Question 18.1: Determining Side-Chain Hydrophobicity/Hydrophilicity Conside......
Determining Side-Chain Hydrophobicity/Hydrophilicity
Consider the structures of phenylalanine and serine in Table 18.3. Which of these two amino acids has a hydrophobic side chain and which has a hydrophilic side chain?
ANALYSIS Identify the side chains. The side chain in phenylalanine is an alkane. The side chain in serine contains a hydroxyl group.
TABLE 18.3 The 20 𝛂-amino acids found in proteins, with their abbreviations and isoelectric points. The structures are written here in their fully ionized forms. These ions and the isoelectric points given in parentheses are explained in Section 18.4.
| Nonpolar , Neutral Side Chains | |
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\begin{matrix} H O\\ H_3\overset{+}{N} — \overset{\mid }{C} — \overset{\mid\mid }{C} — O^-\\ \overset{\mid}{C}H_3\end{matrix} \begin{matrix} H O\\ H_3\overset{+}{N} — \overset{\mid }{C} — \overset{\mid\mid }{C} — O^-\\ \overset{\mid}{H} \end{matrix} \begin{matrix} H O\\ H_3\overset{+}{N} — \overset{\mid }{C} — \overset{\mid\mid }{C} — O^-\\ \overset{\mid}{C}HCH_2 CH_3\\ \overset{\mid}{C}H_3\end{matrix} \begin{matrix} H O\\ H_3\overset{+}{N} — \overset{\mid }{C} — \overset{\mid\mid }{C} — O^-\\ \overset{\mid}{C}H_2CHCH_3\\ \overset{\mid}{C}H_3\end{matrix} Alanine, Ala (6.0) Glycine, Gly (6.0) Isoleucine, Ile (6.0) Leucine, Leu (6.0) \begin{matrix} H O\\ H_3\overset{+}{N} — \overset{\mid }{C} — \overset{\mid\mid }{C} — O^-\\ \overset{\mid}{C}H_2CH_2SCH_3\\ \\ \\ \end{matrix} Methionine, Met (5.7) Phenylalanine, Phe (5.5) Proline, Pro (6.3) Tryptophan, Trp (5.9) Valine, Val (6.0) |
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| Polar , Neutral Side Chains | |
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\begin{matrix} H O\\ H_3\overset{+}{N} — \overset{\mid }{C} — \overset{\mid\mid }{C} — O^-\\ \overset{\mid}{C}H_2C — NH_2\\ \overset{\mid\mid}{O}\end{matrix} \begin{matrix} H O\\ H_3\overset{+}{N} — \overset{\mid }{C} — \overset{\mid\mid }{C} — O^-\\ \overset{\mid}{C}H_2\\ \overset{\mid}{S}H \end{matrix} \begin{matrix} H O\\ H_3\overset{+}{N} —\overset{\mid }{C} — \overset{\mid\mid }{C} — O^-\\ \overset{\mid}{C}H_2CH_2 C — NH_2\\ \overset{\mid\mid}{O}\end{matrix} \begin{matrix} H O\\ H_3\overset{+}{N} — \overset{\mid }{C} — \overset{\mid\mid }{C} — O^-\\ \overset{\mid}{C}H_2\\ \overset{\mid}{O}H\end{matrix} Asparagine, Asn (5.4) Cysteine, Cys (5.0) Glutamine, Gln (5.7) Serine, Ser (5.7) \begin{matrix} H O\\ H_3\overset{+}{N} — \overset{\mid }{C} — \overset{\mid\mid }{C} — O^-\\ \overset{\mid}{C}HCH_3\\ \overset{\mid}{O}H\\ \\ \\ \\ \\ \end{matrix} Threonine, Thr (5.6) Tyrosine, Tyr (5.7) |
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| Acidic Side Chains | Basic Side Chains |
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\begin{matrix} H O\\ H_3\overset{+}{N} — \overset{\mid }{C} — \overset{\mid\mid }{C} — O^-\\ \overset{\mid}{C}H_2C — O^-\\ \overset{\mid\mid}{O}\end{matrix} Aspartic acid, Asp (3.0) (Aspartate) \begin{matrix} H O\\ H_3\overset{+}{N} — \overset{\mid }{C} — \overset{\mid\mid }{C} — O^-\\ \overset{\mid}{C}H_2CH_2C — O^-\\ \overset{\mid\mid}{O}\end{matrix} Glutamic acid, Glu (3.2) |
\begin{matrix}H O \\ H_3\overset{+}{N} — \overset{\mid }{C} — \overset{\mid\mid }{C} — O^- NH_2^+\\ \overset{\mid}{C}H_2CH_2CH_2NH\overset{\mid\mid}{C}NH_2\\ \\ \\ \\ \end{matrix} Arginine, Arg (10.8) Histidine, His (7.6) \begin{matrix} H O\\ H_3\overset{+}{N} — \overset{\mid }{C} — \overset{\mid\mid }{C} — O^-\\ \overset{\mid}{C}H_2CH_2CH_2CH_2\overset{+}{N}H_3\end{matrix}
Lysine, Lys (9.7) |
The hydrocarbon side chain in phenylalanine is an alkane, which is nonpolar and hydrophobic. The hydroxyl group in the side chain of serine is polar and is therefore hydrophilic.