Amino acids: classification and properties
Protein is built from 20 standard amino acids, each with an amino group and a carboxyl group. They are grouped by side chain, nutritional need and metabolic fate.
By side chain (polarity)
| Group | What the side chain (R group) is like | Examples |
|---|---|---|
| Non-polar (hydrophobic) | Avoids water; tucked inside the folded protein | Alanine, valine, leucine, isoleucine, methionine, phenylalanine, tryptophan, proline |
| Polar, no charge | Likes water; carries –OH, –SH or amide groups | Serine, threonine, cysteine, tyrosine, asparagine, glutamine |
| Polar, positive (basic) | Has an extra basic (nitrogen) group; tends to carry a positive charge | Lysine, arginine, histidine |
| Polar, negative (acidic) | Has an extra carboxyl group; carries a negative charge at body pH | Aspartic acid, glutamic acid |
By nutrition and by metabolic fate
| Basis | Group | Amino acids |
|---|---|---|
| Nutrition | Essential: the body cannot make them, so food must supply them | Valine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan (Lippincott and WHO also count histidine, giving 9) |
| Nutrition | Semi-essential: adults make enough, growing children do not | Arginine, histidine |
| Nutrition | Non-essential: the body makes them from other compounds | Glycine, alanine, serine, aspartic acid, glutamic acid, proline and others |
| Metabolic fate | Ketogenic only: form acetyl-CoA or acetoacetate (ketone bodies, fat), never glucose | Leucine, lysine |
| Metabolic fate | Both glucogenic and ketogenic | Isoleucine, phenylalanine, tyrosine, tryptophan |
| Metabolic fate | Glucogenic only: the carbon skeleton can be turned into glucose | All the remaining 14, e.g., alanine, glycine, serine, aspartate, glutamate, methionine, valine |
Chemical properties
Both groups together make the peptide bond: the –COOH of one amino acid joins the –NH₂ of the next, releasing water.
Zwitterion and isoelectric point
In water, the amino group picks up H⁺ (–NH₃⁺) and the carboxyl group loses it (–COO⁻). One molecule now carries both charges. This dipolar ion is called a zwitterion. The pH at which the two charges balance (net charge zero) is the isoelectric point (pI). At its pI an amino acid or protein does not move in an electric field and is least soluble. Kitchen example: when milk sours into curd, lactic acid brings the pH down towards the pI of casein (about 4.6), so casein sets.
Cereals such as rice and wheat are low in lysine. Pulses (dal) are low in methionine but rich in lysine. Eaten together, as in idli, dosa, pongal, khichdi or rice with sambar, each fills the other's gap. This mutual supplementation raises protein quality cheaply. A little milk or curd helps further.
2 marks: “What is a zwitterion?” An amino acid carrying both –NH₃⁺ and –COO⁻ at once; its net charge is zero at the isoelectric point.
5 marks: “Classify amino acids based on nutritional requirement, with examples.” Essential, semi-essential and non-essential with examples; end with lysine in cereals, methionine in pulses and cereal–pulse supplementation.
10 marks: “Classify amino acids and explain their chemical properties.” Three classification tables (side chain, nutrition, metabolic fate), then reactions of the amino and carboxyl groups and the zwitterion.
"PVT TIM HALL": Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Arginine, Leucine, Lysine. Arginine and histidine are the semi-essential pair. Purely ketogenic? Just the two Ls: leucine and lysine.