The crystal and molecular structure of proteinase K was determined by X-ray diffraction data to 0.15-nm resolution. The enzyme belongs to the subtilisin family with an active-site catalytic triad Asp39--His69--Ser224 but is a representative of a subgroup with a free Cys73 close to and 'below' the active His69. Besides this Cys72, proteinase K has two disulfide bonds, Cys34--Cys123 and Cys178--Cys249, which contribute to the stability of the tertiary structure consisting of an extended central parallel beta-sheet decorated by six alpha-helices, three short antiparallel beta-sheets, 18 beta-turns and involving several internal, structurally important water molecules. Proteinase K exhibits two Ca2+-binding sites, one very strong and the other weak, which were the sites of the heavy atoms (Pb2+, Sm3+) used to solve the crystal structure. The weak binding site is liganded to the N and C termini, Thr16 and Asp260, and is only incompletely coordinated by oxygen ligands. The strong binding site is coordinated in the form of a pentagonal bipyramid with the side chain carboxylate of Asp200 and the C = O of Pro175 as apex, and C = O of Val177 and four water molecules in the equatorial plane. Upon removal of this Ca2+, proteinase K loses activity which is interpreted in terms of a local structural deformation involving the substrate-recognition site (Ser132--Gly136), probably associated with a cis----trans isomerization of cis Pro171. Several water molecules are located in the active site. One, W335, is positioned in the 'oxyanion hole' and is displaced by the C = O of the scissile peptide bond of the substrate, as indicated by crystallographic studies with peptide chloromethane inhibitors. Based on these experiments, a reaction mechanism is proposed where the peptide substrate forms a three-stranded antiparallel pleated sheet with the recognition site of proteinase K consisting of Ser132--Leu133--Gly134 on one side and Gly100--Ser101 on the other, followed by expulsion of the oxyanion hole water W335 and hydrolytic cleavage by the Asp39--His69--Serr224 triad. These latter residues display low thermal motion corresponding to well-defined geometry and are hardly accessible to solvent molecules, whereas the recognition-site amino acids are more flexible and partially exposed to solvent.