Dr. Toni Petan
of Molecular and Biomedical Sciences
sPLA2s are Interfacial Enzymes
sPLA2s are interfacial enzymes, meaning that have to bind to an aggregated phospholipid surface (i.e. cell membranes, liposomes) in order to gain access to their substrate molecules.
The contact with the membrane surface is enabled by the interfacial binding site (or IBS), which is a flat surface, surrounding the narrow hydrophobic pocket, extending to the active site of the enzyme.
The putative IBS residues of Ammodytoxin A, a snake venom sPLA2 (Petan et al. (2005) Biochemistry)
The membrane binding step is structurally and kinetically distinct from the subsequent catalytic reaction.
Calcium is necessary for binding of a phospholipid molecule to the active site and for the catalytic step itself, but not for interfacial binding.
It is becoming apparent, that differences in binding affinities of sPLA2s for various membrane surfaces, and not necessarily catalytic site phospholipid specificity, can lead to different physiological functions of these enzymes.