Protein hydrolysis steric effects

In a series of ten morphine 3-benzoates, large differences in rates of enzymatic hydrolysis were seen [122], In 80% human plasma at pH 7.4 and 37°, the unsubstituted 3-benzoate had a tm value of 0.6 h, whereas esters of 2,6-disubstituted benzoic acid were much more resistant to enzymatic attack (t1/2 ranging from 60 h for the dimethylbenzoate to 300 h for the dichloro-and dimethoxybenzoates). Although these results point to marked steric hindrance, electronic effects cannot be excluded but escape characterization because of the limited series. Furthermore, and as mentioned repeatedly in this text, the possibility of binding to plasma proteins is a complicating factor that should be kept in mind. 

Various diastereomeric di-, tri-, and tetrapeptides that carry the sterically demanding trifluoromethyl group instead of the natural a-proton at different positions within these short peptide sequences have been designed, and their stability towards enzymatic hydrolysis has been investigated. The structures of the a-trifluoromethyl (aTfm)-substituted amino acids are shown in Scheme 1. From these studies we gained valuable information on how a-trifluoromethyl-substi-tuted peptides may interact with proteins. The aTfm amino acids used in this study combine the conformational restrictions [49-52] of C -dialkylation with the unique stereoelectronic properties of the fluorine atom and have shown interesting effects on peptide-enzyme interactions [53,54]. 

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