THERMODYNAMIC PROPERTIES OF TRANSIENT INTERMEDIATES AND TRANSITION-STATES IN THE FOLDING OF 2 CONTRASTING PROTEIN STRUCTURES

The N-terminal domain of phosphoglycerate kinase (N-PGK) and domain 1 of the T-cell adhesion protein CD2 (CD2.d1) fold through rapidly forme d and transiently populated intermediate states in reactions which hav e no kinetic complications arising from proline isomerization or disul fide bonding. We have evaluated the thermodynamic parameters (Delta C- p), change in heat capacity; Delta S, entropy change; Delta H, enthalp y change) for each experimentally accessible step in these folding rea ctions. Despite their different topologies and amino acid compositions , the individual steps [U-I (unfolded to intermediate state), I-t (int ermediate to major transition state), and t-F (transition state to the fully folded state)] have closely similar qualitative properties in t he two proteins, For bath, the heat capacity changes are proportional to In-value changes (Delta m) for every step in the reaction, but the ratio Delta C-p/Delta m is lower fur N-PGK, presumably owing to a much larger compliment of aromatic amino acids in the core. According to m easurements of Delta C-p and Delta m, the I-states are highly condense d (65-70% for N-PGK and 40-45% dehydrated for CD2.d1), yet the changes in entropy in the U-to-I transition are small, showing that the entro py gained from desolvation must be balanced by that lost in ordering t he chain. The high degree of conformational order in the I-stale, impl ied by these measurements, Is mirrored by the extensive, native second ary structure revealed by amide exchange measurements [Hosszu, L. L. P ., et al. (1997) Nat. Str?lct. Biol. 4, 801-804; Parker, M. J., et al. (1997) Biochemistry 36, 13396-13405]. At 25 degrees C the transition state barrier has an entirely enthalpic origin. the entropic contribut ion being favorable. The latter observation implies that, during the c onsolidation of structure occurring in the I-to-F step, further dehydr ation (positive Delta S) precedes side-chain locking (negative Delta S ). Only after the transition state is surmounted do we see a net entro pic penalty arising from the widespread ordering of side chains.