2H (neutral) are added as H+ & H-
When a substance is oxidized, the free hydrogen atoms do not remain free in the cell but are immediately transferred to another compound. This is accomplished by Coenzymes . Recall that most catalysts are protein molecules called Enzymes. Some enzymes consist of 2 parts: a protein part and a nonprotein part called a Cofactor. The cofactor may be a metal ion such as Fe, Mg or Ca or an organic molecule called a Coenzyme which are often derived from vitamins.
2 coenzymes are commonly used to carry hydrogen atoms:
Nicotinamide Adenine Dinucleotide (NAD) which is a derivative of vitamin B3 Niacin.
Flavin Adenine Dinucleotide (FAD) which is a derivative of vitamin B2 Riboflavin.
The transfer of hydrogen atoms by these coenzymes can be shown as:
When NAD+ is REDUCED to NADH + H , the NAD gains an H- ion and the H+ is released into the surrounding solution.
When NADH is OXIDIZED to NAD+, the loss of the hydride ion results in one less hydrogen atom.
When FAD is REDUCED to FADH2 it gains a hydrogen ion (H+) and a hydride (H-).
When FADH2 is OXIDIZED to FAD it loses the same two ions.
Oxidation and reduction reactions are ALWAYS coupled ; meaning each time one substance is oxidized, another is reduced. These are called Redox reactions. Using the same example above: When lactic acid is OXIDIZED to form pyruvic acid, the two hydrogen atoms removed in the reaction are used to REDUCE NAD+.
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