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As is true for so many fields of science (see for example the somewhat wistful article' by Gunther Stent on "the molecular biology that was") protein chemistry has recently undergone a large quantum jump. The dramatic progress in the methodology of sequence analysis and crystallography has introduced a new level of sophistication to the field and, except for very large protein molecules, particularly those containing many subunits, the direct approach of x-ray structure analysis combined with elucidation of covalent structure may soon bypass many of the more prosaic problems of protein chemistry. However, the question of the comparative structures of proteins in solution and in the crystalline state is not yet resolved, and the detailed study of the dynamic aspects of catalysis and conformation will undoubtedly require the more classical approaches for years to come. An extracellular nuclease, produced by cultures of Staphylococcus aureus, has proved to be an extremely convenient and cooperative test object in our experimental approach to some of these questions. It has lent itself to a broad study of conformational stability and the mechanism of catalytic action, as well as crystallographic and organic synthetic investigations. Since I would like to spend a fair share of my time on some recent studies directed toward the chemical synthesis of large polypeptides, I shall give only a brief summary of our work on the characterization of the enzyme. Most of these findings were recently reviewed at the Brookhaven Symposium2. The protein, which can be conveniently isolated in large amounts from cultures, contains 149 amino acids and has a molecular weight of 16807. Its complete sequence is shown in Figure 1. This globular enzyme contains no sulphhydryl groups or disuiphide bonds and undergoes rapid and complete renaturation after treatment with denaturants or heat. A single tryptophan residue is buried deeply in the hydrophobic core of the molecule in the absence of deriaturants, and two of the seven tyrosine residues appear to have a unique, cooperative function in the binding, and perhaps hydrolysis, of substrate molecules. Nuclease hydrolyses both DNA and RNA to produce 3'-mnononucleotides and the principal attack is endonucleolytic with preferential cleavage of dXp—dTp and dXp—dAp bonds in DNA and Xp—Ap and Xp— Up bonds in RNA. The reaction catalysed is summarized inFigure 2, which also shows the structures of certain competitive inhibitors. Catalytic activity, as well as inhibitor binding, is completely dependent upon the presence of calcium ions (although Sr2 will support the hydrolysis of DNA but not RNA).

Characterization of Staphylococcal nuclease and the status of studies on its chemical synthesis