Prolyl isomerases discovery

Beyond protein folding, the discovery of peptidyl prolyl isomerases (PPIases) and related proteins has opened the way to novel concepts in biology the notion of chaperone-assisted receptor binding is an emerging field of research which sheds light on receptor function and protein-protein interactions. The recent discovery of a secondary amide peptide bond cis-trans isomerase (APIase) heralds new advances in this field. 

The first enzyme to catalyze prolyl isomerization was identified by Fischer and colleagues (1984). This discovery was possible because they developed an ingenious assay for prolyl isomerases based on the conformational specificity of chymotrypsin. This protease cleaves a chro-mogenic reporter group from a tetrapeptide (such as succinyl-Ala-Ala-Pro-Phe-4-nitroanilide) only when the Ala—Pro bond of this peptide is in the trans conformation. In aqueous solution the assay peptide exists as a 90 10 mixture of molecules with the Ala—Pro bond in trans and cis, respectively. Therefore, in the presence of a high concentration of chymotrypsin, 90% of the peptide molecules are cleaved within the dead time of manual mixing. Hydrolysis of the remaining 10% is slow because it is limited in rate by the cis —trans isomerization of the Ala—Pro bond. Acceleration of this reaction serves as a sensitive probe for prolyl isomerase activities. 

After their discovery in 1984 (Fischer et al, 1984), a huge number of prolyl isomerases have been found in all organisms and all subcellular compartments. It is clear that not all these proteins are involved in de novo protein folding. Rather, they are assumed to participate in many cellular functions. The following section discusses several of these functions. 

Whereas peptidyl prolyl cis-trans isomerases constitute a well-characterized enzyme class comprising well over 1000 members with small sequence variations in the proteins of different species, the discovery of secondary amide peptide bond cis-trans isomerases (APIases) had to await the development of suitable enzyme assays. Fortunately, spectral differences in the UV region between cis and trans isomers of dipeptides could be exploited to identify and quantify isomerization rate-enhancing factors in biological material [149]. 

Conformational restriction played an important role in the discovery that water induces the bioactive conformation of CsA. Cyclosporin A (Sandimmune , CsA, (23.37), Fig. 23.7), is a major drug for preventing rejection of transplanted human organs and has been the subject of many synthetic, conformational and mechanism of action studies. To produce immunosuppression, CsA first binds to cyclophilin A (CyP A), a peptidyl prolyl cis-trans isomerase (PPIase), to form the CsA-CyP complex, which then binds to and inhibits calcineuiin (CaN), a calmodulin-dependent serine/threonine protein phosphatase, thereby inhibiting interleukin-2 (IL-2) synthesis. ... 

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