Polypeptide Net charge

[crazyasian]

So I'm reviewing a BR CBT test 4 and I have trouble with the explanation for this answer and would like some clarification. The question goes "at pH=7 the charge on the eleven-amino acid polypeptide would be:"
Fragment A: Ser-Thr-Gly-Met
Fragment B: Phe-Ala-Pro-Trp
Fragment C: Met-Thr-Cys
The explanation says since there are no acidic amino acids, the net charge will be the terminal ends amino and carbonyl ends of the polypeptide. So does that mean if we had acidic side chains or basic side chains we would add those together? ot do we still only take the terminal ends? Thanks.

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[bullsfan1986]

Ya, I think you have the right reasoning. Depending on whether you have basic or acidic amino acids you would just take the net charge of the amino acid side chains (and terminal ends) of the entire polypeptide depending on the pH of the solution the peptide is in. I'm pretty confident the MCAT will give you all relevant pka's.