Heme-iron in lipid oxidation

Oxygen transport, oxygen storage and oxygen activation in aerobic organisms depend on the iron porphyrin moiety in heme proteins. Under fluctuating oxygen supply and pH decrease the heme pigments like myoglobin and hemoglobin become catalytic in lipid peroxidation by mechanisms different from mechanisms for lipid oxidation by the non-heme-iron lipoxygenase, which is pivotal in energy metabolism. Simpler iron species originating from degradation of heme proteins and other sources bind to negatively charged phospholipids in membranes and catalyze the cleavage of preformed lipid hydroperoxides. Heme initiated lipid peroxidation, involved in pathogenesis in humans, is autocatalytic and forms lipid hydroperoxides and is further linked to cross-linking of proteins. It is also important for quality deterioration of muscle-based food. Heme-iron catalyzed lipid peroxidation is classified into four groups: (i) Fenton-like mechanism; (ii) iron(III)/iron(IV) mechanism; (iii) pseudoperoxidase mechanism; and (iv) iron(II)/iron(IV) mechanism. Partly proteolysed myoglobin becomes catalytic by a Fenton-type one-electron Fe(II)/Fe(III) cycling mechanism, rather than by a pseudoperoxidase mechanism as known from proteolysed cytochromes. Besides hydroxyl-, alkoxyl-, and peroxyl-radicals, nitric oxide is also important in lipid oxidation with a possible role of myoglobin as a nitric oxide dependent antioxidant.