The crystal and molecular structure of a triacylglyceride lipase (EC 3.1.1.3) from the fungus Rhizomucor miehei was analyzed using X-ray single crystal diffraction data to 1.9 A resolution. The structure was refined to an R-factor of 0.169 for all available data. The details of the molecular architecture and the crystal structure of the enzyme are described. A single polypeptide chain of 269 residues is folded into a rather unusual singly wound beta-sheet domain with predominantly parallel strands, connected by a variety of hairpins, loops and helical segments. All the loops are right-handed, creating an uncommon situation in which the central sheet is asymmetric in that all the connecting fragments are located on one side of the sheet. A single N-terminal alpha-helix provides the support for the other, distal, side of the sheet. Three disulfide bonds (residues 29-268, 40-43, 235-244) stabilize the molecule. There are four cis peptide bonds, all of which precede proline residues. In all, 230 ordered water molecules have been identified; 12 of them have a distinct internal character. The catalytic center of the enzyme is made up of a constellation of three residues (His257, Asp203 and Ser144) similar in structure and function to the analogous (but not homologous) triad found in both of the known families of serine proteinases. The fourth residue in this system equivalent to Thr/Ser in proteinases), hydrogen bonded to Asp, is Tyr260. The catalytic site is concealed under a short amphipatic helix (residues 85 to 91), which acts as "lid", opening the active site when the enzyme is adsorbed at the oil-water interface. In the native enzyme the "lid" is held in place by hydrophobic interactions.