Step 1: Determine the total positive charge on the peptide (n) when all acidic a

Question

Step 1: Determine the total positive charge on the peptide (n) when all acidic and basic groups are fully protonated. (Enter as a whole number without the sign.)

The isoelectric point (pl) of a peptide is the pH at which the peptide does not migrate in an electric field. Since the peptide is zwitterionic, there are the same number of positive charges as negative charges on the peptide population. The pl can be estimated fairly accurately (within 0.1 or 0.2 pH units) from the pK values of all the proton dissociable groups in the peptide. Using pK values from the table at the right, estimate the pl value of the following hexapeptide: Amino Acid Chain pKK Side Ar Asp Cys Glu IS Lys Tyr peptide- NH3 peptide- 12.5 3.7 8.2 4.3 6.0 10.5 10.5 8.0 Trp-His-Glu-Tyr-Gly-Asp STRATEGY: Step 1: Determine the total positive charge on the peptide (n) when all acidio Step 2: List the pK values of all ácidic and basic groups in order from lowest Step 3: Calculate the pl as the average of the values for pKn), the proton and basic groups are fully protonated (pKi) to highest. dissociation forming a neutral species from a +1 species, and pK(n+-), 3.4 the proton dissociation forming a 1 species from the neutral species COOH Step 1: Determine the total positive charge on the peptide (n) when all acidic and basic groups are fully protonated. (Enter as a whole number without the + sign.) Amino and carboxy terminal values differ from the amino and carboxy values of a single amino acid Number

Explanation / Answer

Step 1:

Trp => one terminal amino group = +1 charge

His => one imidazole ring, proton acted = +1 charge

Glu => one carboxylic group in side chain, protonated = 0 charge

Tyr => no charged side chain = 0 charge

Gly => no charged side chain = 0 charge

Asp => one terminal, one side chain carboxylic group, both protonated = 0 charge.

Total positive charge = 2

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