Question

Selenocysteine has pKa values for the a-amino, a-carboxyl, and sidechain groups of 10.0, 1.90, and 5.4, respectively. the sidechain looks just like cysteine, but has a selenium in place of the sulfur. In terms of protonation and charge, selenium behaves like oxygen or sulfur. Answer the following.

1) what is the overall charge of selenocysteine at pH 6.2?

Answer 1

Ans. See the attached table to assign approximate charge on the groups.

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The amino group is protonated at pH 6.2 and bears +1 charge.

The carboxyl group is deprotonated at pH 6.2 and bears -1 charge.

The Se-atom acts as a proton acceptor or more like a basic group. However, unlike –NH₂, the Se-atom does NOT have any H-atom to donate to the medium when pH of medium is higher than its pI. Also, Se can’t accept a proton because the medium is basic (pH is higher than its pI, thus pH is relatively basic than its pI) and lacks enough H⁺ to bind with Se. Therefore, Se is neutral at pH 6.2.

[You can also proceed with +0.010 charge on Se using Henderson-Hasselbalch equation. However, there is no need to seeking such accuracies].

# So, overall charge on Selenocysteine at 6.2 = -1 + +1 + 0 = 0.0