ABSTRACT
Since triplet oxygen reacts very slowly or not at all with most organic compounds, the reactions of oxygen in vivo are catalyzed by a huge variety of oxygen metabolizing enzymes.
Although many enzymes are composed of only a protein, others, including all oxidation-reduction enzymes (oxidoreductases), require the presence of a cofactor or coenzyme [387]. These cofactors or coenzymes are essential for the catalytic activity of the enzyme. Some of these coenzymes are tightly bound (covalently) to the protein. These coenzymes are known as prosthetic groups. Another group of cofactors, which are not covalently linked, is consumed in stochiometric amounts during the reaction. These coenzymes are low molecular weight compounds, which in the case of oxidoreductases are electron or hydrogen donors or acceptors. Many oxidations require the cooxidation of substrate and coenzyme. The most commonly used cofactors in these oxidations are NAD+ (nicotineamide-adenine-dinucleotide) or NADP+ (nicotineamide-adenine-dinucleotide phosphate).
