What are amino acids? Chemical Structure
Amino acids are the primary structural unit of proteins. We can therefore imagine amino acids as bricks that, united by a glue called peptide bond, form a long sequence that gives rise to a protein.
Inside the stomach and duodenum these bonds are broken and the individual amino acids reach the small intestine where they are absorbed as such and used by the body.
By a chemical point of view, the amino acid is an organic compound containing a carboxylic group (COOH) and an amine group (NH2). In addition to these two groups, each amino acid is distinguished from the others by the presence of a residue (R) also known as the amino acid side chain.
Classification of amino acids
Only twenty of the different amino acids existing in nature (over five hundred) are involved in protein synthesis. From a nutritional point of view, these amino acids can be divided into two main groups:
essential amino acids and non-essential amino acids.
Essential amino acids are defined as those amino acids that the human organism cannot synthesise in sufficient quantity to meet its needs. For the adult there are eight and more precisely: phenylalanine, isoleucine, lysine, leucine, methionine, threonine, tryptophan and valine.
During the growth period, a ninth, histidine, should be added to the eight mentioned above, because during this period the demand for this amino acid is higher than its synthesis capacity.
Cysteine and tyrosine are considered semi-essential amino acids, as the body can synthesise them from methionine and phenylalanine.
Conditionally essential amino acids (arginine, glycine, glutamine, proline and taurine) are defined as those amino acids that play a fundamental role in maintaining homeostasis and the functions of the body in certain physiological situations. Under certain pathological conditions, these amino acids may not be synthesised fast enough to meet the real needs of the body.
Arginine is becoming very important as a precursor of nitric oxide because of the many functions it performs in cellular activity, transduction of biological signals and immune defence.
ESSENTIAL AMINOACIDS: we can define as complete or noble those proteins that contain all the essential AA in quantity and in balanced ratios.
In general, animal proteins are complete and plant proteins are incomplete.
The term noble associated with vegetable proteins is incorrect and has been introduced to counter the saying that “legumes are the meat of the poor“. In reality, assuming a fair source of vegetable protein in the diet is very important and to further enhance this concept, the term “noble” has been improperly introduced. These deficiencies can be overcome simply by using appropriate food associations such as PASTA and BEANS.
In this case we speak of mutual integration because the amino acids lacking in pasta are supplied by beans and vice versa.
LIMITANT AMINOACID: a protein mixture is the essential amino acid deficient or absent that limits the use of all other amino acids even if present over needs. As we have seen in proteins of vegetable origin, this amino acid is not sufficient to guarantee the needs and must be introduced through the combination with other foods.
CHEMICAL INDEX: it is given by the ratio between the quantity of a given amino acid in one gram of the protein under examination and the quantity of the same amino acid in one gram of the biological reference prwotein (of the egg). The higher this index is, the higher will be the percentage of essential amino acids.
RAMIFIED AMINOACIDS: or BCAA are three essential amino acids (Valine, Isoleucine and Leucine) that under particular conditions, such as intense physical activity, an auxiliary energy substrate of fats and carbohydrates.
Functions of amino acids
The primary function of amino acids is to intervene in protein synthesis, which is necessary to cope with the body’s cellular renewal processes.
Besides this function, called “plastic“, amino acids also have a modest but not negligible importance in energy production (branched amino acids).
Some amino acids are also precursors of compounds that perform important biological functions.
From tryptophan we get niacin (vitamin PP), serotonin (neurotransmitter) and melatonin (regulator of circadian sleep/wake cycle rhythms).
From sulphurous amino acids (methionine and cysteine) we get glutathione, an important antioxidant useful to fight free radicals and keratin, an essential protein for the health of hair, hair and nails.
In addition to those involved in protein synthesis, many other amino acids perform very important functions. Among these, the best known in the sports field are creatine (useful to increase capacity and anaerobic power and lactacidic acid) and carnitine, which facilitates the transport of lipids within the mitochondrion).