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–   In biological systems, REDOX reactions using the OXIDANT ability of Reactive Oxygen Species (ROS) are essential to body's functions maintaining life



When we say Oxygen "OXIDIZES" Food, What Does that Mean?

 

Oxygen Molecule (3 O2) "Activated" to Produce ROS by 2 Different Mechanisms

 

ROS production in the body can be Enzymatically or Non-enzymatically mediated

ROS are produced continuously in cells either as by-products of metabolism or deliberately as in immune system phagocytosis. They are also by-products of circulation, respiration, digestionand assimilation.






REDOX(REDuction/OXidation) REACTION – This chemical term refers to a two-way reaction in which:

–    One atom or molecule (REDUCTANT) - GIVES away an electron (REDuces)

–    Another atom or molecule (OXIDANT)- TAKES an electron(OXidizes).

When we say Oxygen "OXIDIZES" Food, What Does that Mean?

Oxygen is responsible for providing energy for the cell in a process called “cellular respiration”- which takes place in the cell mitochondria (“energy factories').

–    Oxygen (the “oxidant” in this REDOX reaction) “oxidizes“( removes electrons from)glucose (Electrons are transferred from the sun's radiant energy to glucose in food, via chlorophyll) - and then uses the 4 electrons it acquires to produce energy.

Electrons are removed from glucose in a series of enzyme-assisted steps, which pass them along an “Electron Transport Chain”until they are finally accepted by oxygen, producing water. Glucose is the “reductant” in this redox reaction, since glucose “reduces”(gives away electrons to) oxygen.

–   The energy gained in this process is stored (via a process called “oxidative phosphorylation”) as ATP molecules - the form of energy that cells use to function.

 

Cellular Respiration

The Oxygen Molecule ( 3O2 ) May be "Activated" to Produce ROS in the body by 2 Different Mechanisms

(1) By Excitation:

If Atmospheric Oxygen (3 O2) absorbs sufficient energy to reverse the spin of one of its unpaired electrons, it will form the ROS singlet oxygen, in which its two unpaired electrons have opposite spins.

Singlet oxygen does not then have the spin restriction of triplet oxygen, and can thus participate in reactions involving the simultaneous transfer of two electrons (divalent reduction). Since paired electrons are common in organic molecules, singlet oxygen is much more reactive towards organic molecules than 3 O2 .

Singlet oxygen can be introduced into the body by various therapies E.g. Ozone therapy, Hydrogen peroxide therapy, PDT

(Photodynamic Therapy –provides benefit of light therapy on photosensitive molecules purposefully introduced to target areas off body)

(2) By reduction: In the body, ROS are more typically produced by the progressive reductionof Oxygento form Superoxide, Hydrogen Peroxide, Hydroxyl Radical and finally water.

The energy-producing cellular respiration chain in the mitochondria (the energy "factory" of cell)uses more than 90% of the body's oxygen, in a 4-step processthat takes place one electron-transfer at a time, ultimately producing water, carbon dioxide (CO2) and energy. The summary process of how cells produce energy from glucose in the presence of sufficient oxygen is:

C6H12O6 (glucose) + 6 3O2 6 CO2 +6 H2O+ Energy

These partial equations show the intermediate ROS produced during this 4-step process:

1

3 O2+e-O2

Superoxide Radical(mildly reactive)

Adding 1e- to molecular oxygen

2

O2 -- +H2OHO2 +OH--

Hydroperoxyl Radical (highly reactive)

Adding H to Superoxide

3

HO2 +e-+ H H2O2

Hydrogen Peroxide (poorly reactive)

Adding 2 e- to molecular oxygen

4

H2O2+e- OH+OH--

Hydroxyl Radical(extremely reactive)

Adding 1e- to Hydrogen peroxide

ROS production in the body can be Enzymatically or Non-enzymatically mediated

Enzymatic oxidation:

✔ Oxidases: enzymes that catalyze the reaction when oxygen is the acceptor of hydrogen to form water or H2O2

✔ Hydroperoxidases and peroxidases: use H2O or ROOH as oxidizing agent;

✔ Dehydrogenases: catalyse dehydrogenation of a substrate using NAD+ or NADP+

✔ Oxygenases: incorporate oxygen into substrate

A variety of enzyme systems are capable of generating significant amounts of free radicals, including:

✔ Xanthine oxidase –    activated in ischemia / reperfusion (reduction / restoration of blood supply to an organ or tissue caused by constriction or obstruction of the blood vessels, as after a heart attack).

✔  Lipoxygenase enzymes catalyze reactions between 3 O2 and PUFAs (polyunsaturated fatty acids), such as arachidonic acid (AA), containing methylene interrupted double bonds.
When AA is the substrate, these Hydroperoxides are known as HpETEs which can be transformed into Hydroxy products (
HETEs).

✔  Cyclooxygenase (COX) enzymes catalyze the addition of 3 O2 to various PUFAs, converting them into biologically active molecules called endoperoxides(PGG, PGH) - intermediates in the transformation of fatty acids to prostaglandins.

✔ Myeloperoxidase -produced in neutrophils, monocytes and newly activated macrophages, catalyses hydrogen peroxide's oxidation of chloride ions into the powerful oxidant hypochlorous acid (HOCl)

✔ Prostaglandin synthase

✔ Aldehyde oxidase

✔ Amino acid oxidase.

Non-enzymatic (Autoxidation):

Autoxidation is a by-product of the aerobic internal milieu - molecules that undergo autoxidation include:

✔ Catecholamines

✔ Haemoglobin

✔ Myoglobin (in heavily exercised muscles)

✔  Rreduced cytochrome C

✔ Thiol.

Autoxidation of any of the above molecules in a reaction results in the reduction of3O2 and the formation of ROS -Superoxide is the primary radical formed.

A ferrous ion (Fe II) also, can have its electron stolen from it by oxygen to produce superoxide and Fe III - by the process of autoxidation (Fridovich, 1983 and 1995).

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