Nonheme oxygen-binding iron proteins

Whereas oxygen binding in humans and many other animals involves heme units, not all life-forms bind and carry dioxygen in this way. Hemerythrin is a nonheme iron protein used by sipunculid and brachiopod marine invertebrates for oxygen transfer and/or storage. 

Ionic forms of iron (referred to hereafter as iron) also participate in a variety of enzymatic reactions as nonheme irons, which are present as iron-sulfur clusters (e.g., mitochondrial electron transport). There are also both storage and transportable forms of iron that are bound to proteins. Under normal physiological conditions only trace amounts of free iron exist. In the body, if iron exceeds the sequestration capacity of the iron-binding proteins present in different physiological compartments, the free iron can cause tissue damage. Cellular injury is caused by reactive oxygen species that are produced from H2O2 in a reaction catalyzed by iron. Thus, iron homeostasis in the body is in a delicate balance. Either the deficiency or the excess results in abnormalities and presents as a common cause of human diseases. 

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