Lipoxygenase catalytic cycle

In this chapter the generation of free radicals, mainly superoxide and nitric oxide, catalyzed by prooxidant enzymes will be considered. Enzymes are apparently able to produce some other free radicals (for example, HO and N02), although their formation is not always rigorously proved or verified. The reactions of such enzymes as lipoxygenase and cyclooxygenase also proceed by free radical mechanism, but the free radicals formed are consumed in their catalytic cycles and probably not to be released outside. Therefore, these enzymes are considered separately in Chapter 26 dedicated to enzymatic lipid peroxidation. 

Lipoxygenases (LOX), cycloxygenases (COXs), and xanthine oxidase (XO) are metalloen-zymes whose catalytic cycle involves ROS such as lipid peroxyl radicals, superoxide, and hydrogen peroxide. LOXs and COXs catalyze important steps in the biosynthesis of leuco-trienes and prostaglandins from arachidonic acid, which is an important cascade in the development of inflammatory responses. XO catalyzes the ultimate step in purine biosynthesis, the conversion of xanthine into uric acid. XO inhibition is an important issue in the... 

Figure 5 Catalytic cycles of horseradish peroxidase (a) and of soybean lipoxygenase (b). ROOH = H202 or an alkylhydroperoxide DH = electron-donor substrate of peroxidase LH = linoleic acid LOOH = 13-hydroperoxide of linoleic acid (P) Fe(III) = Fe (III) (protoporphyrin IX).

Low concentrations of calcium ion (1-2 pM) are required for maximal activity of purified 5-LO, but the major role of calcium appears to be that of increasing lipophilicity of 5-LO through a C-2-like domain in order to translocate to the nuclear envelope. ATP has a stimulatory effect on 5-LO at 20 nM and lipid hydroperoxides are important to initiate the 5-LO catalytic cycle since they readily form Fe(III) from inactive 5-LO with Fe(II) by the pseudoperoxidase mechanism. Microsomal membranes as well as phosphatidylcholine vesicles can stimulate purified 5-LO activity since 5-LO performs the oxidation of AA at the interface between the membrane and cytosol in a manner similar to that of cPLAja. Calcium ions increase the association of 5-LO with phosphatidylcholine vesicles that likely recapitulate events within the cell where 5-LO becomes associated with the nuclear membrane which is rich in phosphatidylcholine [25]. At the nuclear membrane AA is thought to be presented by a protein which has been termed 5-lipoxygenase activating protein (FLAP) [24]. In cells, such as the neutrophil and mast cells where 5-LO is found in the cytosol, 5-LO is catalytically active only when bound to a membrane, typically the nuclear membrane. In fact, in some cells 5-LO is found to be constitutively associated with the nuclear membrane, likely a result of a process of cellular activation while 5-LO is found in the alveolar macrophage within the nucleus itself [25]. 

Scheme 2 Catalytic cycles of the anaerobic and aerobic lipoxygenase reactions...

In the third example, no evidence has been obtained, despite serious effort [56], for direct interaction of oxygen with the ferrous iron in lipoxygenase, but NO forms a complex with a pH-dependent Kd in the range 10-95 pM [49]. Instead, the lipoxygenase iron-NO complex (Figure 15) could be viewed as a model of an intermediate in the catalytic cycle ... 

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