CASE-STUDY OF PROTEIN-STRUCTURE, STABILITY, AND FUNCTION - NMR INVESTIGATIONS OF THE PROLINE RESIDUES IN STAPHYLOCOCCAL NUCLEASE

We have used NMR spectroscopy to determine peptide bond configurations and to measure the rates and equilibria of interconversion at individ ual Xaa-Pro peptide bond linkages in staphylococcal nuclease and sever al variants produced by site-directed mutagenesis. In general, tertiar y interactions, rather than short-range interactions, have been found to be critical for stabilizing the cis linkage at Lys116-Pro117 which predominates in the wild-type enzyme. A correlation has been found bet ween the position of the cis/trans equilibrium at the Lys116-Pro117 pe ptide bond and thermal stability of the variant. Enthalpic interaction s that stabilize the folded protein appear to be present when the pept ide bond is cis but not when it is trans. Hydrogen exchange protection factors correlate with the mole fraction of the protein that is in th e cis configuration. Nuclease variants in which the peptide bond is pr edominantly cis are more stable against denaturation (by heat, pressur e, or guanidinium chloride) than those that are predominantly trans. D isulfide bridges have been engineered and introduced by mutagenesis th at stabilize certain conformational states; one of these shows couplin g between the oxidation state of the engineered cysteine pair and the cis/trans configuration about the Lys116-Pro117 peptide bond. A rough correlation is seen between the catalytic activities of mutants and th e cis/trans ratio; the effect is primarily on k(cat) rather than on K( m).