Answer the following seven questions (a through g) based upon a tripeptide seque
ID: 578711 • Letter: A
Question
Answer the following seven questions (a through g) based upon a tripeptide sequence with the following amino acids (one-letter symbols are given): Q V R a) What are the full names of the amino acids? Spelling counts, but capitalization does not. For an acidic amino acid, assume it is ionized and use the - ate ending instead of - ic acid. Q: V: R: bJ What are the three-eter abreviations for the amino acids? Q: V: R: c) What is the overall charge of the most abundant tripeptide species at neutral pH (pH 7.0)? Number Enter your answer as a whole number. d) At physiological pH, what is the best description of the chemical properties of the side chain (R group)? Q Selec answer Select anowerRSolet anawer Q: Select answer V: Select answer continued below... e) What is the overall charge of the most abundant tripeptide species below pH 1.0?Explanation / Answer
a) The one letter codes are given; we identify the amino acids as
Q = Glutamine
V = Valine
R = Arginine
b) The three letter codes are easily obtained.
Q = Glu
V = Val
R = Arg
c) Determine the pKas of the ionizable groups of the amino acids as shown in the table below.
Amino Acid
pKa of alpha carboxyl group
pKa of alpha amino group
pKa of side chain
pI of amino acid
Q
2.17
9.13
5.65
V
2.32
9.62
5.96
R
2.17
9.04
12.48
10.76
The alpha amino group of Q and the alpha carboxyl group of R are free at pH 7.0. The side chain of R is free at pH 7.0.
At pH 7.0, the alpha carboxyl group of R is deprotonated (carries charge 1-) while the alpha amino and side amino groups of Q and R are protonated (hence carries charge 1+). The overall charge is (-1 + 1 + 1) = 1+.
d) At physiological pH, the side chain (amino side chain) of R is protonated and hence, carries charge 1+. The side chain is positively charged and carries an extra proton.
e) Below pH 1.0, all the ionizable groups are protonated (hence, the alpha carboxyl group carries charge 0) and hence, the overall charge is (0 + 1 + 1) = 2+.
f) At pH 11.5, all the ionizable groups are deprotonated (hence, the amino groups carry charge 0) and hence, the net charge on the tripeptide is (-1 + 0 + 0) = 1-.
Amino Acid
pKa of alpha carboxyl group
pKa of alpha amino group
pKa of side chain
pI of amino acid
Q
2.17
9.13
5.65
V
2.32
9.62
5.96
R
2.17
9.04
12.48
10.76
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