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 six amino acid residues, as shown below. Is the pI of human insulin lower than or higher than that of duck insulin?

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Amino acid residue A8 A9 A10 B B2 B27 Human Duck Thr Ser lePhe Val Thr Glu Asn Pro Ala Ala Ser