Refined crystal structure of lysozyme from the rainbow trout (Oncorhynchus mykiss)

Lysozymes (E.C. 3.2.1.17) are well characterized ubiquitous enzymes that have an antibacterial effect. The lysozymes from rainbow trout (RBTL) (Oncorhynchus mykiss) could be particularly interesting in aquaculture since they show higher activity than egg-white lysozyme and lysozymes from other fish species against a variety of pathogenic bacteria. Two lysozymes, I and II, differing only in a single amino acid, were purified from the kidney of rainbow trout and shown to belong to the c-type class of lysozymes. The type II form was shown to be much more potent against a variety of bacteria than the type I enzyme. We have grown crystals from a mixture containing about 80% type I and 20% type II lysozyme from rainbow trout, and solved the X-ray crystal structure. The crystals are trigonal with a = 76.68, c = 54.46 A and space group P3(1)21. The phase problem was solved by the molecular-replacement method, and the structure was refined to an R-factor of 17.4% using data to 1.8 A resolution. The crystal structure shows that the three-dimensional structure of rainbow trout lysozyme is very similar to the previously solved structures of other c-type lysozymes. The single polypeptide of 129 amino acids is folded into two domains separated by a deep cleft which contains the active site. Secondary-structure elements, four alpha-helices and a three-stranded beta-sheet, are located in the same sequential positions as in the hen, turkey and human enzymes. The beta-sheet is found to be common for structures of both c- and g-type lysozymes. We suggest that differences in antibiotic activity of the two forms of RBTL are probably due to small differences in the hydophobicity of a small surface region.