Peptidyl-prolyl cis-trans isomerase does not affect the Pro-43 cis-trans isomerization rate in folded calbindin D9k

The calcium-binding protein calbindin D9k has previously been shown to exist in two folded forms only differing in the proline cis-trans isomerism of the Gly-42-Pro-43 amide bond. This bond is located in a flexible loop connecting the two EF-hand Ca2+ sites. Calbindin D9k therefore constitutes a unique test case for investigating if the recently discovered enzyme peptidyl-prolyl cis-trans isomerase (PPIase) can affect the cis-trans exchange rate in a folded protein. The 1H NMR saturation transfer technique has been used to measure the rate of interconversion between the cis and trans forms of calbindin in the presence of PPIase (PPIase:calbindin concentration ratio 1:10) at 35 degrees C. No rate enhancement could be detected.