The animal itself is capable of synthesising about half of the amino acids. These amino acids are termed non-essential amino acids. However, about 10 amino acids (depending on the species) can not be synthesised by the animal and a source must be supplied in
the feed (see Table 3). For this reason they are termed the essential
amino acids. A number of non-essential amino acids can only be synthesised
from essential amino acids and are called semi-essential.
Table 3 – Essentiality of amino acids in poultry
Essential : Arginine(1), Histidine, Isoleucine, Leucine, Lysine, Methionine,
Phenylalanine, Threonine, Tryptophan, Valine.
Semi-essential: Cystine (Cysteine)(2), Tyrosine
Non-essential
Alanine, Asparagine, Aspartic Acid, Glutamic Acid, Glutamine,
Glycine, Proline, Serine.
The classification of amino acids into essential and non-essential
should not be taken to imply that non-essential amino acids are not
required for the synthesis of protein. The terminology non-essential
simply indicates that the animal is able to synthesise these non-essential
amino acids or convert them from one amino acid into another.
To undertake such amino acid inter-conversions the animal requires
sources of carbohydrates and suitable nitrogen compounds.
Sulphur containing amino acids
The two sulphur containing amino acids methionine and cysteine
each contain a sulphur atom and they are present in animal and plant
proteins in varying proportions. Methionine is an essential amino
acid whereas cysteine is semi-essential. Depending on the species
of animal cysteine may be responsible for up to 50% of the dietary
methionine requirement. Scientific findings show that this proportion
is less than 50% in high performance animals. In addition to its essential
role as a protein building block and precursor of cysteine, methionine
is also involved in a number of other biosynthetic pathways.
Methionine is involved in metabolic pathways of compounds such as
choline, creatine and adrenaline via the release of a methyl group and
the formation of S-adenosyl-methionine in the methyl donor pathway.
Thus methionine indirectly plays an important role in intermediate metabolism.
In the organism, cysteine is produced from methionine via
S-adenosyl-methionine-cystathionine. Cysteine is subsequently further
metabolised into taurine or via a number of intermediate stages
transformed into sulphate.
Basic and aromatic amino acids
Basic amino acids are listed in Table 2. Amongst those, lysine is one of the key essential amino acids, while arginine is essential only in very young animals.
Tryptophan, phenylalanine and tyrosine are aromatic amino acids.
Besides being a constituent of body protein, tryptophan plays other
important roles in metabolism. Tryptophan is involved in feed intake
regulation, in the immune response and in coping with stress situations.
Phenylalanine and tyrosine are interchangeable. The total requirement
for phenylalanine and tyrosine can be met by phenylalanine
alone but the converse does not occur for tyrosine. The limited
evidence available indicates that tyrosine can only provide part of the
requirement for phenylalanine.