Abstract
THE hydrolysis of a substrate by the proteolytic enzymes, papain and ficin, has been shown to involve the formation of an acyl-enzyme intermediate through the sulphydryl group of a cysteine residue1. These cysteine residues must therefore be in the catalytic site of the enzymes and a knowledge of their environment could facilitate our understanding of the mechanism of action of these enzymes. Cysteine-25 has been identified as the active centre cysteine residue of papain, by irreversible inhibition with [14C]-iodoacetate2 and with the “active site directed” irreversible inhibitor, chloroketone 
The amino-acid sequence around this cysteine residue is shown in Fig. 1. [14C]-Iodoacetate has also been used to label and identify the amino-acid sequence around the reactive cysteine of ficin4 (see Fig. 1). The structural similarity between these two sequences leaves little doubt that this must be the active cysteine residue of ficin.
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LOWE, G. Structural Relationships between some Plant and Animal Proteases. Nature 212, 1263–1264 (1966). https://doi.org/10.1038/2121263b0
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DOI: https://doi.org/10.1038/2121263b0
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