[sazzy]

A mutant enzyme containing cysteine in place of selenocysteine has 1000-fold less activity at pH 7 than selenocysteine. Explain these differences in activity using the following information:
selenocys sidechain is pKa 5.1
cysteine sidechain is pKa 8.3

Does it have something to do with the fact that selenocysteine will become deprontonated at pH 7. So because the mutant can't accept any protons it's structure isn't the same & the enzyme can't work because it needs to be specific?

[Babcock_Hall]

It is possible that the active form of the enzyme is E-Se(1-) and that E-Se-H is catalytically inactive.  This is a reasonable assumption if the selenium atom is a nucleophile in the reaction.  See if you can work from this idea.