Essential amino acids

Proteins are biopolymers of acids, so named because the amino group is bonded to the carbon atom, next to the carbonyl group. The physical and chemical properties of a protein are determined by its constituent amino acids.

Amino acids are the fundamental constituents of body proteins and serve as substrates for protein synthesis. There are about 300 amino acids occur in nature. Only 20 of them occur in proteins.

Proteins are synthesized from the set of 20 L-α-amino acids encoded by nucleotide triplets called codons. Common amino acids are those for which at least one specific codon exists in the DNA genetic code.

Humans can synthesize about half of the amino acids needed to make proteins. Other amino acids, called the essential amino acids, must be provided in the diet.

The essential amino acids are arginine (required for the young, but not for adults), histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. These amino acids are required in the diet.

Proteins that provide all the essential amino acids in about the right proportions for human nutrition are called complete proteins. Examples of complete proteins are those in meat, fish, milk, and eggs. About 50 g of complete protein per day is adequate for adult humans.
Essential amino acids