SYNTHETIC, STRUCTURAL AND BIOLOGICAL STUDIES OF THE UBIQUITIN SYSTEM - SYNTHESIS AND CRYSTAL-STRUCTURE OF AN ANALOG CONTAINING UNNATURAL AMINO-ACIDS

Ubiquitin is a 76-amino acid protein involved in the targeting for des truction of proteins in the cell. The protein can readily be synthesiz ed chemically affording an extra dimension to studies of protein stabi lity, Ubiquitin with various modifications to the hydrophobic core has been synthesized. In particular, two core amino acids have been repla ced by aminobutyric acid (Val-26) and norvaline (for Ile-30) and the p roduct crystallized. The refined crystal structure shows an overall co ntraction of the molecule and the side chain of Nva-30 rotates relativ e to Ile-30. However, the side chain rotation is not sufficient to com pensate for the effect of the loss of the methyl group and hence a sma ll cavity is introduced into the structure, which decreases the stabil ity of the protein. The biological behaviour of the modified protein i s unaltered. The observed changes in stability are of the magnitude ex pected for the removal of methyl groups from the hydrophobic core of a protein. Interestingly, the effect appears to be independent of the p osition of the removed methyl group. The intact structure, but not its stability, is important for recognition by the biological conjugating system.