General characteristics and main subclasses
Enzymes of the oxidoreductase class specialize exclusively in catalyzing redox reactions. Depending on the mechanism of action, the type of transferred groups, and the nature of the electron acceptor, several important subclasses are distinguished within this broad class. The most significant for clinical biochemistry are dehydrogenases, oxidases, and oxygenases (or hydroxylases). Each of these subclasses has its specific coenzymes and strict rules for interacting with substrates.
Subclass: Dehydrogenases
Dehydrogenases catalyze dehydrogenation reactions — the process of removing hydrogen atoms followed by the transfer of electrons from the oxidized substrate to a specialized acceptor.
For successful functioning, dehydrogenases require coenzymes that act as electron acceptors. The main coenzymes here are NAD⁺, NADP⁺, FAD, and FMN.
Typical examples of enzymes in this subclass:
- Malate dehydrogenase
- Isocitrate dehydrogenase
- Succinate dehydrogenase
- Alpha-ketobutyrate dehydrogenase
Reaction example: Dehydrogenation of malate. In this reaction, malate is the substrate and oxaloacetate (oxaloacetic acid) is the product. The reaction is catalyzed by the enzyme malate dehydrogenase, while the NAD⁺ coenzyme is reduced to NADH + H⁺.
Subclass: Oxidases
Unlike dehydrogenases, oxidases catalyze oxidation reactions in which molecular oxygen acts as the direct electron acceptor.
A classic example of such an enzyme is cytochrome c oxidase, a vital component of the mitochondrial respiratory chain. It catalyzes the transfer of electrons to oxygen with the formation of water.
The equation of the reaction catalyzed by cytochrome c oxidase: $$O_2 + 4H^+ + 4e^- \rightarrow 2H_2O$$
Subclass: Oxygenases (hydroxylases)
Oxygenases catalyze oxidation reactions by directly incorporating an oxygen atom into the hydroxyl group of the substrate molecule. This reaction also proceeds with the obligatory participation of molecular oxygen ($O_2$).
The mechanism of action of oxygenases strictly distributes the oxygen atoms:
- One oxygen atom attaches to the substrate (forming a hydroxyl group).
- The second oxygen atom participates in the formation of a water molecule.
Reaction example: Hydroxylation of phenylalanine. The substrate is the amino acid phenylalanine (Phe), which is converted into tyrosine (Tyr) by the action of the enzyme phenylalanine hydroxylase. The hydrogen donor in this complex reaction is the coenzyme tetrahydrobiopterin ($H_4B$), which is oxidized to dihydrobiopterin ($H_2B$) during the process.
Clinical significance: connection with vitamin PP
Understanding the function of oxidoreductases is of direct clinical importance, especially in the context of vitamin metabolism. A clear logical chain can be built from the enzyme class to a specific disease:
- Oxidoreductases catalyze redox processes.
- Their subclass, dehydrogenases, carries out the removal of hydrogen atoms.
- For the functioning of many dehydrogenases, the coenzyme NAD⁺ (nicotinamide adenine dinucleotide) is critically essential.
- The NAD⁺ molecule is synthesized in the body based on nicotinic acid — vitamin PP (or vitamin B₃).
- With a deficiency of nicotinic acid, the function of the dehydrogenase pool is disrupted, which clinically manifests as a severe disease — pellagra.
Pellagra is characterized by a specific triad of symptoms: dermatitis, diarrhea, and dementia. It is important to distinguish it from other avitaminoses (for example, scurvy in vitamin C deficiency or macrocytic anemia in vitamin B₁₂/folic acid deficiency).
Briefly: answers to topical questions
From which vitamins are the coenzymes FAD and FMN synthesized?
The coenzymes FAD (flavin adenine dinucleotide) and FMN (flavin mononucleotide) are formed based on vitamin B₂ — riboflavin.
In which complex of the mitochondrial respiratory chain does cytochrome c oxidase function?
Cytochrome c oxidase functions in Complex IV of the mitochondrial respiratory chain. It catalyzes the reduction of molecular oxygen to water: O₂ + 4H⁺ + 4e⁻ → 2H₂O.
What enzymes belong to the oxidase subclass, besides cytochrome c oxidase?
According to the cited sources, the oxidase section directly mentions only cytochrome c oxidase. Other enzymes explicitly classified under the oxidase subclass are not specified in the sources.
The triad of pellagra symptoms (deficiency of vitamin PP, an oxidoreductase coenzyme) is the "3Ds": Dermatitis, Diarrhea, Dementia.
Class and exam questions
What is the main difference between oxidases and oxygenases?
Oxidases use molecular oxygen solely as an electron acceptor (often forming water), whereas oxygenases (hydroxylases) directly incorporate an oxygen atom into the substrate structure, forming a hydroxyl group.
Which enzyme and coenzyme are involved in the conversion of malate to oxaloacetate?
The reaction is catalyzed by malate dehydrogenase (a dehydrogenase subclass). NAD⁺ acts as the coenzyme and electron acceptor, being reduced to NADH + H⁺.
What role does tetrahydrobiopterin play in the phenylalanine hydroxylation reaction?
Tetrahydrobiopterin ($H_4B$) acts as a specific hydrogen-donor coenzyme. During the conversion of phenylalanine to tyrosine, it transitions into its oxidized form — dihydrobiopterin.
What disease does a deficiency of the NAD+ precursor lead to?
Deficiency of vitamin PP (nicotinic acid), from which NAD+ is built, causes pellagra — a disease manifesting with dermatitis, diarrhea, and dementia.
What else to study on this topic
- Structure and functions of coenzymes NAD+, NADP+, FAD, FMN
- The role of malate dehydrogenase, isocitrate dehydrogenase, and succinate dehydrogenase in the Krebs cycle
- Cytochrome c oxidase as a component of the mitochondrial respiratory chain
- Detailed mechanism of phenylalanine hydroxylation and the tetrahydrobiopterin cycle
- Biochemical pathogenesis of pellagra at the cellular level
Master the entire topic in the "Biochemistry" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Oxidoreductases" and 3,616 more subject pages
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 115 video lessons on the subject
Other topics in the "Enzymology" section
Free video lessons: biochemistryShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.