Catalytic domain, repeat 2, of vertebrate phospholipases, PLD1 and PLD2, yeast PLDs, and similar proteins
Catalytic domain, repeat 2, of vertebrate phospholipases D (PLD1 and PLD2), yeast phospholipase D (PLD SPO14/PLD1), and other similar eukaryotic proteins. These PLD enzymes 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. The vertebrate PLD1 and PLD2 are membrane associated phosphatidylinositol 4,5-bisphosphate (PIP2)-dependent enzymes that selectively hydrolyze phosphatidylcholine (PC). Protein cofactors and calcium may be required for their activation. Yeast SPO14/PLD1 is a calcium-independent PLD, which needs PIP2 for its activity. Instead of the regulatory calcium-dependent phospholipid-binding C2 domain in plants, most mammalian and yeast PLDs have adjacent Phox (PX) and the Pleckstrin homology (PH) domains at the N-terminus, which have been shown to mediate membrane targeting of the protein and are closely linked to polyphosphoinositide signaling. The PX and PH domains are also present in zeta-type PLD from Arabidopsis, which is more closely related to vertebrate PLDs than to other plant PLD types. In addition, this subfamily also includes some related proteins which have either PX-like or PH domains in their N-termini. Like other members of the PLD superfamily, the monomer of mammalian and yeast PLDs consists of two catalytic domains, each containing one copy of the conserved HKD motif (H-x-K-x(4)-D, where x represents any amino acid residue). Two HKD motifs from the 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.
Jiyao W et al.(2023). "The conserved domain database in 2023.",