- All the enzymes that are involved in biological oxidation belong to the class oxidoreductases.
- In biochemistry, an oxidoreductase is an enzyme that catalyzes the transfer of electrons from one molecule called the reductant, also known as the electron donor, to another called the oxidant ( also known as the electron acceptor).
- These are further categorized into four groups.
- These enzymes are responsible for catalyzing the elimination of hydrogen from the substrates which is accepted by oxygen to form mostly water.
- Cytochrome oxidase, tyrosinase, mono-amine oxidase (H2O2 formed instead of H2O) are its examples.
- Cytochrome oxidase transfers the electrons (obtained from the oxidation of substrate molecules by dehydrogenases) to the final acceptor oxygen.
- It is the terminal component of electron transport chain.
- Some Flavoproteins containing FAD or FMN also belong to this category. E.g. L-amino acid oxidase (FMN), xanthine oxidase (FAD).
- This enzyme cannot utilize oxygen as hydrogen acceptor.
- They catalyze the reversible transfer of hydrogen from one substrate to another which brings about oxidation-reduction reactions.
- There are a large number of enzymes belonging to this group.
- Alcohol dehydrogenase, glycerol 3-phosphate dehydrogenase are NAD+ dependent dehydrogenases.
- HMG CoA reductase, enoyl reductase are NADP+ dependent dehydrogenases.
- NADH dehydrogenase is FMN dependent dehydrogenase.
- Succinate dehydrogenase, acyl CoA dehydrogenase is FAD dependent dehydrogenase.
- All the electrons of electron transport chain (b, c1 and c) except the terminal cytochrome oxidase (a+a3) belong to this group.
- Hydrogen peroxide acts as the substrate for these enzymes.
- There is a constant production of H2O2 in the reactions catalyzed by the aerobic dehydrogenases.
- The harmful effect of H2O2 gets reduced by:
2H2O2 → 2H2O + O2
- This group of enzymes catalyzes the direct incorporation of oxygen into the substrate molecules.
i) Dioxygenases (true oxygenases)
- They are responsible for the incorporation of both the atoms of oxygen (O2) into the substrate.
- E.g. L-tryptophan pyrrolase, homogentisate oxidase.
ii) Monooxygenases (mixed function oxidases)
- They catalyse the incorporation of one atom of oxygen (1/2 O2) while the other oxygen atom is reduced to H2O.
- NADPH usually provides the reducing equivalents.
- E.g. cytochrome P450 monooxygenase system of microsomes is responsible for the metabolism of many drugs (amino pyrine, morphine, aniline, etc).
- It is also responsible for biosynthesis of steroid hormones (from cholesterol).
- The action of Cyt P450 is depicted here as below:
Enzymes involved in biological oxidation