Principles of Biochemistry, Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Electron Transport, Oxidative Phosphorylation, and Regulation of ATP Synthesis
Cells also contain other enzymes that utilize oxygen as an electron acceptor.

In almost all Cells, about 90% of total oxygen consumption is reduced through the mediation of mitochondrial cytochrome oxidase. However, certain Tissues contain Other types of Enzymes that catalyze specific redox reactions, incorporating oxygen atoms directly into the substrate molecule to form, for example, a new hydroxyl or carboxyl group. These enzymes are termed oxygenases. Although these specialized reactions consume only a small fraction of the total oxygen taken up by cells, they play a vital role in the Organism.

There are two classes of oxygenases: Dioxygenases and Monooxygenases. Dioxygenases catalyze reactions in which both atoms of an oxygen molecule are incorporated into the organic substrate molecule. A classic example is pyrocatechase, an enzyme that catalyzes The oxidation of catechol by molecular oxygen, accompanied by ring Cleavage:

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If the reaction involves molecular oxygen labeled with the ¹⁸O isotope, the label (highlighted in red) is found exclusively in the carboxyl groups of the product.

Monooxygenases (which are relatively abundant in tissues and exhibit more complex behavior) catalyze reactions in which only one atom of the oxygen molecule is incorporated into the organic substrate molecule, while the second atom is reduced to H₂O. Monooxygenases require two substrates that act as reducing agents for the two oxygen atoms of O₂. The principal substrate incorporates one of the two oxygen atoms, whereas the cosubstrate supplies H atoms to reduce the second oxygen atom to H₂O. The overall equation for monooxygenase-catalyzed reactions is of the form

where AH is the principal substrate incorporating one oxygen atom, and BH₂ is the cosubstrate supplying H atoms for the reduction of the second oxygen atom to H₂O. Because the principal substrate in monooxygenase-catalyzed reactions is typically hydroxylated, this group of enzymes is also referred to as hydroxylases. They are occasionally called mixed-function oxygenases because they simultaneously oxidize two different substrates.

Monooxygenases are subdivided into several classes depending on The Nature of the cosubstrate involved in the reaction, which supplies the two H atoms for The formation of H₂O. Some monooxygenases utilize reduced flavin NUCLEOTIDES (FMNH₂ or FADH₂) as cosubstrates for this purpose, others use NADH or NADPH, and still others use α-ketoglutarate. A prominent example among monooxygenases is the enzyme that catalyzes the hydroxylation of the aromatic ring of phenylalanine, yielding Tyrosine (Chapter 19). An inherited defect in The activity of this enzyme underlies the genetic disorder known as phenylketonuria.

The most numerous and particularly complex monooxygenase reactions involve cytochrome P-450, a member of the hemoprotein group. This cytochrome is typically located in The Endoplasmic reticulum rather than in the Cell/35.html">Mitochondria. Much like mitochondrial cytochrome oxidase, cytochrome P-450 is capable of interacting with both oxygen and carbon monoxide. It differs from cytochrome oxidase, however, in that the complex of its reduced form with carbon monoxide exhibits a strong Light absorption band at 450 nm.

Cytochrome P-450 catalyzes hydroxylation reactions in which an organic substrate RH is hydroxylated to R-OH at the expense of one oxygen atom from O₂, whereas the second oxygen atom is reduced to H₂O through The addition of reducing equivalents from NADH or NADPH, or more frequently from an iron-sulfur protein. This reaction is depicted in simplified form in Fig. 17-30; in reality, it proceeds via a series of intermediates that are not yet fully understood. For instance, cytochrome P-450 participates in steroid hydroxylation during The Biosynthesis of Adrenocortical Hormones. Furthermore, cytochrome P-450 plays a crucial role in the hydroxylation of various drugs and other xenobiotics, particularly those that are relatively Water-insoluble. Hydroxylation increases the aqueous solubility of such foreign compounds, thereby greatly facilitating their detoxification and excretion from the body (Chapter 24). Cytochrome P-450 exists in multiple isoforms exhibiting distinct substrate specificities.

Fig. 17-30. Hydroxylation of the lipid-soluble drug RH by cytochrome P-450 functioning as a monooxygenase. The reaction product, R—OH, has a higher water solubility and is consequently more readily excreted from the organism. The cosubstrate supplying the H atoms (shaded gray) for the reduction of the second oxygen atom to water is NADPH + H⁺.