Amino Acids & Proteins

Amino acids are the building blocks of proteins. Every amino acid shares the same basic structure: a central carbon (the alpha carbon) bonded to four groups — an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom, and a variable side chain called the R group. It is the R group that makes each amino acid unique. R groups differ in size, shape, charge, and chemical reactivity. There are 20 standard amino acids used to build proteins in the human body. Amino acids are classified by their R groups: Non-polar (hydrophobic) — e.g. glycine, alanine, valine, leucine, phenylalanine. Found buried in the interior of proteins away from water. Polar (uncharged) — e.g. serine, threonine, asparagine, glutamine. Can form hydrogen bonds with water. Charged (acidic) — e.g. aspartate, glutamate. Carry a negative charge at physiological pH. Charged (basic) — e.g. lysine, arginine, histidine. Carry a positive charge at physiological pH. Special cases — cysteine (forms disulfide bonds), proline (introduces rigid kinks), glycine (smallest — fits anywhere).

Proteins fold into precise 3D shapes that determine their function. There are four levels of structure: Primary (1°) — the sequence of amino acids. A single change here can cause disease (e.g. sickle cell anaemia). Secondary (2°) — local folding into alpha helices or beta sheets, held by hydrogen bonds. Tertiary (3°) — the overall 3D shape, determined by R group interactions including hydrophobic forces, ionic bonds, and disulfide bridges. Quaternary (4°) — multiple polypeptide chains assembled together. Example: haemoglobin (4 subunits).

Of the 20 amino acids, 9 are essential — they cannot be made by the body and must come from food: Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Valine. (Mnemonic: PVT TIM HaLL) Clinically important: PKU (Phenylketonuria) — the body cannot break down phenylalanine. It builds up and damages the brain. Screened at birth. Sickle cell anaemia — just one wrong amino acid in haemoglobin causes red blood cells to deform and block blood vessels.