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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 around 1.9 A Ê resolution. The D99A mutant is isomorphous to the orthorhombic recombinant enzyme, space group P2 1 2 1 2 1 . The H48Q and the calciumloop mutant D49E are isomorphous to the trigonal recombinant enzyme, space group P3 1 21. The two active-site mutants show no major structural perturbations. The structural water is absent in D99A and, therefore, the hydrogen-bonding scheme is changed. In H48Q, the catalytic water is present and hydrogen bonded to Gln48 N, but the second water found in native His48 is absent. In the calcium-loop mutant D49E, the two water molecules forming the pentagonal bipyramid around calcium are absent and only one O atom of the Glu49 carboxylate group is coordinated to calcium, resulting in only four ligands.