The electrostatic fields in the active-site clefts of actinidin and papain

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Abstract
The active sites of actinidin (EC 3.4.22.14) and papain (EC 3.4.22.2) display different reactivity chracteristics to probes targeted at the active-site cysteine residue despite the close structural similarity of their active sites. The calculated elctrostatic fields in the active-site clefts of actinidin and papain differ significantly and may explain the reactivity characteristics of these enzymes. Calculation of electrostatic potential also focuses attention on the electrostatic properties that govern formation of the active-site thiolate-imidazolium ion-pair. These calculations will guide the modification of the pH-activity profile of the cysteine proteinases by site-directed mutagenesis.