The term amino acid has been around for years, from pure scientific research and biology classes to sports supplements where people talk about amino acid supplementation.
However, it is unknown what these compounds really are since on many occasions the way to explain it is complex. In this article we will try to explain everything necessary about amino acids in depth and at the same time in a simple and understandable way. Quite a challenge for which we are going.
What are amino acids?
The amino acids are organic molecules, ie, are chemical compounds primarily consist of a group of carbon, called carboxyl group (-COOH), and by a nitrogen group (-NH2), called amino group, a side chain and a hydrogen. In image 2 you can see these joints:
Amino acids differ from each other in the side chain, which is a variable branching of more or less more carbon, nitrogen, or sulfur groups.
What is the function of amino acids?
The best known function of amino acids is to be part of proteins . However, there are hundreds of amino acids while only 22 are those that make up proteins. These are basic for life since they develop all the functions necessary for the survival of living beings.
In addition, being part of proteins they have other functions such as:
- neurotransmitters: glycine, asparagine, glutamine , GABA (derived from g-amino butyric acid), dopamine …
- hormones: thyroxine, triiodothyronine
- intermediates in the formation of other amino acids that are part of proteins
- precursors of other fundamental molecules for the functions of living beings (nucleotides, chlorophyll …)
- and participate in the regulation of metabolic pathways or as intermediaries (carnitine …)
Classification of amino acids
There are different ways to classify them, however, there are three classifications that are the most used:
According to origin: essential / non-essential amino acids
This classification is based on whether or not they are produced in the body. The essential amino acids are those that can not be synthesized in the body and are not essential that the body can be manufactured. Those that are or are not essential depend on each species.
The essential amino acids for humans are: Val, Leu, Thr, Lys, Trp, His *, Ile, Phe, Arg * and Met. Also, tyrosine is considered essential in pregnant women. They are very important since if they are not ingested through the diet, the tissues that die cannot be replaced and therefore can limit the development of the organism.
According to the position of the amino group
It depends on which carbon the amino group is attached to. In amino acids, the carboxyl group called alpha carbon is counted as the first carbon. Starting from the second carbon, the amino group can be linked:
- Alpha-amino acids (α-amino acids): The amino group is located on the second carbon in the chain (as can be seen in image 2). These are what make up proteins. They are encoded in the genetic code in codons .
- Beta-amino acids (β-amino acids): The amino group is located on the third carbon in the chain.
- Gamma-amino acids (γ-amino acids): The amino group is located on the fourth carbon in the chain.
According to the properties of the side chain
The side chain is different in each one, and this difference is what gives them the different properties that will later mark their function in organisms.
They may be:
- Polar, polar or hydrophilic neutrals: can react with water.
- Nonpolar, apolar or hydrophobic neutrals: they do not react with water.
- Negatively charged or acids.
- With positive or basic letter.
- Aromatics have an aromatic group attached to their side chain as an indole group, a benzene ring.
What are the 22 types of amino acids that make up proteins?
The amino acids that make up proteins, as we have seen previously, are called alpha-amino acids and are the following:
When we talk about essential amino acids, we mean that they are essential in children.
As we said before, the amino acids that make up proteins are encoded in DNA following a sequence of three nucleotides (codon) that encode a single amino acid. In this graph we leave you the codons of the amino acids.
How do amino acids bind to each other? The peptide bond
The union between two amino acids is called a peptide bond although it is an amide bond. Peptide bonds are always made between the carboxyl group of one amino acid and the amino group of another.
In these bonds, the hydroxyl (OH) of the carboxyl group is replaced by the amino group (NH2), so that OH and H are lost, remaining united in a bond between carbon and nitrogen (CO-NH) and releasing a molecule of water.
As we will see at the point of how a peptide bond breaks, it is a very strong and durable bond that is not easy to destroy.
Characteristics of the peptide bond
There is an aspect that is important to take into account, although conceptually it is not easy. In organic molecules, the bond is very important but the structure of the bond, the position of the same and the ability to turn are important. These characteristics can confer properties on the molecules and therefore the particularities of the peptide bond are important.
As we have seen, the peptide bond is an amide bond but it has some differences from the traditional bond:
- The CN bond that is made in a peptide is shorter than other CN bonds
- The CN bond has a partial double bond character. Due to this, since it is stabilized by resonance, this bond does not present the possibility of turning and the carbon and nitrogen are in the same plane, so it has a certain rigidity.
- The rigidity of the peptide bond makes the spatial conformation of the peptide limited. There are two conformational possibilities:
- trans: the alpha carbons are on the different side of the double bond. This is the most common configuration.
- cis: the alpha carbons are on the same side of the double bond. This configuration only occurs when proline is involved.
- As we mentioned in the previous point, the four atoms of the bond (CO-NH) are in the same plane, with the Oxygen and Hydrogen in the trans position. However, the rest of the bonds (NC and CC) are true single bonds, with which there could be twisting and this is what allows other conformations in the peptides, although not all twists are possible.
Each of the three-dimensional structures that a peptide can have is called its conformation and is very important since it is what gives it the properties of amino acids.
How can a peptide bond be broken?
Peptide bonds are broken by adding water, thanks to hydrolysis that occurs without energy input. However, despite the fact that this is a process that does not require energy, it is very slow. At a temperature of 25ºC each bond takes between 350 and 600 years to break, releasing 8-16 kJ / mol of Gibbs free energy.
As living beings do not live 300 years on average and amino acids are degraded every day, something must help. In this case, they are the peptidase or protease enzymes that catalyze the hydrolysis of the peptide bonds without degrading the amino acids.
Other ways to break the peptide bond are:
- Acid hydrolysis: it is the boiling of the protein with strong acid solutions such as hydrochloric and sulfuric acid for a long time. This method is very aggressive, destroying some amino acids completely like tryptophan and part of threonine and serine.
- Basic hydrolysis: It respects the amino acids that are destroyed by the previous hydrolysis. It is the same process as above but using sodium hydroxide (caustic soda) or Barium hydroxide.
- Hydrolysis by temperature: under normal conditions, the high temperature does not destroy peptide bonds. It can have other effects by altering other characteristics of proteins such as secondary, tertiary and quaternary structure. From 110º for 48 hours it is possible to destroy the peptide bond.
What reactions can amino acids carry out?
The functions of amino acids and proteins are conditioned by the peptide bond, and by the carboxyl and amino groups. Here is a breakdown of the reactions that can occur in amino acids:
- Decarboxylation : when the amino acid is decarboxylated at the alpha carbon, that is, it loses the carboxyl group, resulting in an amine. Some examples of amines produced by the decarboxylation of amino acids such as histamine (trigger of allergic reactions), thiamine (or vitamin B1), …
- Formation of amides : they are formed when the carboxyl group combines with ammonia, giving rise to amides such as glutamine (intervenes in metabolism and in muscles), asparagine (participates in N-glycosylation) …
- Transamination : it is one of the most important functions in the body related to amino acids. This reaction serves for the interconversion of amino acids and the synthesis of non-essential amino acids. Transamination consists of the amino group being transferred to an alpha-ketoacid, forming new amino acids and alpha-ketoacids. The most important alpha-keto acids are pyruvic acid that participates in glycolysis and alpha-ketoglutarate that participates in the Krebs cycle.
- Oxidative deamination : the amino group of the alpha carbon is removed from the amino acid to form a keto acid and ammonia.
- Formation of carbamino compounds: A carbon dioxide is added to the amino group of amino acids to create carbamino compounds such as hemoglobin.
- Formation of methylene compounds : they occur when formaldehyde reacts with the amino group to form a methylene compound.
Modifications of amino acids and their derivatives
- Hydroxylation : Addition of a hydroxyl group, for example the addition of a hydroxyl group to lysine generates hydroxylysine which is present in collagen.
- Carboxylation : a carboxyl group is added to the alpha carbon. For example, γ-carboxyglutamate that participates in the binding of calcium ions.
- Methylation : a methyl group is added, as in the case of 6-N-methylisine that participates in muscle fibers.
- Phosphorylation : a phosphate group is added as in the case of phosphoserine, which is an intermediary in metabolic pathways.
How many amino acids does a protein contain?
As we said at the beginning of the article, amino acids make up proteins that are also called peptides or polypeptides.
When several amino acids come together they form a chain called a peptide or polypeptide. When a peptide chain reaches 100 amino acids or when the total molecular mass exceeds 1000 amu or they have a defined stable three-dimensional structure, they are called proteins. When a peptide chain is short it is called an oligopeptide.
Therefore, all proteins are polypeptides but not all polypeptides are proteins since peptides of even two amino acids are called dipeptides.
The amino acids that are part of the peptides are called residues.
What foods contain amino acids?
One of the greatest sources of amino acids is found in the proteins in foods, so meat and fish are foods that provide a large amount of protein and are easier to assimilate. Dairy and eggs are also high in amino acids and proteins.
Vegetables that are a source of proteins and essential amino acids, although some have more than others and with different degrees of assimilation. Legumes and nuts have a high concentration of protein. Soy deserves a prominent place for its large amount of protein. Cereals are also a source of protein and amino acids, with a higher concentration in their wholegrain versions. Vegetables also contribute although to a lesser extent.
On the other hand, quinoa is a food that contains all 20 essential amino acids.
As we can see, amino acids are found in all foods, so a healthy person with a balanced diet does not have to take any amino acid supplement even if he is an amateur athlete.