Catalytic domain, repeat 1, of plant phospholipase D and similar proteins
Catalytic domain, repeat 1, of plant phospholipase D (PLD, EC 3.1.4.4) and similar proteins. Plant PLDs have broad substrate specificity and can hydrolyze the terminal phosphodiester bond of several common membrane phospholipids such as phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylglycerol (PG), and phosphatidylserine (PS), with 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. Most plant PLDs possess a regulatory calcium-dependent phospholipid-binding C2 domain in the N-terminus and require calcium for activity, which is unique to plant PLDs and is not present in animal or fungal PLDs. Like other PLD enzymes, the monomer of plant 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. Plant PLDs 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. This subfamily includes two types of plant PLDs, alpha-type and beta-type PLDs, which are derived from different gene products and distinctly regulated. The zeta-type PLD from Arabidopsis is not included in this subfamily.
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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