Amino acids and their classification
α-Amino acids carry an amino group and a carboxyl group on the same carbon. In water they exist as zwitterions, so they behave like salts.
What amino acids are
Amino acids contain an amino group (-NH₂) and a carboxyl group (-COOH). Proteins on hydrolysis give only α-amino acids, in which both groups are joined to the same carbon atom: R-CH(NH₂)-COOH. The side chain R is different in each amino acid. Each has a common name, a three-letter symbol and a one-letter code: glycine (Gly, G) has R = H, and alanine (Ala, A) has R = CH₃.
Classification of amino acids
| Basis | Class | Examples |
|---|---|---|
| Number of NH₂ and COOH groups | Neutral: equal numbers | Glycine, alanine, valine |
| Number of NH₂ and COOH groups | Acidic: more COOH than NH₂ | Aspartic acid, glutamic acid |
| Number of NH₂ and COOH groups | Basic: more NH₂ than COOH | Lysine, arginine |
| Need in the diet | Essential: the body cannot make them | Valine, leucine, isoleucine, threonine, methionine, phenylalanine, tryptophan, lysine, arginine, histidine |
| Need in the diet | Non-essential: the body can make them | Glycine, alanine, serine, glutamic acid |
Some amino acids to know
| Name | Symbol and code | Side chain R |
|---|---|---|
| Glycine | Gly, G | H |
| Alanine | Ala, A | CH₃ |
| Valine | Val, V | (CH₃)₂CH- |
| Serine | Ser, S | HOCH₂- |
| Cysteine | Cys, C | HSCH₂- |
| Phenylalanine | Phe, F | C₆H₅CH₂- |
| Tyrosine | Tyr, Y | p-HO-C₆H₄-CH₂- |
| Lysine | Lys, K | H₂N(CH₂)₄- |
| Glutamic acid | Glu, E | HOOC-CH₂CH₂- |
Zwitterion and amphoteric behaviour
- 1H₂N-CH(R)-COOH ⇌ H₃N⁺-CH(R)-COO⁻In water, COOH loses a proton and NH₂ gains one: a dipolar ion (zwitterion).
- 2H₃N⁺-CH(R)-COO⁻ + H⁺ → H₃N⁺-CH(R)-COOHWith an acid, the COO⁻ group accepts a proton.
- 3H₃N⁺-CH(R)-COO⁻ + OH⁻ → H₂N-CH(R)-COO⁻ + H₂OWith a base, the NH₃⁺ group gives up a proton.
- 4★So amino acids are amphoteric: they react with both acids and bases.Being ionic, they are colourless crystalline solids with high melting points, soluble in water, and behave like salts rather than like simple amines or acids.
★ key step
Optical activity of amino acids
In every α-amino acid except glycine (R = H), the α-carbon carries four different groups, so it is asymmetric. These amino acids are optically active and exist in D and L forms. Most natural amino acids have the L-configuration. In the Fischer projection of an L-amino acid, with COOH at the top, the NH₂ group is written on the left.
Dal and rice eaten together supply more essential amino acids than either alone: pulses are rich in lysine, and cereals provide more methionine. That is why dal-chawal and idli make good vegetarian protein meals.
1 marks: “Which α-amino acid is optically inactive, and why?” Glycine; its α-carbon carries two H atoms, so it is not asymmetric.
2 marks: “Lysine is called an essential amino acid, but glycine is not. Explain, and name one more amino acid of each type.” Essential amino acids cannot be made in the body and must come from food (lysine, valine). Non-essential ones are made in the body (glycine, alanine).
2 marks: “What is a zwitterion? Show how glycine in its zwitterion form reacts with an acid and with a base.” A dipolar ion: H₃N⁺-CH₂-COO⁻. With H⁺: H₃N⁺-CH₂-COOH. With OH⁻: H₂N-CH₂-COO⁻ + H₂O.
Ten essentials: 'PVT TIM HALL': phenylalanine, valine, threonine, tryptophan, isoleucine, methionine, histidine, arginine, leucine, lysine.