PROSTHETIC GROUP: MAIN GROUPS AND THEIR FUNCTIONS - BIOLOGY - 2024

A prosthetic group is the fragment of a protein that does not have an amino acid nature. In these cases, the protein is called "heteroprotein" or conjugated protein, where the protein portion is called apoprotein. In contrast, molecules made up of amino acids only are called holoproteins.

Proteins can be classified according to the nature of the prosthetic group: when the group is a carbohydrate, a lipid or a heme group, the proteins are glycoproteins, lipoproteins and hemeproteins, respectively. In addition, prosthetic groups can be very varied: from metals (Zn, Cu, Mg, Fe) to nucleic acids, phosphoric acid, among others.

In some cases, proteins need extra components to perform their functions successfully. In addition to the prosthetic groups there are the coenzymes; The latter bind loosely, temporarily and weakly to the protein, while the prosthetic groups are firmly anchored to the protein portion.

Main prosthetic groups and their functions

Biotin

Biotin is a hydrophilic vitamin of the B complex that participates in the metabolism of different biomolecules, including gluconeogenesis, amino acid catabolism and lipid synthesis

It acts as a prosthetic group for various enzymes, such as acetyl-CoA carboxylase (in the forms found in the mitochondria and in the cytosol), pyruvate carboxylase, propionyl-CoA carboxylase, and b-methylcrotonyl-CoA carboxylase.

This molecule is capable of attaching itself to said enzymes through a lysine residue and is responsible for the transport of carbon dioxide. The function of biotin in organisms goes beyond its role as a prosthetic group: it participates in embryogenesis, in the immune system and in gene expression.

Raw egg white has a protein called avidin, which suppresses the normal use of biotin; Therefore, the consumption of cooked egg is recommended because heat denatures avidin, thus losing its function.

Heme group

The heme group is a molecule of porphyrin nature (a large heterocyclic ring) that has in its structure iron atoms capable of reversibly binding to oxygen or of giving up and taking electrons. It is the prosthetic group of hemoglobin, a protein responsible for the transport of oxygen and carbon dioxide.

In functional globins, the iron atom has a +2 charge and is in a ferrous oxidation state, thus it can form five or six coordination bonds. The characteristic red color of the blood is due to the presence of the heme group.

The heme group is also the prosthetic group of other enzymes, such as myoglobins, cytochromes, catalases, and peroxidases.

Flavin mononucleotide and flavin adenine dinucleotide

These two prosthetic groups are present in flavoproteins and are derived from riboflavin or vitamin B ₂. Both molecules have an active site that undergoes reversible oxidation and reduction reactions.

Flavoproteins have very varied biological roles. They can participate in dehydrogenation reactions of molecules such as succinate, participate in the transport of hydrogen in the electron transport chain or react with oxygen, generating H ₂ O ₂.

Pyrroloquinoline quinone

It is the prosthetic group of quinoproteins, a class of dehydrogenase enzymes such as glucose dehydrogenase, that participates in glycolysis and other pathways.

Pyridoxal phosphate

Pyridoxal phosphate is a derivative of vitamin B ₆. It is found as a prosthetic group of the amino transferase enzymes.

It is the prosthetic group of the enzyme glycogen phosphorylase and is linked to it by means of covalent bonds between the aldehyde group and the ε-amino group of a lysine residue in the central region of the enzyme. This group helps in the phosphorolytic breakdown of glycogen.

Both the flavin mononucleotide and flavin adenine dinucleotide mentioned above are indispensable for the conversion of pyridoxine or vitamin B ₆ to pyridoxal phosphate.

Methylcobalamin

Methylcobalamin is an equivalent form of vitamin B ₁₂. Structurally, it has an octahedral cobalt center and contains metal-alkyl bonds. Among its main metabolic functions is the transfer of methyl groups.

Thiamine pyrophosphate

Thiamine pyrophosphate is the prosthetic group of enzymes involved in major metabolic pathways, such as α-ketoglutarate dehydrogenase, pyruvate dehydrogenase, and transketolase.

In the same way, it participates in the metabolism of carbohydrates, lipids and branched-chain amino acids. All enzymatic reactions that require thiamine pyrophosphate involve the transfer of an activated aldehyde unit.

Thiamine pyrophosphate is synthesized intracellularly by phosphorylation of vitamin B ₁ or thiamine. The molecule consists of a pyrimidine ring and a thiazolium ring with a CH azide structure.

Thiamine pyrophosphate deficiency results in neurological diseases known as beriberi and Wernicke – Korsakoff syndrome. This occurs because the only fuel in the brain is glucose, and since the pyruvate dehydrogenase complex requires thiamine pyrophosphate, the nervous system does not have energy.

Molybdopterin

Molybdopterins are derivatives of pyranopterin; They are made up of a pyran ring and two thiolates. They are prosthetic groups or cofactors found in enzymes that have molybdenum or tungsten.

It is found as a prosthetic group of thiosulfate reductase, purine hydroxylase, and formate dehydrogenase.

Lipoic acid

Lipoic acid is the prosthetic group of lipoamide and is covalently attached to the protein moiety by a lysine residue.

In its reduced form, lipoic acid has a pair of sulfhydryl groups, while in the oxidized form it has a cyclic disulfide.

It is responsible for the reduction of cyclic disulfide in lipoic acid. Furthermore, it is the prosthetic group of transcetylase and a cofactor of different enzymes involved in the citric acid cycle or Krebs cycle.

It is a component of great biological importance in the dehydrogenases of the alkatoacids, where the sulfhydryl groups are responsible for transporting hydrogen atoms and acyl groups.

The molecule is a derivative of octanoic fatty acid and consists of a terminal carboxyl and a dithional ring.

Nucleic acids

Nucleic acids are the prothetic groups of nucleoproteins found in cell nuclei, such as histones, telomerase, and protamine.

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