|MadSci Network: Biochemistry|
I am not surprised that copper ions didn't have an effect on either pepsin or alcalase. Copper ions usually inhibit proteases 1) directly by binding to an important amino acid residue at the active site of the enzyme, or 2) by catalyzing oxidation of an important group in the enzyme. This is most often seen with the group of proteases called cysteinyl (or cysteine) proteases (such as papain) that have the -SH group of a cysteine residue (a sulfur containing amino acid) at their active site.
Pepsin belongs to another class of proteases, the aspartic proteases, which have two aspartic acid (another amino acid) residues in their active site. Alcalase is a member of yet another class of proteases, the serine proteases, which have a serine residues at their active site. Neither the aspartic nor the serine proteases are particularly susceptible in general to inhibition by copper ions, so I am not surprised that your pepsin was not inhibited.
I think the real question here is why didn't your alcalase do anything? I am guessing that you carried out the incubation of the enzymes with the film at an acid pH also. If that is the case, the alcalase would be far from its pH optimum, and we would expect it to have very little or no activity in digesting the gelatin of the photograpghic film. On the other hand, pepsin would be near it's pH optimum, and would be active in digesting the gelatin.
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