$$\begin{array}{l}
\text { Exhibit } \mathbf { 3 A } \text { (for this chapter) }\
\begin{array} { l l l l }
\text { Amino acid } & \boldsymbol { \alpha } \text {-carboxyl group pK a } & \boldsymbol { \alpha } \text {-amino group pK a } & \text { R-group pK } \
\hline \text { Ala } & 2.3 & 9.7 & \
\text { Arg } & 2.2 & 9.0 & 12.5 \
\text { Asn or Gln } & 2.1 & 9.0 & 4.0 \
\text { Asp or Glu } & 2.2 & 9.8 & 8.3 \
\text { Cys } & 1.7 & 10.8 & 6.0 \
\text { His } & 1.8 & 9.2 & \
\text { Ser } & 2.2 & 9.2 & 10.1
\end{array}
\end{array}$$
-Refer to Exhibit 3A. Calculate the pI of ASN:

Question

$$\begin{array}{l}
\text { Exhibit } \mathbf { 3 A } \text { (for this chapter) }\
\begin{array} { l l l l } 
\text { Amino acid } & \boldsymbol { \alpha } \text {-carboxyl group pK a } & \boldsymbol { \alpha } \text {-amino group pK a } & \text { R-group pK } \
\hline \text { Ala } & 2.3 & 9.7 & \
\text { Arg } & 2.2 & 9.0 & 12.5 \
\text { Asn or Gln } & 2.1 & 9.0 & 4.0 \
\text { Asp or Glu } & 2.2 & 9.8 & 8.3 \
\text { Cys } & 1.7 & 10.8 & 6.0 \
\text { His } & 1.8 & 9.2 & \
\text { Ser } & 2.2 & 9.2 & 10.1
\end{array}
\end{array}$$
-Refer to Exhibit 3A. Calculate the pI of ASN:

Quizplus · Accepted Answer

pI is the pH at which the amino acid or protein has no net charge. It can be calculated by taking the mean of the pKa values of the ionizable groups that can be deprotonated or protonated (i.e. the $\alpha$-carboxyl and $\alpha$-amino groups, as well as any side chain groups with pKa values within 1 pH unit of the pH being determined).

For ASN, the two ionizable groups are the $\alpha$-carboxyl group (pKa = 2.1) and the $\alpha$-amino group (pKa = 9.0). Since there is no R-group pKa value within 1 pH unit of the pI, it does not need to be considered.

To determine the pI, we take the average of the pKa values of the two ionizable groups:
pI = (pKa of $\alpha$-carboxyl group + pKa of $\alpha$-amino group)/2
pI = (2.1 + 9.0)/2
pI = 5.55

Therefore, the best choice is C) 5.5.