Question

A novel hydrolase is found to have a kcat=50 s-1 and KM =1.4x10-4 M at 37 degrees Celcius. It is proposed that the side chain of Tyr155 contributes a hydrogen bond to the transition state of this enzyme. The kinetic parameters for the Y155F mutant of this enzyme are kcat =0.02 s-1 and KM =2.0x10-4 M.

A. Why mutate the tyrosine to a phenylalanine instead of another amino acid?

B. By how much (in kJ/mol) does Tyr155 stabilize the transition state at 37 oC?

Answer 1

(A) Mutate the tyrosine to a phenylalanine instead of another amino acid because there is little difference between them as phenylalnine differs from tyrosine by the absence of the ζ-OH group only. Therefore a minimum disruptive mutation to explicitly test the effect of the hydroxyl group.

(B) As Arrhenius equation shows :

kcat= A exp(-Ea/RT)

Ea= -RT ln(kcat/A) = -RT ln(kcat) + RT ln(A) Ea(WT) – Ea(Y155F) = -RT ln(kcatWT) + RT ln(A) + RT ln(kcatY155F) – RT ln(A) = RT ln(kcatY155F) – RT ln(kcatWT) = 8.3 ×310 ×ln(0.02) – 8.3 ×310 ×ln(50) = -10066 – 10066 = -20100 J mol-1or -20 kJ mol-1