I need help w/ a question: Insulin protein has 2 polypeptides A and B. Human and duck insulins have the same sequence except for six below. Is the pI of human insulin higher or lower than duck insulin?
AA seq. A8 A9 A10 B1 B2 B27
Human Thr Ser Ile Phe Val Thr
Duck Glu Asn Pro Ala Ala Ser
To determine whether the pI (isoelectric point) of human insulin is higher or lower than duck insulin, we need to understand the concept of pI and how it can be influenced by specific amino acid residues.
The pI of a protein is the pH at which it carries no net electrical charge. At its pI, a protein neither donates nor accepts protons, and therefore, it has an overall neutral charge. The pI of a protein can be calculated based on the pKa values of its constituent amino acids.
In this case, we are comparing the amino acid sequences of two insulin proteins: human and duck insulin. We are specifically looking at the differences in six amino acid residues: A8, A9, A10, B1, B2, and B27.
To determine the effect of these differences on the pI, we need to focus on the nature of the amino acids in terms of their pKa values. The pKa value is a measure of the acidity or basicity of an amino acid side chain.
For the human insulin sequence:
A8 = Thr (Threonine)
A9 = Ser (Serine)
A10 = Ile (Isoleucine)
B1 = Phe (Phenylalanine)
B2 = Val (Valine)
B27 = Thr (Threonine)
For the duck insulin sequence:
A8 = Glu (Glutamic acid)
A9 = Asn (Asparagine)
A10 = Pro (Proline)
B1 = Ala (Alanine)
B2 = Ala (Alanine)
B27 = Ser (Serine)
Now, we can analyze the pKa values of these amino acids involved in the sequence differences.
For human insulin, the amino acids involved have the following pKa values:
- Thr (Threonine): pKa ~ 7.6
- Ser (Serine): pKa ~ 14
- Ile (Isoleucine): pKa ~ 9.6
- Phe (Phenylalanine): pKa ~ 5.5
- Val (Valine): pKa ~ 9.6
For duck insulin, the amino acids involved have the following pKa values:
- Glu (Glutamic acid): pKa ~ 4.2
- Asn (Asparagine): pKa ~ 13.9
- Pro (Proline): pKa ~ 1.95
- Ala (Alanine): No ionizable side chain
- Ala (Alanine): No ionizable side chain
- Ser (Serine): pKa ~ 14
Now, let's consider the pI calculation. To calculate the pI of a protein, we average the pKa values of the ionizable groups. However, since Alanine (Ala) has no ionizable side chain, it does not contribute to the pI calculation.
Based on the pKa values of the amino acids involved, we can conclude the following:
1. Human insulin has three amino acids with basic side chains (Threonine, Serine, and Isoleucine), which can accept protons at high pH (basic conditions), making the overall pI of human insulin lower.
2. Duck insulin has one amino acid with an acidic side chain (Glutamic acid) and another with a basic side chain (Serine), which can both donate protons at low pH (acidic conditions), making the overall pI of duck insulin higher.
Therefore, based on the analysis of the amino acid sequences and their pKa values, we can conclude that the pI of human insulin is lower than that of duck insulin.