Coenzymes in Oxidation-Reduction Reactions
These coenzymes function alongside oxidoreductases—enzymes that catalyze the transfer of electrons and protons. Key representatives of this group include nucleotide derivatives and specific acids.
- Nicotinamide adenine dinucleotide (NAD+). This is the primary coenzyme for anaerobic dehydrogenases. It is required 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 include glucose-6-phosphate dehydrogenase, fatty acid synthase, and glutathione reductase.
- Flavin adenine dinucleotide (FAD). Works with aerobic dehydrogenases and oxidases. Essential examples include succinate dehydrogenase, acyl-CoA dehydrogenase, and xanthine oxidase.
- Flavin mononucleotide (FMN). Found in the structure of NADH dehydrogenase (Complex I of the electron transport chain) and also functions with L-amino acid oxidase.
In addition to nucleotides, oxidation-reduction reactions involve:
- Lipoic acid (in the form of lipoamide). This is required for the oxidative decarboxylation of $\alpha$-keto acids. It operates 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 critical 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 operate in tandem with the transferase class.
- Tetrahydrofolic acid (THF). Specializes in the transfer of single-carbon groups, such as methyl and methylene groups. The function of thymidylate synthase and methionine synthase depends entirely on THF.
- Coenzyme A (CoA-SH, acyl coenzyme). The primary carrier of acyl groups. It functions with acyltransferases and is involved in the pyruvate dehydrogenase complex, thiolase, and acyl-CoA synthetase.
- Thiamine pyrophosphate (TPP). This coenzyme has a dual function. On one hand, it works with transferases to mediate the transfer of glycolaldehyde groups (e.g., in transketolase of the pentose phosphate pathway). On the other hand, TPP acts as a coenzyme for oxidoreductases during the decarboxylation of $\alpha$-keto acids, serving as part of pyruvate dehydrogenase (E1 subunit).
Coenzymes with Multiple or Specific Actions
Some coenzymes possess narrow specialization or, conversely, are capable of serving multiple enzyme classes simultaneously.
Pyridoxal phosphate (PLP) is one of the most versatile coenzymes. It participates in the functioning of three enzyme classes:
- Transferases: serves aminotransferases such as ALT (alanine aminotransferase) and AST (aspartate aminotransferase).
- Lyases: operates with decarboxylases (e.g., glutamate decarboxylase).
- Isomerases.
Biotin. Functions in the body as biocytin (biotin tightly bound to a lysine amino acid residue). It is a strictly specific coenzyme for ligases (carboxylases). Its primary task is the attachment of a carbon dioxide molecule ($CO_2$). Essential enzymes utilizing biotin include pyruvate carboxylase and acetyl-CoA carboxylase.