Question

1) Draw the polypeptide: Ser-Gln-His-Ser II) Estimate the net charge of the polypeptide that has the sequence of Glu-His-Trp-Ser-Gly-Leu-Arg-Pro-Gly 1) At pH 7 2) At PH 11 Pka values are given in table 3.2

Answer 1

2. At pH 7 the amino group of glu has a positive charge and the side chain carboxylic acid donates a proton since side chain pKa is 4.2 ,hence net charge is zero , in histidine side chain pKa is 6 where it donates a proton and becomes neutral,Trp,ser,gly and leu do not have side chains and their amino and carboxylic groups are involved in peptide bonds hence they do not carry a charge, arginine has a side chain whose pKa is 12.4 hence at pH 7 it will be positively charged, pro again doesn't have a side chain and is thus neutral, in the last glycine residue the amino group is involved in the peptide bond and the carboxylic group is negatively charged at pH 7 .This cancels out the positive charge on arginine hence the peptide is neutral at pH 7.

At pH 11, the amino group of glu is neutral and the side chain carboxyl group is negatively charged, histidine also donates a proton from the side chain and is neutral,trp,ser,gly and leu are neutral,since arginine has a side chain with pKa 12.4 it still has a positive charge on its side chain at pH 11, which cancels out the negative charge on the first glu residue.Pro is neutral and the free carboxyl group of gly is negatively charged at pH 11 as it has donated its proton. Hence net charge on the peptide at pH 11 is -1.

I H₂N-ċ-C-N-c-c-N-c-c - K-¢-COOH CH2 СН. - 5 - 5 CH₂ OH Se G - sen