Consider the peptide: NH 2 -GAPAGPAGTGKTETTKDLAKSMALLCWFNCS-COOH @ pH 9, net cha

Question

Consider the peptide:
NH2-GAPAGPAGTGKTETTKDLAKSMALLCWFNCS-COOH

@ pH 9, net charge is not 0 or -1

@ pH 12, net charge is not -3

Consider the peptide: NH2-GAPAGPAGTGKTETTKDLAKSMALLCWFNCS-COOH Table I. pK values of some amino acids pK of side chain or terminal group uncharged 12.5 uncharged 3.9 8.3 4.3 uncharged 6.0 uncharged 10.8 uncharged uncharged uncharged 10.9 uncharged 7.8 3.6 amino acid Alanine (Ala or A) Arginine (Arg or R) Asparagine (Asn or N) Aspartic acid (Asp or D) Cysteine (Cys or C) Glutamic acid (Glu or E) Glycine (Gly or G) Histidine (His or H) Leucine (Leu or L) Lysine (Lys or K) Phenylalanine (Phe or F) Serine (Ser or S) Tryptophan (Trp or W) Tyrosine (Tyr or Y) Valine (Val or V) peptide N terminus peptide C terminus Using Table I above, determine the net charge of the peptide at pH7. Submit Answer Tries 0/3 Determine the net charge of the peptide at pH9 -1 Submit Answer Incorrect. Tries 1/3 Previous Tries Determine the net charge of the peptide at pH12 Submit Answer Tries 0/3

Explanation / Answer

The peptide sequence is NH2-GAPAGPAGTGKTETTKDLAKSMALLCWFNCS-COOH

Using three letter code we can rewrite the peptide sequence,

NH2-Gly-Ala-Pro-Ala-Gly-Pro-Ala-Gly-Thr-Gly-Lys-Thr-Glu-Thr-Thr-Lys-Asp-Leu-Ala-Lys-Ser-Met-Ala-Leu-Leu-Cys-Trp-Phe-Asn-Cys-Ser-COOH

Determination of the net charge of the peptide at pH 7:

At very low pH, acid-base groups are fully protonated and at very high pH, these groups are unprotonated. At intermediate pH values, the basic groups tend to be protonated and the acidic groups tend to be unprotonated.

Therefore at pH=7, the amino-terminal of the peptide will be in the ionized form because its pKa value is greater than the pH value. This group will carry +1 charge. Because of the same reason, at pH=7, the ionizable group of the side chain of three Lys will carry +3 charge.

At pH=7, the carboxy-terminal group will be in the ionized form because its pKa value is less than the pH value. It will carry -1 charge. Because of the same reason, the ionizable group of the side chain of Asp will carry -1 charge and the ionizable group of the side chain of Glu will carry -1 charge.

The net charge of the peptide at pH 7= {(+1)+(+3)+(-1)+(-1)+(-1)}= +1. [Answer]

Determination of the net charge of the peptide at pH 9:

Because of the above-mentioned reason, at pH 9 the amino-terminal of the peptide will be in the unionized form. So, this group will not carry any charge. The ionize group of the side chain of three Lys will carry +3 charge.

At pH 9, the carboxy-terminal group of the peptide, the ionized group of the side chain of Asp, the ionized group of the side chain of Glu will carry -1 charge each, and the ionized group of two Cys will carry -2 charge.

The net charge of the peptide at pH 9= {(+3)+(-1)+(-1)+(-1)+(-2)}= -2. [Answer]

Determination of the net charge of the peptide at pH 12:

Again because of the above-mentioned reason, at pH 12 the amino-terminal of the peptide and the ionized group of the side chain of three Lys both will be in the unionized form. They will not carry any charge.

At pH 12, the carboxy-terminal group of the peptide, the ionized group of the side chain of Asp, the ionized group of the side chain of Glu will carry -1 charge each, and the ionized group of two Cys will carry -2 charge.

The net charge of the peptide at pH 12= {(-1)+(-1)+(-1)+(-2)}= -5. [Answer]

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