the substances of which protein is made. Twenty-four of them are known to be involved in the synthesis of protein, of which 20 are important. These are glycine, alanine, proline, valine, leucine, isoleucine, methionine, phenylalanine, tryptophan, aspartic acid, glutamic acid, tyrosine, cysteine, serine, threonine, asparagine, glutamine, histidine, lysine, and arginine. The names in italics are those of the 10 ‘essential’ amino acids for humans. These cannot be made in the body, and have to form part of the diet. The others can be made by altering other amino acids. Other animals have different sets of essential acids.
Amino acids have part of their chemical structure in common. Their general structure is R-CH-(NH2)-COOH, where R is a group of atoms which varies from one acid to another; in the simplest, valine, this is simply one hydrogen atom. The -NH2 group is known as an amino group, the -COOH group as a carboxyl group; the latter is a characteristic part of organic acids.
Proteins consist of chains of linked amino acids. When one acid is joined to another the amino group of one acid loses one hydrogen atom, and the carboxyl group of the other acid loses one hydrogen and one oxygen atom. The two groups then link thus: -NH-CO-. This is known as a ‘peptide bond’. The two hydrogen and one oxygen atoms join to form a molecule of water, H2O.
The peptide bond can be broken by putting the two parts of the water molecule back in, so that the amino and carboxyl groups resume their original form. This is known as hydrolysis, and is fundamental to the cooking and digestion of protein.
The molecules of living things are ‘chiral’—they can exist in either of two forms which are mirror images of each other, known as ‘laevo-’ (left-handed, L for short) and ‘dextro-’ (right-handed, D) forms. As a rule only L forms are found in nature, and this applies to all amino acids except glycine, which is too simple to have more than one form.
The food industry uses some amino acids on their own. L-glutamic acid and its salt monosodium glutamate are both used as flavour enhancers. The sweetener aspartame is made from aspartic acid.
Ralph Hancock is an encyclopedist with a special interest in food history and food science.
