Mechanism of Peptide Bond Cis-Trans Isomerases

In conclusion, the biochemical properties of the APIase function of hsp70 molecular chaperones allow kinetic proofreading of folding by rapid scanning of APIase-sensitive sites of unfolded polypeptide chains. 

Catalytic Mechanism of Peptide Bond Cis-Trans Isomerases 

Despite the amount of data and the simplicity of the chemical reaction catalyzed, the molecular basis of the catalytic mechanism of PPIases and APIases is still only poorly understood [155]. The considerable degree of amino acid sequence dissimilarity between the subgroups of peptide bond cis-trans isomerases also raises the challenging question of the mechanistic relatedness among the enzymes. At present there is a lack of detailed mechanistic investigations on APIases and multidomain PPIases. Thus, prototypic PPIases of all three families serve as the bases for unraveling catalytic pathways. One or more potential transition-state structures for enzyme-catalyzed prolyl isomerizations, alone or in combination, are consistent with the acceleration of the spontaneous rate of prolyl isomerization (Fig. 10.4). 

A mechanistic model has been proposed for PPIase catalysis in which a twisted peptide bond, a structure involving substrate strain, is stabilized by noncovalent interaction with the enzyme [156], However, catalytic antibodies generated to transition state analogs containing twisted carbonyl moieties do not show a PPIase-like catalytic efficiency [157,158], Consequently, small detergent micelles and phosphatidylcholine membranes are able to catalyze CTI of typical PPIase substrates in a manner reminiscent of that observed for catalytic antibodies [159]. Apparently, sequestration of hydrophobic substrates within the enzyme may account for both a small portion of the catalytic power of FKBP and the acceleration of CTI by catalytic antibodies. Despite overall amino acid sequence dissimilarity the structural features making up the active sites of prototypic enzymes such as Cypl8 and ParlO proved to be similar (Fig. 10.6). 

Although the X-ray structure of a Pinl-dipeptide complex implicated covalent catalysis, a lack of nucleophilic assistance in the catalytic pathway can be derived from the covalent modification of parvulins by 5-OH-l,4-naphthoquinone (juglone). A juglone-inactivated E. coli ParlO contains two inhibitor molecules that are covalently bound to the side-chains of Cys41 and Cys69 because of a Michael 

---

Catalytic Mechanism of Peptide Bond Cis-Trans Isomerases I 217... 

---