16. BIOLOGICAL OXIDATION

Introduction to Biological Oxidation:

Biological oxidation refers to the oxidation reactions that occur in living systems to release and utilize energy. These reactions are exergonic, meaning they release energy as substrates are oxidized from a higher energy state to a lower one.

The released energy is converted into a usable chemical form through the synthesis of ATP (adenosine triphosphate). This conversion of energy from oxidation into ATP is known as oxidative phosphorylation.

Key Concept: The process of ATP formation from ADP and inorganic phosphate (Pi), coupled with oxidation, is called biological oxidative phosphorylation.

Coenzyme System Involved in Biological Oxidation

Biological oxidation involves a complex interplay of enzymes and coenzymes that mediate the transfer of hydrogen or electrons during metabolic reactions.

1. Enzymes Involved:

These enzymes belong to the class of oxidoreductases and include:

Oxidases
Dehydrogenases
Hydroperoxidases
Oxygenases

2. Coenzymes Involved:

Coenzymes serve as electron carriers in oxidation-reduction reactions. The major coenzymes are:

Nicotinamide adenine dinucleotide (NAD⁺)
Nicotinamide adenine dinucleotide phosphate (NADP⁺)
Flavin mononucleotide (FMN)
Flavin adenine dinucleotide (FAD)

Oxidases

Oxidases catalyze the removal of hydrogen from a substrate and transfer it to molecular oxygen (O₂), forming either water (H₂O) or hydrogen peroxide (H₂O₂).

Examples of Oxidases:

Cytochrome oxidase: Involved in the final step of the electron transport chain, where O₂ is reduced to water.
L-amino acid oxidase: Found in the liver and kidney; catalyzes oxidative deamination of amino acids.
Xanthine oxidase: Converts xanthine to uric acid during purine metabolism.

Dehydrogenases

Dehydrogenases catalyze the removal of hydrogen atoms from a substrate but do not directly use oxygen as an electron acceptor. Instead, they transfer hydrogen to specific coenzymes.

There are two main types of coenzymes involved with dehydrogenases:

1. Nicotinamide Coenzyme-Linked Dehydrogenases (NAD⁺ or NADP⁺)

These enzymes use NAD⁺ or NADP⁺ derived from vitamin niacin (vitamin B₃).
They catalyze reactions of the type:

MH₂ + NAD⁺ → M + NADH + H⁺
MH₂ + NADP⁺ → M + NADPH + H⁺

NAD-linked dehydrogenases participate in energy-yielding oxidative pathways such as glycolysis, TCA cycle, and the mitochondrial electron transport chain.

NADP-linked dehydrogenases are used in biosynthetic (reductive) reactions, such as fatty acid and cholesterol synthesis.

Note: NADH contributes to ATP generation, while NADPH is used for anabolic processes, not for direct energy production.

2. Flavin Coenzyme-Linked Dehydrogenases (FMN or FAD)

FMN and FAD are derived from riboflavin (vitamin B₂).
These coenzymes accept both hydrogen atoms from substrates:

MH₂ + FAD → M + FADH₂
MH₂ + FMN → M + FMNH₂

FMN-linked enzymes include NADH dehydrogenase (Complex I of ETC). FAD-linked enzymes include succinate dehydrogenase (TCA cycle) and acyl-CoA dehydrogenase (β-oxidation of fatty acids).

Hydroperoxidases

Hydroperoxidases are enzymes that detoxify hydrogen peroxide (H₂O₂) by reducing it to water. They protect cells from oxidative damage.

Types of Hydroperoxidases:

Peroxidases: Catalyze the reduction of hydrogen peroxide using various electron donors.
Catalases: Decompose hydrogen peroxide into water and oxygen to prevent tissue injury caused by oxidative stress.

2H₂O₂ → 2H₂O + O₂

Oxygenases

Oxygenases are enzymes that incorporate one or both atoms of molecular oxygen (O₂) into a substrate. Unlike oxidases and dehydrogenases, these enzymes are not involved in energy production but in metabolic conversions.

1. Monooxygenases

Also called mixed-function oxidases.
They insert one oxygen atom into the substrate as a hydroxyl group (-OH), while the other oxygen atom is reduced to water.
Example: Phenylalanine hydroxylase, which converts phenylalanine to tyrosine.

2. Dioxygenases

These enzymes incorporate both oxygen atoms into the substrate molecule.
Examples:
- Homogentisate oxidase (in tyrosine metabolism)
- 3-Hydroxyanthranilate oxidase and L-tryptophan dioxygenase (in tryptophan metabolism)
- Cyclooxygenase (in prostaglandin synthesis)

Summary of Enzymes in Biological Oxidation

Enzyme Type	Function	Example
Oxidases	Remove hydrogen using oxygen as the acceptor	Cytochrome oxidase, Xanthine oxidase
Dehydrogenases	Remove hydrogen without using oxygen directly	Succinate dehydrogenase, Lactate dehydrogenase
Hydroperoxidases	Detoxify hydrogen peroxide	Catalase, Peroxidase
Oxygenases	Incorporate oxygen atoms into substrates	Phenylalanine hydroxylase, Cyclooxygenase

Detailed Notes:

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