Amino acids

Proteins are the most abundant biomolecules in living systems. Milk, cheese, pulses, peanuts, fish and meat are rich sources. The name comes from Greek proteios, of prime importance. Every protein is a polymer of α-amino acids.

Amino acids carry –NH₂ and –COOH. By where the amino group sits relative to the carboxyl they are α, β, γ, δ and so on; protein hydrolysis gives only α-amino acids, R–CH(NH₂)–COOH, where R is the side chain.

They have trivial names and three-letter (sometimes one-letter) symbols. Glycine (Gly, G) is named for its sweet taste (Greek glykos) and tyrosine (Tyr, Y) for cheese (Greek tyros). Table 10.2 lists twenty.

Side chains from Table 10.2: glycine H; alanine –CH₃; serine HO–CH₂–; cysteine HS–CH₂–; lysine H₂N–(CH₂)₄–; aspartic acid HOOC–CH₂–; phenylalanine C₆H₅–CH₂–.

Classification by charge-bearing groups: equal numbers of amino and carboxyl groups make a neutral amino acid, more amino groups a basic one (lysine, arginine), more carboxyl groups an acidic one (aspartic, glutamic acid).

Non-essential amino acids can be made in the body; essential ones cannot and must come from food. The ten essential ones in Table 10.2 are valine, leucine, isoleucine, arginine, lysine, threonine, methionine, phenylalanine, tryptophan and histidine.

Amino acids are colourless, crystalline, water-soluble solids with high melting points, behaving more like salts than like simple amines or acids.

Reason: in water the –COOH gives up a proton and the –NH₂ takes one, forming a dipolar zwitter ion, H₃N⁺–CHR–COO⁻. It is neutral overall but carries both charges, and it reacts with acids and bases alike: it is amphoteric.

Every naturally occurring α-amino acid except glycine (R = H) has an asymmetric α-carbon and is optically active. Both D and L forms exist, but most natural ones are L, drawn with –NH₂ on the left.