Amino acids: Protein building blocks

All the proteins we know are formed from the union of 20 amino acids, which are the most basic and elemental structures of proteins. These are formed by a carbon atom in position a (immediately next to the carboxyl), to which a carboxyl acid group (-COOH) and a basic amino acid are joined (-NH2). The other two valences of the carbon are saturated with a hydrogen atom (H) and a radical (R) chain giving specificity to the range of proteins.

Different chains give rise to different amino acids:

• Aliphatic, which have a hydrocarbonated chain (glycine, alanine, isoleucine, leucine and valine; leucine, isoleucine and valine form part of the branched chain, all are essential.
• Aromatic: these have an aromatic ring, and include phenylalanine, tryptophan and tyrosine.
• Sulphured, because they contain sulphur (S) in their chain (methionine and cystine).
• Hydroxylates, because they contain a hydroxyl group (OH) (serine and threonine).
• Basic: lysine, arginine, histidine.
• Acids: aspartic acid, glutamic acid.

 

Of the 20 amino acids in the complete range of proteins, eight are considered essential amino acids in human beings (leucine, isoleucine, tryptophan, phenylalanine, methionine, lysine, valine, threonine) and in babies up to six months old, histidine. Some studies suggest that this is also essential for adults.

The term essential refers to a substance which cannot be synthesised or manufactured in the body because it does not have precursors and must be taken in food. In some physiological situations (such as metabolic stress) some of the 20 amino acids may become essential or conditionally essential (proline, serine, arginine, tyrosine, cystine, taurine, glycine and glutamine).

 

The biological quality or value of a protein is the capacity of a food protein to contribute all the essential amino acids in the required quantities for the formation of our own proteins.

 

When an essential amino acid is not in the required quantity it is said to be a limiting amino acid because it limits the quality of this protein, i.e. its use in protein synthesis by our body. This is the case of lysine in cereals or methionine in pulses.