Influence of size and polarity of residue 31 in porcine pancreatic phospholipase A2 on catalytic properties

Residue 31 of porcine pancreatic phospholipase A2 (PLA2) is located at the entrance to the active site. To study the role of residue 31 in PLA2, six mutant enzymes were produced by site-directed mutagenesis, replacing Leu by either Trp, Arg, Ala, Thr, Ser or Gly. Direct binding studies indicated a three to six times greater affinity of the Trp31 PLA2 for both monomeric and micellar substrate analogs, relative to the wild-type enzyme. The other five mutants possess an unchanged affinity for monomers of the product analog n-decylphosphocholine and for micelles of the diacyl substrate analog rac-1,2-dioctanoylamino-dideoxy-glycero-3-phosphocholine. The affinities for micelles of the monoacyl product analog n-hexadecylphosphocholine were decreased 9-20 times for these five mutants. Kinetic studies with monomeric substrates showed that the mutants have Vmax values which range between 15 and 70% relative to the wild-type enzyme. The Vmax values for micelles of the zwitterionic substrate 1,2-dioctanoyl-sn-glycero-3-phosphocholine were lowered 3-50 times. The Km values for the monomeric substrate and the Km values for the micellar substrate were hardly affected in the case of five of the six mutants, but were considerably decreased when Trp was present at position 31. The results of these investigations point to a versatile role for the residue at position 31: involvement in the binding and orientating of monomeric substrate (analogs), involvement in the binding of the enzyme to micellar substrate analogs and possibly involvement in shielding the active site from excess water