Structures of the catalytic site mutants D99A and H48Q and the calcium-loop mutant D49E of phospholipase A(2)

Crystal structures of the active-site mutants D99A and H48Q and the calcium -loop mutant D49E of bovine phospholipase A(2) have been determined at arou nd 1.9 Angstrom resolution. The D99A mutant is isomorphous to the orthorhom bic recombinant enzyme, space group P2(1)2(1)2(1), The H48Q and the calcium -loop mutant D49E are isomorphous to the trigonal recombinant enzyme, space group P3(1)21, The two active-site mutants show no major structural pertur bations. The structural water is absent in D99A and, therefore, the hydroge n-bonding scheme is changed. In H48Q, the catalytic water is present and hy drogen bonded to Gln48 N, but the second water found in native His48 is abs ent. In the calcium-loop mutant D49E, the two water molecules forming the p entagonal bipyramid around calcium are absent and only one O atom of the Gl u49 carboxylate group is coordinated to calcium, resulting in only four lig ands.