Biological oxidation

Many metabolic reactions in the living systems are oxidation-reduction reactions.

Chemically,

Oxidation is defined as addition of oxygen or removal of electrons.

Reduction is defined as removal of Oxygen or gain of electrons Oxidation is always accompanied by reduction of an electron acceptor. These reactions are also known as redox reactions.

In redox reactions free energy change is proportionate to the tendency of reactants to donate or accept electrons. Hence, the change in the free energy can be expressed as redox potential.

Redox reactions can be divided into half reactions. A half reaction consists of an electron donor and its conjugate electron acceptor.

Eg. Fe³⁺ + Cu^{+ ______________}> Fe²⁺ + Cu²⁺ can be divided as

Fe³⁺ + e^- <^{______________}> Fe²⁺ (reduction)

Cu⁺ <^{__________________}> Cu²⁺ + e^_ (oxidation)

For electrons to be trasferred both half reactions should occur simultaneously. e^- are half reaction's common intermediate.

The two half reactions of a redox reaction, each consisting of a conjugate redox pair, can be physically separated to form an electrochemical cell.

In such a device each half reaction takes place in separate half cell, and e- are passed between half cells as an electric current in the wire connecting their two electrodes. A salt bridge completes the electric circuit by providing a conduit for ions to migrate and there by maintaining neutrality.

The free energy of a redox reaction can be determined by measuring the voltage difference between its two half cells.

The equation to measure redox potential is formulated by Walther Nernst in 1881.

Consider

A_oxⁿ⁺ + B_red <^{________________}> A_red + B_oxⁿ⁺

in which n electrons per mole of reactants are transferred from reductant ( B_red ) to oxidant ( A_oxⁿ ). The equation for the redox potential can be written as

∆G= ∆G⁰' + RT ln ([ A_red ][B_oxⁿ⁺ ] / [ A_ox^n+]] [ B_red ])

∆G= -nF∆E

∆E=∆E⁰' - RT/nF In([ A_red ][B_oxⁿ⁺ ] / [ A_ox^n+]] [ B_red ])

where E= reduction potential

∆E= Electromotive force

( electron pressure that the electrochemical cell exerts)

The reduction potential when all components are in their standard state is called standard reduction potential.

The redox potential of a redox couple is estimated by measuring the EMF of sample half cell connected to a standard half cell. The sample half cell contains 1molar solution each of the reductant and oxidant. The reference half cell has 1M H⁺ solution in equilibrium with hydrogen gas at 1atm pressure. The reference half cell has a reduction potential of 0V. But, in biological systems, the redox potential is normally expressed at pH 7.0, at which hydrogen electode is 0.421V

Redox biologically important systems

Negative and positive redox potential:

when a substrate has lower affinity for electrons than hydrogen, it has a negative redox potential.

If the substance has a positive redox potential, it has a higher affinity for electrons than hydrogen.

Thus, NADH, a strong reducing agent has a negative redox otential where as a sterong oxidant like Oxygen has a positive redox potential.

Enzymes involved in oxidation reduction reactions are called oxidoreductases. They can be classified as

      1. Oxidases

      2. Dehydrogenases

3. Hydroperoxidases
4. Oxygenases

Oxidases:

Oxidases catalyze the removal of Hydrogen from substrate which is accepted byOxygen. They form mostly water, sometimes Hydrogen peroxide as a reaction product.

                    

AH_{2  +} ½ O_{2  ------------------>     A  +  H2O}   

AH_2 + O_{2  --------------------->} A + H₂O₂

This group includes Cytochrome oxidase, Tyrosinase, polyphenol oxidase, catachol oxidase and monoamine oxidase etc.

Cytochrome oxidase

It is a haemoprotein having heme as the prosthetic group. It is the terminal component of electron transport chain, and transfers electrons (obtained from the oxidation of substrate molecules by dehydrogenases) to the their final acceptor, Oxygen.

The enzyme is poisoned by Carbon monoxide, cyanide and Hydrogen sulfide. It used to be termed as cytochrome a₃ which is now termed as cytochrome aa₃. It contains two molecules of heme, each having one Fe atom that oscillates between Fe³⁺ and Fe²⁺. It also contains two atoms of Copper.

Some oxidases are flavo proteins. They contain FMN ( Flavin mono nucleotide) or FAD (Flavin adenine dinucleotide) as prosthetic groups. Riboflavin is required for the formation of FMN and FAD in body.

Examples of flavoprotein enzymes are L-aminoacid oxidases, xanthine oxidase and aldehyde dehydrogenase

Dehydrogenases:

these enzymes catalyze the removal of hydrogen from a substrate but oxygen can not act as the Hydrogen acceptor. They catalyze the reversible transfer of Hydrogen from one substrate to another thus bringing abou oxidation and reductions. The often require co enzymes as acceptors of Hydrogen atoms.

A large number of enzymes belong to this group.

NAD⁺ (Nicotinamide adenine dinucleotide) dependent dehydrogenases:

NAD⁺ is derived from nicotinic acid, a member of vitamin B-complex. When NAD⁺ accepts two Hydrogen atoms, one f the hydrogen atom is removed from the substrate as such. The other Hydrogen atom is split into one Hydrogen ion and one electron. The elctron is accepted by NAD⁺ to neutralize the positive charge on the co enzyme molecule. The remaining Hydrogen ion is released into the surrounding medium.

H₂ -----------------> H + H⁺ + e^-

AH₂ + NAD⁺ -----------------> A + NADH +H⁺

NAD⁺ linked dehydrogenases are

• Glyceraldehyde-3- phosphate dehydrogenase
• Isocitrate dehydrogenase
• Malate dehydrogenase
• Glutamate dehydrogenase
• beta hydroxy acyl co A dehydrogenase
• Pyruvate dehydrogenase
• α keto glutarate dehydrogenase

NADP⁺ (Nicotinamide adenine dinucleotide phosphate) linked dehydrogenases:

they take part in reductive biosynthetic reactions like extra mitochondria pathway of fattyacid synthesis and steroid synthesis. They aso articipate in pentose phosphate pathway.

Eg. HMG coA reductase, enoyl reductase

FAD linked dehydrogenases:

Unlike NADP⁺ and NAD⁺ dehydrogenases, FAD accepts both the Hydrogen atoms.

eg. Succinate dehydrogenase, acyl coA dehydrogenase

Cytochromes:

All the cytochromes of electron transport chain except cytochrome aa₃ belong to this group.

All cytochromes are heme proteins having iron atom.

Hydroperoxidases:

They use Hydrogen peroxide and any organic peroxide as substrate. Two types of enzymes fall in this category.

1. Peroxidases
2. catalases

Hydroperoxidases protect the body against harmful peroxidases. Accumulation of peroxides leads to the generation of free radicals which may cause cancer and atherosclerosis.

Peroxidases: they reduce peroxides using various electron acceptors such as ascorbate, quinones, and cytochrome C. The reaction catalyzed by peroxides is complex but the overall reaction is as follows:

H₂O₂ + AH₂ ------Peroxidase-----> 2H₂O

Examples of peroxidases are glutathione peroxidase in RBC which contains Selenium as prosthetic group, leucocyte peroxidase and Horse radish peroxidase.

Catalases: It is a heme protein containing four heme groups. Catalases use H₂O₂ as both electron donor and acceptor.

2H₂O ------Catalase------> 2H₂O + O₂

Caalase is fund in blood, bone marrow, mucus membrane, kidney and liver. It destructs the H₂O₂ found by the action of oxidases.

Peroxisomes are subcellular organelles having both oxidase and catalase activity.

Oxygenases:

This group of enzymes catalyze direct direct transfer and incorporation of Oxygen into the substrate molecule.

The catalytic reaction of oxygenase occurs in two steps:

1. Oxygen is bound to enzyme at active site
2. the bound oxygen is transferred to substrate

Oxygenases may be subdivided into

a. monooxygenases

b. dioxygenases

a. Monooxygenase:

They are also known as mixed function oxidases. They catalyze the incorporation of one atom of oxygen, while the other atom is reduced to H₂O. These enzymes are also called Hydroxylases because OH group is incorporated into the substrate. NADPH usually provides reducing equivalents.

 

Examples of monooxygenases are

1. phenyl alanine hydroxylase,
2. hydroxylase,
3. tryptophan hydroxylase
4. Nitricoxide synthase
5. Mitochondrial cytochrome P 450 monooxygenase
   450 denotes that it absorbs light at 450nm, when the heme combines with Carbon Monoxide. It is required for steroid hydroxylation in adrenal cortex, testis and ovary

b. Dioxygenases:

They incorporate both atoms of molecular Oxygen into the substrate.

A + O₂ ----------------->AO₂

Superoxide dismutase:

Transfer of single electron to O₂ generates Superoxide free radical which is potentially damaging by giving rise to free radical chain reaction. The enzyme superoxide dismutase is responsible for the removal of superoxide anion.

The enzyme occurs in all major tissues in the mitochondria and cytosol.