Amino acids are generally organic molecules containing carboxyl (COOH) and amino (NH2) group. They can be sequentially linked to peptides (up to about 50 amino acids) and polypeptides (more). Twenty of amino acids (called proteinogenic) constitute building material for proteins (protein is one or more linked polypeptides). Each of these 20 amino acids also corresponds to a particular DNA code.
Proteinogenic amino acids are:
- branched-chain essential (BCAA): leucine, isoleucine, valine (3pcs) – they are often used by athletes to protect muscle mass
- other essential: phenylalanine, tryptophan, lysine, methionine, threonine, plus histidine is sometimes mentioned (total 6pcs)
- conditionally (under special conditions) essential: arginine, cysteine, glycine, glutamine, proline, tyrosine (6pcs)
- nonessential: alanine, aspartic acid, asparagine, glutamic acid, serine (5pcs)
The 21st amino acid selenocysteine (although not directly encoded in DNA) also plays a proteinogenic role in humans. In some other organisms there is pyrrolysin (22) and, in some bacteria also n-formylmethionine (23). Therefore, sometimes the number of proteinogenic amino acids is mentioned from 21 to 23.
From a nutritional point of view, every food (protein source) has its own amino acid profile, which tells us what amino acids (especially essentials) does the food contain and how much of it does the food contain. There are two important terms related to amino acid profile:
- completeness (whether it contains all the essential amino acids or some are missing) – in general all animal sources (plus soy and quinoa) are complete, the varied plant diet is also OK. However, with increased demand for protein intake, it is appropriate to combine plant sources (eg cereals, nuts, and seeds are poorer in lysine and legumes have lower tryptophan and methionine levels). See also the usability below
- usability (what proportion of the consumed proteins can the body really use) – if the amino acid profile is too “scattered” (a large excess of some essential AK on the one hand and lack on the other), the body can use the food only to the extent that it corresponds to the amino acids in lower concentrations (the others are “discarded”). Therefore, even complete sources may have low usability. The most usable are animal foods, somewhat less certain plant combinations (cereals or nuts and seeds combined with legumes)
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