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Phospholipase D (PLD) is an ubiquitous enzyme found in bacteria, fungi, plants
and mammals. PLD is involved in vesicle formation, protein transport, signal
transduction and mitosis. It catalyzes the breakdown of phosphatidylcholine
into choline and the putative second messenger phosphatidic acid, which in
turn is broken down into two second messengers (diacylglycerol and
lysophosphatidic acid). PLD also catalyzes a phosphatidyl transfer reaction
using primary alcohols as nucleophilic acceptors to produce
phosphatidylalcohols. Sequence analysis revealed that PLD belongs to a
superfamily that includes cardiolipin synthases, phosphatidylserine synthase,
poxvirus envelope proteins, a Yersinia murine toxin and several endonucleases.
All members of this superfamily of phosphodiesterases contain the sequence
motif H-x-K-x(4)-D-x(6)-G-S-x-N denoted 'HKD'. Despite the distinct substrate
specificities of the superfamily members, the consensus HKD motif appears to
be essential for their enzymatic activity. Most of the enzymes in the
superfamily contain two copies of this consensus sequence, but the bacterial
endonuclease contain only a single copy [1,2,3]. Two HKD motifs are supposed
to associate to produce a single active site. It has been proposed that upon
substrate binding, one of the histidine residues function as the nucleophile
attacking the phosphodiester bond, while the other histidine could serve as a
general acid protonating the oxygen atom of the leaving group [4,5].
The profile we developed is centered around the phospholipase D
phosphodiesterase active site.
April 2002 / First entry.
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