Question

Blitzein is an enzyme that requires a protonated histidine in its catalytic site to function. Experiments show that at least 90% or more of the enzymes must have the protonated histidine for catalysis of the chemical reaction. Use the Henderson-Hasselbalch equation to determine which pH will allow blitzein to catalyze the chemical reaction.

Henderson-Hasselbalch equation: pH = pK_a + log (conjugate base / acid)
pK_a of histidine R group = 6.0

A. 7.5

B. 6.5

C. 6.0

D. 5.5

E. 4.5

Answer 1

Histidine    +     H+   ----> Protonated histidine
base                                         conjugate acid

Here since base and conjugate acid is present we need to use below Henderson-Hasselbalch equation

pOH = pKb + log (conjugate acid / base)

pkb = 14 - pKa = 14 - 6 = 8
conjugate acid / base = 90/10 = 9 because 90% are protonated and 10 % is base

pOH = pKb + log (conjugate acid / base)
pOH = 8 + log (9)
          = 8.95

pH = 14 - pOH
       = 14 - 8.95
       = 5.05

Answer should be 5.05