Some observations on the basic principles of design in protein molecules

Abstract

The last stage in the biosynthesis of an enzyme involves the intramolecular coiling and folding of the extended polypeptide chain into a configuration that embodies a unique active center. The arrangement of amino acids along the chain must be such that the final tertiary structure is assumed with a high degree of reproducibility and must also contain information governing the correct pairing of half-cystine residues and the formation of other stabilizing covalent cross-linkages. Experiments on ribonuclease are discussed which suggest that all the information for the production of the fully cross-linked and folded molecule is contained in the amino acid sequence alone. In these experiments, the four SS bounds of RNase were reduced to yield a derivative containing eight SH groups. The oxidation of this reduced derivatives to yield native enzyme is quantitative under proper conditions of protein concentration and pH. Various substances and environmental conditions were tested for their ability to interfere with normal pairing of half-cystine residues, thus furnishing information on the nature of the interactions that govern this process. It is suggested that the process of mutation and natural selection has lead to the “design” of amino acid sequences so uniquely constructed that the formation of active centers and three-dimensional conformations follow automatically and reproducibly without additional genetic information.