Prolyl bonds RNase

The prolyl isomerases catalyze isomerizations only at prolyl bonds and not at nonprolyl peptide bonds. The refolding of the P39A variant of RNase Tl, which is limited in rate by the very slow trans —> cis reisomerization of the Tyr38-Ala39 bond (see Section IV.B), is not catalyzed by cyclophilins, FKBPs, or parvulins. These enzymes are also unable to catalyze amide bond isomerizations in the proline-free model peptide Ala-Ala-Tyr-Ala-Ala (Scholz etal., 1998b). 

The fraction of Us molecules depends on the number of proline residues and on their isomeric state in the native protein. In particular, the presence of cts-prolyl peptide bonds in the folded molecules leads to a high fraction of Us, since in unfolded proteins the cis state is populated to a small extent only. Adler and Scheraga (1990) showed by NMR that in heat-unfolded RNase A the nonnative trans isomers predominate at both Pro93 and Proll4. The Up molecules dominate in the unfolded state of proteins that have only tram-prolyl peptide bonds, such as lysozyme (Kato et ai, 1981, 1982), cytochrome c (Ridge el ai, 1981 Nall,... 

In in vitro experiments prolyl isomerase accelerates the oxidative folding of reduced RNase T1 (i.e., folding coupled with formation of the disulfide bonds) and the catalysis of disulfide bond formation by PDl is markedly improved when PPI is present simultaneously (Schonbrunner and Schmid, 1992). The oxidative folding of RNase T1 in the presence of a mixture of reduced and oxidized glutathione is a slow process and it can be followed by the increase in tryptophan fluorescence (Fig. 7). Folding is strictly linked to disulfide bond formation under the conditions... 

Together, the results for RNase T1 and -lactamase show that the trans —> cis isomerizations of nonprolyl peptide bonds control the folding of proteins with such cis bonds. The trans —> cis isomerizations of prolyl and nonprolyl bonds show similar rates, the reverse cis —> trans isomerizations are 50-100-fold faster, however, for the nonprolyl peptide bonds. 

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