Question

Give the pka values for the protonatable groups of cysteine, and then calculate its pi. (answers within ± 0.3 ph units will be accepted).

Answer 1

`pKa_1=1.71\\pKa_2=10.78\\pKa_3=8.33\\pI=5.02`

Explanation: Cysteine is a special amino acid having 3 protonable groups which means this group will have 3 pKa's.

The structures wit their pKa values are given in the image below.

pI or isoelectric point is the pH at which the amino acid have no net charge.

Therefore, in the case of cysteine the net charge is 0 in the structure B, so pI will be present between
`pKa_1` and
`pKa_3`.

pKa values of cysteine are:

`pKa_1=1.71\\pKa_2=10.78\\pKa_3=8.33`

pI will be

`pI=(pKa_1+pKa_3)/(2)`

`pI=(1.71+8.33)/(2)`

pI = 5.02