Protein Design with Fluoroprolines: 4,4-Difluoroproline Does Not Eliminate the Rate-Limiting Step of Thioredoxin Folding.

C4 -substituted fluoroprolines (4R)-fluoroproline ((4R)-Flp) and (4S)-fluoroproline ((4S)-Flp) have been used in protein engineering to enhance the thermodynamic stability of peptides and proteins. The electron-withdrawing effect of fluorine can bias the pucker of the pyrrolidine ring, influence the conformational preference of the preceding peptide bond, and can accelerate the cis/trans prolyl peptide bond isomerisation by diminishing its double bond character. The role of 4,4-difluoroproline (Dfp) in the acceleration of the refolding rate of globular proteins bearing a proline (Pro) residue in the cis conformation in the native state remains elusive. Moreover, the impact of Dfp on the thermodynamic stability and bioactivity of globular proteins has been seldom described. In this study, we show that the incorporation of Dfp caused a redox state dependent and position dependent destabilisation of the thioredoxin (Trx) fold, while the catalytic activities of the modified proteins remained unchanged. The Pro to Dfp substitution at the conserved cisPro76 in the thioredoxin variant Trx1P did not elicited acceleration of the rate-limiting trans-to-cis isomerization of the Ile75-Pro76 peptide bond. Our results show that pucker preferences in the context of a tertiary structure could play a major role in protein folding, thus overtaking the rules determined for cis/trans isomerisation barriers determined in model peptides.