Explanation for the enzyme activity changes

RNAse is an enzyme that cleaves the P–O^5′ bond in RNA. it has two His residues in the active site. Suggest a plausible explanation why the enzyme activity changes when pH is increased or decreased from ~pH 6.0,

Choose all the true statements

a.) the RNAse reaction is an example of metal ion catalysis with a positively charged metal
b.) both His residues are deprotonated at pH higher than 6.5
c.) one His residue abstracts a proton from the substrate at pH 6.0, whereas the other denotes a proton
d.) there are two His residues, and both are deprotonated below pH 4.2
e.) the substrate is affected by the change in pH, and is inactive (+ charged) when pH is above ~6.0


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