You are working with an enzyme that catalyzes the deamidation of Gln residues in proteins. You have developed two different tripeptide substrates for the enzyme: GlyGlnGly and TyrGlnGly. Assaying the enzyme, you get the following results:
Substrate KM (M) kcat (s -1)
GlyGlnGly 2.7 x 10 -2 8.3 x 10 2
TyrGlnGly 1.2 x 10 -3 6.2 x 10 2
A. For which substrate is the enzyme more catalytically efficient?
B. Which is more affected by the structural difference between the two tripeptide substrates, KM or kcat?
C. Which is more likely to be affected by the structural difference between the two tripeptide substrates, their binding to the enzyme or some step in the catalytic mechanism? Explain your reasoning.