The most important chemical reactions in metabolism

The metabolic pathways of the human organism form an extensive network of interconnected reactions that often share common intermediates. Chemical transformations of individual substances are usually classified according to a general mechanism common to all substances undergoing a given reaction. For example, decarboxylation is the splitting of CO2 from the carboxyl group, where the substrate may be different carboxylic acids.

Alcohols, carbonyl compounds and carboxylic acids
Alcohols, carbonyl compounds and carboxylic acids are important substrates for many reactions of metabolic pathways of organisms.

 Alcohols contain the functional group −OH. Depending on the number of OH groups in the molecule, alcohols can be one-, two- or polybasic. Furthermore, depending on which carbon atom the OH group binds to, we distinguish between primary, secondary and tertiary alcohols. Aldehydes with ketones form a group of carbonyl compounds. The functional group of aldehydes is the group −CHO, in ketones −C=O. Of this group of substances, the most important substrates are probably the reactions of  carboxylic acids, characterized by the presence of the functional group −COOH, and their 'derivatives''.

Significant reactions of alcohols, aldehydes and carboxylic acids

 * 1) Formation of anions and acyls derived from carboxylic acids
 * 2) Dehydrogenation and hydrogenation (oxidation and reduction)
 * 3) Esterification

Formation of anions and acyls derived from carboxylic acids
The carboxyl group is capable of dissociation', with the degree of dissociation for individual acids given by dissociation constant. Carboxylic acids are ''weak, which means that their dissociation is only partial. The acid thus gives rise to the anion (group −COO−'''). After splitting off the whole −OH group from the carboxyl group, its acyl is formed.

Dehydrogenation and hydrogenation (oxidation and reduction)
During the chemical reaction, dehydrogenation, the  H is removed from the molecule'''. The obtained hydrogen can then be used for the formation of a proton gradient in mitochondria and for energy gain (ATP). The introduction of hydrogen into a molecule is called 'hydrogenation. In the body, dehydrogenation and hydrogenation occur, for example, in the following processes:


 * Oxidation of single bonds to double bonds:
 * −CH 2−CH 2− −CH=CH− + 2 H+ +2 e−undefined
 * These reactions occur, for example, in  Krebs cycle, at  β-oxidation of fatty acids or desaturation reactions, which aim at the synthesis of unsaturated fatty acids.


 * Mutual conversion of alcohols, aldehydes / ketones and carboxylic acids
 * Alcohols, carbonyl compounds and carboxylic acids  form a series differing from each other in the degree of oxidation / reduction.
 * The general scheme of their mutual transformation is as follows (oxidation takes place towards the carbonyl compound and carboxylic acid, reduction in the opposite direction):
 * Primary alcohol aldehyde  carboxylic acid
 * R−CH2−OH R−CHO  R−COOH
 * Secondary alcohol ketone
 * R 1undefined−CH(OH)−R 2R 1−CO−R2
 * Tertiary alcohol
 * "Slight" oxidation does not take place (it can be oxidized only with simultaneous splitting of the carbon chain).

An example of oxidation is the formation of dihydroxyacetone phosphate (DHA-P) from glycerol-3-phosphate (cofactor is  FAD), through which glycerol enters glycolysis or gluconeogenesis according to the current needs of the organism.

Esterification
Esterification is the reaction of a carboxylic acid with an alcohol, producing an ester and water:

Hydroxy acids and keto acids
Hydroxy acids in addition to the −COOH group also contain the −OH group replacing one −H. Keto acids or oxo acids contain in the molecule in addition to the group −COOH also the group =O replacing one −H. Their mutual conversion is relatively common in metabolic pathways.

An example is the relatively common keto-enol tautomery in metabolism. It converts two forms of organic compounds: The mutual transformation of the two forms represents the migration of the hydrogen atom or the proton, accompanied by the swapping of the single bond and the adjacent double bond.
 * ketoform (or oxoform) contains double-bonded oxygen as a group =O,
 * enolform, which contains a double bond between carbons and one of them binds −OH group (i.e. contains the structure R1−CH=C(OH)−R2).

Amino acids and oxo acids
Amino acids and oxo acids are substitution derivatives of carboxylic acids. Amino acids contain in the molecule in addition to the −COOH group also the group −NH 2, oxo acids group =O. Their mutual transformations are frequent in the organism, for example, there is a NH2 group for group =O and vice versa. These transformations occur mainly in two processes:
 * Transamination
 * In this reaction, the amino acid is a donor −NH2 of the oxo acid group. From the corresponding oxo acid, an amino acid is formed, and the original amino acid becomes an oxo acid:
 * AK1 + OxoK2 OxoK1 + AK2


 * Oxidative deamination
 * It is the formation of oxo acid from the amino acid by removing −NH2 group, which is released as ammonia (NH3). Oxidative deamination is one of the important reactions through which amino acids initiate the process of their degradation. In the human body, they take place mainly in the liver, and the released ammonia is broken down in  urea synthesis.
 * This reaction is mainly catalyzed by glutamate dehydrogenase.

Decarboxylation and carboxylation
Decarboxylation removes the carboxyl group, which is released as a CO2 molecule and replaced by a proton. They are significant, for example, for
 * conversion of amino acids to biogenic amines (e.g. in the synthesis of many neurotransmitters),
 * dehydrogenation of 2-keto acids – pyruvate dehydrogenase reaction and two reactions of the Krebs cycle.

Carboxylation is the opposite reaction, involving the introduction of a COOH group into the molecule. It occurs, for example, in
 * fatty acid synthesis,
 * gluconeogenesis.