Question 28.1: Naming a Polypeptide What is the name of the polypeptide who...
Naming a Polypeptide
What is the name of the polypeptide whose structure is shown below?
\begin{matrix} \ \ \ \ \quad\quad\quad\quad\quad \underset{||}{O} \ \ \quad\quad\quad\quad\quad\quad \ \ \underset{||}{O} \quad\quad\quad\quad \quad\quad \ \ \ \ \ \underset{||}{O} \ \ \ \ \quad \\ H_2N-CH_2-C-NH-CH-C-NH-CH-C-OH \\\qquad \qquad \qquad \quad\overset{|}{\qquad CH_3}\quad\quad \ \ \overset{|}{\quad \qquad CH_2OH} \ \\ \quad\quad\quad\quad (a) \quad\quad\quad\quad\quad\quad \ \ (b) \quad\quad\quad\quad\quad\quad\quad (c) \end{matrix}
Analyze
We can identify the three amino acids in this tripeptide by using Table 28.3: (a) = glycine; (b) = alanine; and (c) = serine. The C-terminal amino acid is serine.
| TABLE 28.3 Some Common Amino Acids | |||
| Name | Symbol | Formula | pI^a |
| Neutral Amino Acids | |||
| Glycine | Gly | HCH \left( NH _2\right) CO _2 H | 6.03 |
| Alanine | Ala | CH _3 CH \left( NH _2\right) CO _2 H | 6.10 |
| \text{Valine}^b | Val | \left( CH _3\right)_2 CHCH \left( NH _2\right) CO _2 H | 6.04 |
| \text{Leucine}^b | Leu | \left( CH _3\right)_2 CHCH _2 CH \left( NH _2\right) CO _2 H | 6.04 |
| \text{Isoleucine}^b | Ileu or Ile | CH _3 CH _2 CH \left( CH _3\right) CH \left( NH _2\right) CO _2 H | 6.04 |
| Serine | Ser | HOCH _2 CH \left( NH _2\right) CO _2 H | 5.70 |
| \text{Threonine}^b | Thr | CH _3 CH ( OH ) CH \left( NH _2\right) CO _2 H | 5.6 |
| \text{Phenylalanine}^b | Phe | C _6 H _5 CH _2 CH \left( NH _2\right) CO _2 H | 5.74 |
| \text{Methionine}^b | Met | CH _3 SCH _2 CH _2 CH \left( NH _2\right) CO _2 H | 5.71 |
| Cysteine | Cys | HSCH _2 CH \left( NH _2\right) CO _2 H | 5.05 |
| Cystine | (Cys)_2 | \left[ SCH _2 CH \left( NH _2\right) CO _2 H \right]_2 | 5.1 |
| Tyrosine | Tyr | 4- HOC _6 H _4 CH _2 CH \left( NH _2\right) CO _2 H | 5.70 |
| \text{Tryptophan}^b | Trp |  |
5.89 |
| \text{Proline}^c | Pro |  |
6.21 |
| Acidic Amino Acids | |||
| Aspartic acid | Asp | HO _2 CCH _2 CH \left( NH _2\right) CO _2 H | 2.96 |
| Glutamic acid | Glu | HO _2 CCH _2 CH _2 CH \left( NH _2\right) CO _2 H | 3.22 |
| Basic Amino Acids | |||
| \text{Lysine}^b | Lys | H _2 N \left( CH _2\right){ }_4 CH \left( NH _2\right) CO _2 H | 9.74 |
| Arginine | Arg | H _2 NC (= NH ) NH \left( CH _2\right)_3 CH \left( NH _2\right) CO _2 H | 10.73 |
| Histidine | His |  |
7.58 |
_{}^{a}\text{pH} of isoelectric point.
_{}^{b}\text{Essential} amino acids. In addition, arginine and glycine are required by the chick, arginine by the rat, and histidine by human infants.
_{}^{c}\text{The} secondary amino group makes proline an \alpha-imino acid. Nevertheless, it is commonly listed with amino acids.
The name is glycylalanylserine (Gly-Ala-Ser).
Assess
When naming polypeptides, we always begin from the N-terminal and finish at the C-terminal. Remember to replace the –ine with –yl for all the amino acids except the last one, which retains the –ine.