Question

a.Which amino acid has the lowest pI value?

b.Which amino acid has the highest pI value?

c.Which amino acid has the greatest amount of negative charge at pH = 6.20?

d.Which amino acid has a greater negative charge at pH = 6.20, glycine or methionine?

Short answer

(a) Aspartate (pI =2.98) has the lowest pI value.

(b) Arginine (pI = 10.76) has the highest pI value.

(c) Aspartate has the greatest amount of negative charge at pH = 6.20because its pI is lower than that of glutamate.

(d) Methionine has a greater negative charge at pH = 6.20.

Step-by-step solution

Step-by-Step Solution Step 1: Isoelectronic point (pI)

The isoelectric point is defined as the pH of the solution at which the net charge of protein becomes neutral or will carry no units of positive or negativecharge. In other words, it is the pH at which the amount of positive charge on an amino acid or protein exactly balances the amount of negative charge:

pI = pH at which there is no net charge

Identifying amino acid (a) and (b)

(a) Aspartate-

$\begin{array}{l}\mathrm{pI}=\frac{2.09+3.86}{2}\\ =\frac{5.95}{2}\\ =2.98\end{array}$

Aspartate (pI =2.98) has the lowest pI value

(b). Arginine-

$\begin{array}{l}\mathrm{pI}=\frac{9.04+12.48}{2}\\ =\frac{21.52}{2}\\ =10.76\end{array}$

Arginine (pI = 10.76) has the highest pI value.

Identifying acid (c) and (d)

(c) Aspartate has the greatest amount of negative charge at pH = 6.20, because its pI is lower than that of glutamate. Therefore pH = 6.20 is farther away from the pH at which it has no net charge. As we know, the farther an amino acid with no net charge moves to get to the given pH value, the more charge it will gain.

(d) Methionine has a greater negative charge at pH = 6.20, because methionine is farther away from the pH at which it has no net charge.

As,

pI of methionine = 5.75

pI of glycine = 5.97