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

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 CYS?

Quizplus · Accepted Answer

The pI (isoelectric point) of Cys is calculated by averaging the pKa values of the groups that can be protonated or deprotonated around the neutral form. For Cys, these are the α-carboxyl group (pKa 1.7) and the R-group (pKa 6.0). The pI is (1.7 + 6.0) / 2 = 3.85, which is closest to 5.0 among the given options.