The structure of thermolysin: An electron density map at 2.3 Å resolution

Abstract

An electron density map at 2.3 Å resolution has been determined for the thermostable protease thermolysin. Crystallographic details are given for the three isomorphous heavy-atom derivatives which were used in the structure determination. The course of the polypeptide chain could be followed through the electron density map without reference to the chemically determined amino-acid sequence and 53% of the amino acids identified correctly. The electron density map, when combined with the amino-acid sequence determined by Titani, Hermodson, Ericsson, Walsh & Neurath (1972), revealed the conformation of the thermolysin molecule in considerable detail. The molecule is folded into two distinct lobes, with β-structure predominating in one half and helices in the other. The essential zinc atom lies between the two lobes in a deep cleft and has as ligands two histidines and a glutamic acid as in carboxypeptidase A. Some other elements of the active site are similar to that of carboxypeptidase A, but there are also some striking differences. The over-all structures of thermolysin and carboxypeptidase A are quite different. It is shown that thermolysin binds four calcium ions, which may be replaced to varying degrees by ions of strontium, barium and the rare earth metals.