Question

1. which of the following tripeptides would be closest to its pI isoelectric point in a solution at pH=5

A)HisLysArg B)HisGluAla C)TyrAspGluHis D)GluAspGly

2. Describe the coordination of the Fe2+ ( name the coordination ligands) in both the deoxy and oxymyoglobin states.

Answer 1

1) List down the amino acids in the tripeptides, their pK_a and pI values.

Amino Acid	pKa1 (α-COOH)	pKa2 (α-NH2)	pKa3 (Side Chain)	pI
His	1.78	8.97	5.97	7.64
Lys	2.20	8.90	10.28	9.47
Arg	2.18	9.09	13.2	10.76
Glu	2.19	9.67	4.25	3.08
Ala	2.35	9.87		6.11
Tyr	2.20	9.11	10.07	5.63
Asp	1.88	9.60	3.65	2.98
Gly	2.34	9.60		6.06

Next, let us consider the charges on the tripeptides at pH 5 as below.

Tripeptide	Charge on α-COOH at pH = 5	Charge on α-NH2 at pH = 5	Charge on Side Chain (1) at pH 5	Charge on Side Chain (2) at pH 5	Charge on Side Chain (3) at pH 5	Net Charge
His-Lys-Arg	-1	+1	+1	+1	-1	+1
His-Glu-Ala	-1	+1	+1	-1		0
Tyr-Asp-Glu-His	-1	+1	+1	-1	-1	-1
Glu-Asp-Gly	-1	+1	-1	-1		-2

Only His-Glu-Ala has charge 0 at pH 5 and hence, we shall find out the pI value of the tripeptide. We will assume the pI value to be the average of the closest pK_a values on either side of pH = 5. The closest values are 4.25 and 5.97 and the approximate pI of the tripeptide is ½*(5.97 + 4.25) = 5.11 (ans).