Enzymes involved in biological oxidation

March 13, 2019

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.

A) Oxidases

B) Dehydrogenases

C) Hydroperoxidases

D) Oxygenases

A) Oxidases

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).

B) Dehydrogenases

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.

C) Hydroperoxidases

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

D) Oxygenases

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 P₄₅₀monooxygenase 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: