Coenzymes of redox reactions
These coenzymes function in conjunction with oxidoreductases, which are enzymes that catalyze the transfer of electrons and protons. Key representatives of this group are nucleotide derivatives and specific acids.
- Nicotinamide adenine dinucleotide (NAD+). This is the main coenzyme of anaerobic dehydrogenases. It is essential for the function of enzymes such as lactate dehydrogenase (LDH), malate dehydrogenase, and alcohol dehydrogenase.
- Nicotinamide adenine dinucleotide phosphate (NADP+). Unlike NAD+, this coenzyme specializes in pentose phosphate pathway dehydrogenases and reductases involved in lipid synthesis. Examples of enzymes: glucose-6-phosphate dehydrogenase, fatty acid synthase, and glutathione reductase.
- Flavin adenine dinucleotide (FAD). Works with aerobic dehydrogenases and oxidases. Crucial examples: succinate dehydrogenase, acyl-CoA dehydrogenase, and xanthine oxidase.
- Flavin mononucleotide (FMN). Found in the structure of NADH dehydrogenase (Complex I of the respiratory chain) and also works with L-amino acid oxidase.
In addition to nucleotides, the following participate in redox reactions:
- Lipoic acid (in the form of lipoamide). It is essential for the oxidative decarboxylation of $\alpha$-keto acids. It functions as part of the pyruvate dehydrogenase complex (E2 subunit) and the $\alpha$-ketoglutarate dehydrogenase complex.
- Ascorbic acid (ascorbate). Acts as a coenzyme for hydroxylases (hydroxylation reactions). Notable examples include prolyl hydroxylase, which is critically important for collagen synthesis, and dopamine $\beta$-hydroxylase.
Group-transfer coenzymes (Transferases)
This group of coenzymes ensures the transfer of various carbon radicals from one molecule to another. They work in pairs with the transferase class.
- Tetrahydrofolic acid (THFA). Specializes in the transfer of single-carbon groups, such as methyl and methylene groups. The function of thymidylate synthase and methionine synthase is impossible without THFA.
- Coenzyme A (CoA-SH, acyl coenzyme). The main carrier of acyl (acid) groups. It functions with acyltransferases. It is encountered in the action of the pyruvate dehydrogenase complex, thiolase, and acyl-CoA synthetase.
- Thiamine diphosphate (TDP). This coenzyme has a dual function. On the one hand, it works with transferases, carrying out the transfer of glycolaldehyde groups (e.g., in transketolase of the pentose phosphate pathway). On the other hand, TDP acts as a coenzyme for oxidoreductases during the decarboxylation of $\alpha$-keto acids, being part of pyruvate dehydrogenase (E1 subunit).
Coenzymes with multiple or specific actions
Some coenzymes have a narrow specialization or, conversely, are capable of serving several classes of enzymes at once.
Pyridoxal phosphate (PLP) is one of the most versatile coenzymes. It participates in the function of three classes of enzymes at once:
- Transferases: serves aminotransferases such as ALT (alanine aminotransferase) and AST (aspartate aminotransferase).
- Lyases: works with decarboxylases (e.g., glutamate decarboxylase).
- Isomerases.
Biotin. In the body, it functions as biocytin (biotin tightly bound to a lysine amino acid residue). It is a strictly specific coenzyme of ligases (carboxylases). Its main task is the addition of a carbon dioxide ($CO_2$) molecule. Crucial enzymes utilizing biotin include pyruvate carboxylase and acetyl-CoA carboxylase.
Briefly: answers to topical questions
Which coenzymes are part of the pyruvate dehydrogenase complex (PDH complex)?
The pyruvate dehydrogenase complex includes the coenzymes thiamine diphosphate, lipoic acid, coenzyme A, and NAD⁺.
- Thiamine diphosphate (TDP) is the coenzyme of pyruvate decarboxylase (E1) and participates in the transfer of the acetyl residue.
- Lipoic acid (Lipoamide) participates in oxidative decarboxylation and is tightly bound to the complex (E2).
- Coenzyme A (HS-CoA / CoA-SH) is the acyl coenzyme, incorporated into the complex only at the moment of the reaction.
- Nicotinamide adenine dinucleotide (NAD⁺) is incorporated into the multienzyme complex at the moment of the reaction and released as NADH + H⁺.
Which vitamins are precursors of the NAD+ and FAD coenzymes?
The precursors of the NAD⁺ and FAD coenzymes are the water-soluble vitamins PP and B₂.
- Vitamin PP (B₃, niacin, nicotinic acid/nicotinamide) is the precursor of NAD⁺. The active part of the coenzyme is the nicotinamide ring.
- Riboflavin (vitamin B₂) is the precursor of FAD. The basis of the riboflavin molecule is isoalloxazine.
Pyridoxal phosphate (PLP) works with TLI enzymes: Transferases, Lyases, Isomerases.
Class and exam questions
In which complex of the respiratory chain does FMN function?
Flavin mononucleotide (FMN) works in Complex I of the respiratory chain (NADH dehydrogenase).
Which coenzyme is required for CO2 attachment?
Biotin in the form of biocytin is required for carbon dioxide addition (carboxylation) reactions.
Which coenzymes are part of the pyruvate dehydrogenase complex?
Thiamine diphosphate (TDP) on the E1 subunit, lipoic acid on E2, and coenzyme A (CoA-SH) participate in the function of the PDH complex.
What is the difference between the functions of NAD+ and NADP+?
NAD+ predominantly works with anaerobic dehydrogenases, whereas NADP+ is used by pentose phosphate pathway dehydrogenases and lipid synthesis reductases.
What else to study on this topic
- Structure and principles of action of the pyruvate dehydrogenase complex
- Oxidative decarboxylation of $\alpha$-keto acids
- Mechanism of transamination reactions involving ALT and AST
- The role of ascorbate in collagen synthesis (hydroxylation reactions)
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