3. The protein insulin consists of two polypeptides termed the A and B chains. I

Question

3. The protein insulin consists of two polypeptides termed the A and B chains. Insulins from different organisms have been isolated and sequenced. Human and duck insulins have the same amino acid sequence with the exception of 6 amino acid residues, as shown below. A) Is the pl of human insulin lower or higher than that of duck insulin? Please justify your answer. B) Given these sequence for their respective organism? differences, is it possible for these proteins to perform the same function Amino acid residue A8 A9 A10 BI B2 B27 Thr Ser le Phe Val Thr Glu Asn Pro Ala Ala Ser Human Duck

Explanation / Answer

A. At position 8 humans have Thr which is neutral while the duck has Glu which is a negatively charge amino acid at neutral pH. The other amino acids present in duck will not affect the pI as they are uncharged. So, the human insulin has a higher pI than the duck insulin.

B. In their respective organisms, they perform similar functions. The amino acid residue at A8 position is mainly responsible for insulin binding site to the insulin receptor. The Glu at A8 in duck increases the thermodynamic stability but it decreases the affinity for insulin receptors. It has been proved that duck insulin has a lower affinity for rat liver membrane.

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