Catalysis of Secretory Phospholipases

Catalysis by SPLA2S can be conceptually divided into three phases general base-mediated attack on productively bound substrate, forma- 

In the TSA complexes, the sn-1 and sn-2 alkyl substituents lie roughly parallel in hydrophobic channels that extend 11-14 A from the catalytic sites to openings just above the conserved a helices (Figs. 2 and 4). The left wall of the channel is formed by a potentially mobile hydrophobic flap that is only secured firmly once the substrate is productively bound. In 

Ndl of His-48, and the remaining two will coordinate the primary calcium ion (one equato-rially and one axially). The proposed role of a supplemental electrophile is shown in A and B. The catalytic mechanism of class III enzymes is thought to be identical despite the absence of a structural equivalent of Tyr-73 (copyrighted 1990 by the American Association for the Advancement of Science). 

Several uninhibited SPLA2S, as well as the inhibited class III bee venom 

The proposed mechanism of sPLAg hydrolysis is similar to that reported for the serine proteases (Hunkapillar et ai, 1973 Carter and Wells, 1988) with three major exceptions. The first difference is that SPLA2S lack an acyl enzyme intermediate, because a conserved water molecule hydrogen bonded to the catalytic histidine serves as the source of the nucleophile. In the serine proteases, the hydroxyl of the active site serine is deprotonated to create a nucleophile in the acylation step. Virtually any dispersed potential nucleophile (e.g., water, alcohol, hy-droxylamine) can then subsequently attack the carbonyl of the acyl enzyme during the deacylation step. 

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