Coenzyme

The coenzymes are thermostable, non-protein organic cofactors that, together with an apoenzyme, constitute the holoenzyme or catalytically active form of the enzyme. They generally have low molecular mass (at least compared to the apoenzyme) and are key in the catalytic mechanism, for example, accepting or donating electrons or functional groups, which transport from one enzyme to another.

Unlike enzymes, coenzymes are modified during a chemical reaction; for example, NAD⁺ is reduced to NADH when it accepts two electrons (and a proton) and is therefore exhausted; when NADH releases its electrons, NAD⁺ is recovered, which again can act as a coenzyme.

Mechanism of action of coenzymes

The basic mechanism of action of coenzymes is as follows:

1. Coenzyme binds to an enzyme.
2. The enzyme captures its specific substrate.
3. The enzyme attacks that substrate, transferring some of its electrons. In reality the binding of substrate and enzyme produces a new substance. This substance is unstable, which causes its separation in different parts: enzyme, product, and the reduced form of coenzyme, which was left with some electrons (by rusting the substrate is reduced) by presenting greater molecular attraction force.
4. The enzyme yields to coenzyme such electrons from the substrate.
5. Coenzyme accepts such electrons and is detached from the enzyme.
6. The reduced coenzyme goes to the electron transport chain, in which ATP and H are generated₂Or (cellular breath), by "leading" there their electrons is through a shuttle, returns to their initial state.

This last step is essential in order not to deplete a cell's endowment of coenzymes since the enzymes together with which it acts cannot carry out the chemical reaction without the assistance of its coenzyme.

Some coenzymes are strongly and permanently attached to their enzyme, effectively constituting a prosthetic group; such is the case of FMN to the enzyme NADH dehydrogenase or FAD to succinate dehydrogenase.

Each coenzyme is specialized in accepting and transporting a certain type of atoms; some accept hydrogens, others acetyl groups, amino, etc. However, coenzymes are not at all specific with respect to the enzymes to which they bind, so that the same coenzyme can bind to a large number of different enzymes and that is why the number of different coenzymes is relatively low.

Main coenzymes

Coenzima A.

• FAD (flavin-adenin dinucleotide): transfer of electrons and protons.
• FMN (mononucleotide valve): transfer of electrons and protons.
• NAD⁺(syntin-adenin dinucleotide): transfer of electrons and protons.
• NADP⁺ (syntin-adenin dinucleotide phosphate):
• Coenzima A: transfer of acetyl groups (e.g., in the descarboxylation of piruvic acid) and acile groups in general.
• Coenzima Q: Electron transfer in the respiratory chain.
• Coenzima B₁₂: transfer of methyl or hydrogen groups between molecules.
• TPP (thyamine spirophosphate): transfer of aldehyde groups; it is part of, among others, the dehydrogenous piruvate complex.
• Vitamin C.
• PLP (piridoxal phosphate): transfer of amino groups.
• PMP (piridoxamine phosphate): transfer of amino groups.
• FH4 (Tetrahydropholic acid): transfer of formyl, methyl and methylene groups.
• Biocitin: transfer of carbon dioxide.
• Lipoic acid: transfer of hydrogens, acile groups and metilamine.

Coenzymes and vitamins

Many vitamins, or their derivatives, act as coenzymes:

• Vitamin B1 or thyamine: its derivative, thyamine pyrophosphate is essential for the energetic metabolism of glucids.
• Vitamin B2 or riboflavin: its derivatives are coenzymatic nucleotides with great reducing power such as FAD and FMN.
• Vitamin B3 or niacin: its derivatives are coenzymatic nucleotides with great reducing power such as NAD⁺ or NADP⁺.
• Vitamin B5 or pantothenic acid: its main derivative is coenzyme A (Co-A), with great importance in various metabolic processes.
• Vitamin B6 or pyridoxine. Its main derivatives are PLP (pyridoxal phosphate) and PMP (pyridoxamine phosphate), essential in the metabolism of amino acids.
• Vitamin B7 or biotin (vitamin H or vitamin B8). Its derivative, biocitin, is essential for the operation of numerous carboxylates (enzymes).
• Vitamin B9 or folic acid (vitamin M). Its derivative, the FH₄ is essential in the synthesis of purins.