Question

5. Tyrosine has a side-chain with an ionisable phenol group. i) explain in less than 20 words the generl definition of pl; i) state in a complete sentence the pl of tyrosine; and ili) explain in about 50 words how the involved chemicals group determine the pl of tyrosine (in your explanation name the charges that are present on the ionisable groups) (1 mark)

Answer 1

1. Isoelectric point (pI) is the pH at which a molecule (here amino acids) has no net electrical charge (equal number of positive and negative charge on the molecule).

Less than 20 words so don't include the phrases in brackets in your answer.

2. Isoelectric point of tyrosine is 5.66

3. In amino acids with hydrophobic side chains the isolelectric points are more towards neutral pH, with acidic side chains the isolelectric point is very low (2-4), and for basic side chains the isolelectric point is higher (8-10).

For tyrosine

The pKa1 for -COOH group is 2.2.

The pKa2 for -NH2 group is 9.1

Average of these gives (2.2+9.1)/2= 5.66 which is the pI.

Contribution of side chain here is not significant. It's effectis accounted for in the pKa values above. It matters in acidic and basic side chain amino acids.

The zwitterionic structure with the charges aare given in image below.

CO0 3 CH 2 OH