Biochemistry - The Chemical Reactions of Living Cells, Volume 2 - D. Metzler 1980

How electrons meet oxygen, how ATP is formed in the process, and related phenomena
Oxygenases and hydroxylases
Dioxygenases

Among the oxygenases that incorporate both atoms of O₂ into the product, the best known are the Enzymes that cleave the double bonds of Aromatic Compounds at positions adjacent to or between OH groups, as shown in equation (10-43). Other dioxygenases cleave aliphatic compounds. A well-known example is the Cleavage of ß-carotene to yield vitamin A (Box 12-B). Several dioxygenases have been isolated in crystalline form from Bacteria. They were found to be Non-heme iron Proteins, with a subunit Molecular Weight of 50,000 or greater. These proteins typically contain Fe(II) but lack labile sulfur [1]. On the other hand, Tryptophan dioxygenase (tryptophan pyrrolase; Chapter 14, Section II) is a heme enzyme. It catalyzes the reaction described by equation (10-45). The oxygen atoms marked with an asterisk originate from O₂.

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Like non-heme oxygenases, tryptophan dioxygenase is active when the iron incorporated into its molecule is in the ferrous state. It is natural to assume that an essential first step in the reaction is The formation of a complex between Fe(II) and O₂. However, tryptophan must be present in the system prior to complex formation with O₂. At 5°C, the enzyme, tryptophan, and O₂ form a complex with an altered spectrum resembling that of compound III in peroxidases [equation (10-6)]. This oxygenated complex may subsequently convert into an Fe(III) complex containing a peroxide ion [equation (10-46)]. There is evidence (based on the inhibitory

effect of superoxide dismutase on enzymes from certain organisms [137]) that it is the superoxide anion radical which attacks the substrate, initiating the hydroxylase reaction [equation (10-47)]. Note that in the first step, one electron is returned to the Fe(III) form of the enzyme, regenerating the initial Fe(II) form.

There is a plant enzyme classified among the dioxygenases known as lipoxygenase (lipoxidase). Lacking both a metal and any other known prosthetic group, lipoxygenase catalyzes The oxidation of polyunsaturated Fatty acids in Lipids [equation (10-48)] [138]. The Formation of the hydroperoxide product is accompanied by a double-bond shift and its transition from the cis- to the trans-configuration. The enzyme appears to generate free radicals, though The Mechanism of their formation remains unclear. By producing a hydroperoxide as its product, lipoxygenase bears a certain resemblance to the Monooxygenases discussed in the following subsection.