Chapter 2, Problem 19P

Interpretation Introduction

(a) Interpretation:

If the enzyme is more active at pH 3.5 or 4.5 should be explained.

Concept Introduction:

The acidic group in amino acid is protonated at lower pH value and basic group is protonated at high pH value. Thus, for a amino acid, if ${\text{pK}}_{\text{a}}>\text{pH}$, acidic or −COOH group is protonated and if ${\text{pK}}_{\text{a}}<\text{pH}$ then basic group is protonated.

Interpretation Introduction

(b) Interpretation:

The fraction of the enzymes active at pH 4.07 should be determined.

Concept Introduction:

The Henderson-Hasselbalch equation that relates pH and ${\text{pK}}_{\text{a}}$ of solution is as follows:

$\text{pH}={\text{pK}}_{\text{a}}+\log \frac{\left[{\text{A}}^{-}\right]}{\left[\text{HA}\right]}$

Here, pH is negative log of hydrogen ion, ${\text{pK}}_{\text{a}}$ is negative log of acid dissociation constant, $\left[\text{HA}\right]$ is concentration of weak acid and $\left[{\text{A}}^{-}\right]$ is concentration of conjugate base.

Interpretation Introduction

(c) Interpretation:

The pH value at which enzyme show 78% of maximal activity.

Concept Introduction:

The pH of solution can be calculated from Henderson-Hasselbalch equation as follows:

$\text{pH}={\text{pK}}_{\text{a}}+\log \frac{\left[{\text{A}}^{-}\right]}{\left[\text{HA}\right]}$

Here, pH is negative log of hydrogen ion, ${\text{pK}}_{\text{a}}$ is negative log of acid dissociation constant, $\left[\text{HA}\right]$ is concentration of weak acid and $\left[{\text{A}}^{-}\right]$ is concentration of conjugate base.