All amino acids have two ionizable functional groups: an alpha-amino group (aver

Question

All amino acids have two ionizable functional groups: an alpha-amino group (average pK_a of 9.4) and an alpha_carboxylic acid group (average pK_a of 2.2). Cysteine has an ionizable side chain (R group) with a pKa of -8.3. One of the possible ionization states of cysteine is shown below. At what pH would the above structure be the predominant ionization state? Note: Consider the ionization state of all three of the functional groups. pH = The protonated form of the R group of cysteine is shown in the structure above. The ratio of the protonated form to the charged (deprotonated) form depends on the pK_a of the R group and the pH of the solution. Select all the pH values at which the charged form of the R group would predominate. 6.3 8.3 9.8 11.8

Explanation / Answer

Cysteine has three acidic groups with pKa values 2.2 for COOh. 8.3 for SH and 9.4 for NH3+ groups.

We know the aminoacids behave as buffers and the charge existing on the AA depends upon the pH of the solution, which can be observed by using Henderson equation.

pH = pKa + log ( [conjugate base]/[acid])

Q1) The given structure is completely protonated form (monocationic form) .This structrue is possible only in strong acidic solutions that is when the pH = about 1.0 . At such low pH, all the acidic groups are completely protonated.

Q2)When the pH = 8.3, cysteine loses the COOH proton completely and the SH is lost 50% making the monocationic form and monoanionic form equal. thus if the pH > 8.3 the monoanionic form with [ COO-, S_ and NH3+ groups] is dominant

To have the charged or completely deprotonated form predominant the pH must be greater than the highest pKa value that is 9.4 where all the three groups exist as COO-, NH2 and S- making it dianionic form.

Thus the pH values at which cysteine exists as completely deprotonated and charged are 9.8 and 10.8

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