The talk will be in two parts. First, results based on structural and mutagenic analysis of T4 lysozyme will be reviewed. A variety of studies have shown that the protein is surprisingly tolerant of amino acid substitution, even including the insertion of additional amino acids within alpha-helices. Typically the structural changes associated with an amino acid replacement are modest and are localized to the immediate vicinity of the site of the replacement. For the same reason, it is usually not possible to propagate a large-scale conformational change within the body of a protein. Recently, however, by using specifically designed tandem substitutions it has been possible to propagate large-scale structural changes over 20-30 Angstroms.

The second part of the talk will deal with the use of flash-freezing to facilitate X-ray crystallographic structure determination. This procedure has become routine. Analysis with crystals of beta-galactosidase, however, show that flash-freezing can induce structural changes on the surface of the protein. The pros and cons of flash-freezing will be discussed.