-x-K-x(4)-D, where x represents any amino acid residue). Two HKD motifs from two domains form a single active site. These 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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