Exam 1: pI calculation

Question 1

Glutamic acid has three ionizable groups: alpha-carboxyl, alpha-amino, and side chain carboxyl group. Their pKa values are 2.19, 9.67, and 4.25, respectively.

Calculate the pI value of glutamic acid.

A) (2.19 + 9.67)/2
B) (2.19 + 4.25)/2
C) (9.67 + 4.25)/2
D) (2.19 + 9.67 + 4.25)/3

Answer 1

B) (2.19 + 4.25)/2

Question 2

Histidine has three ionizable groups, with pKa values of 1.82 (alpha-carboxyl group), 9.17 (alpha-amino group), and 6.0 (the side chain).

Calculate its isoelectric point.

A) (1.82 + 9.17)/2
B) (9.17 + 6.0)/2
C) (1.82 + 6.0)/2
D) (1.82 + 9.17 + 6.0)/3

Answer 2

B) (9.17 + 6.0)/2

Question 3

Arginine has three ionizable groups: α-carboxyl, α-amino, and side chain. Their pKa values are 2.17, 9.04, and 12.48, respectively.

Calculate the isoelectric point of arginine.

A) 2.17 + 9.04
B) (2.17 + 9.04) / 2
C) (9.04 + 12.48) / 2
D) (2.17+9.04+12.48) /3

Answer 3

C) (9.04 + 12.48) / 2