Thiol Proteases Cleave Peptide Bonds by a Mechanism Similar to That

Thiol proteases cleave peptide bonds by a mechanism similar to that of serine proteases, such as trypsin and chymotrypsin, but they contain cysteine instead of serine.In which way does the sulfhydryl group of cysteine participate in the reaction?

A) It participates in substrate binding and thereby helps determine the substrate specificity of the enzyme.
B) It reacts covalently with the side chain of an acidic amino acid residue in the substrate.
C) It attacks the C=O portion of a peptide bond and forms a thioester with it.
D) It relays electrons to the peptide bond of the substrate, thereby facilitating the formation of a tetrahedral intermediate.
E) It forms a transient disulfide bond with a cysteine side chain of the substrate.