MULTIPLE CONFORMATIONS AND PROLINE CIS-TRANS ISOMERIZATION IN SALMON-CALCITONIN - A COMBINED NUCLEAR-MAGNETIC-RESONANCE, DISTANCE GEOMETRY,AND MOLECULAR MECHANICS STUDY

The relationship between multiple conformations and proline cis-trans isomerization in salmon calcitonin (sCT) lias been investigated by H-1 -NMR, distance geometry, and molecular mechanics. In different solvent s, the isomerization of Pro(23) induces resonance heterogeneity for am ino acids adjacent to it. NOESY experiments have related such heteroge neity to two different isoforms of the hormone, which interconvert int o each other in polar solvents as well as in SDS micelles. The cis-tra ns ratio was found to depend upon the structure of the hormone. In wat er, the random-coil sCT showed a 35% cis population, while 16% cis was observed in the folded SDS-bound hormone. Such a decrease contributes , per se, a Gibbs free energy stabilization of 3.10 kJ mol(-1). Except for the 21-25 region, a common NOE pathway was found for both isomers . This is in agreement with the common secondary structure for both is oforms: a central helix and an extended C-terminal segment interacting with it. Calculations indicated that while the trans isomer is helica l in the Thr(6)-Tyr(22) region, a shorter helix (Thr(6)-Lys(18)) is pr esent in the cis isoform. It is concluded that isomerization of Pro(23 ) does not alter the three-dimensional structure of the hormone, altho ugh trans structures show a lower average NOE root mean square deviati on and a higher relative stability. Both cis and trans structures sati sfactorily reproduce the experimental data, although they do not fulfi ll the whole set of NOE restraints, pointing out the danger involved i n structure calculation whenever the cis-trans equilibrium does not af fect the global fold.