Oxidation states in the context of "FADH₂"

In biochemistry, flavin adenine dinucleotide (FAD) is a redox-active coenzyme associated with various proteins, which is involved with several enzymatic reactions in metabolism. A flavoprotein is a protein that contains a flavin group, which may be in the form of FAD or flavin mononucleotide (FMN). Many flavoproteins are known: components of the succinate dehydrogenase complex, α-ketoglutarate dehydrogenase, and a component of the pyruvate dehydrogenase complex.

FAD exists in two common oxidation states, the fully oxidized form (FAD) and the fully reduced, dihydrogenated form, FADH₂. Other oxidation states also exist, including the N-oxide and semiquinone states. FAD, in its fully oxidized form, accepts two electrons and two protons to become FADH₂. The semiquinone (FADH) can be formed by either reduction of FAD or oxidation of FADH₂ by accepting or donating one electron and one proton, respectively. Some proteins, however, generate and maintain a superoxidized form of the flavin cofactor, the flavin-N(5)-oxide.

The iron cycle (Fe) is the biogeochemical cycle of iron through the atmosphere, hydrosphere, biosphere and lithosphere. While Fe is highly abundant in the Earth's crust, it is less common in oxygenated surface waters. Iron is a key micronutrient in primary productivity, and a limiting nutrient in the Southern ocean, eastern equatorial Pacific, and the subarctic Pacific referred to as High-Nutrient, Low-Chlorophyll (HNLC) regions of the ocean.

While iron can exist in a range of oxidation states from −2 to +7; however, on Earth it is predominantly in its +2 or +3 redox state. It is a primary redox-active metal in nature. The cycling of iron between its +2 and +3 oxidation states is referred to as the iron cycle. This process can be entirely abiotic or facilitated by microorganisms, especially iron-oxidizing bacteria. The abiotic processes include the rusting of metallic which, in addition to oxidation of the metal, involves oxidation of Fe(II) in the presence of oxygen. Another type of abiotic process is the reduction of Fe to Fe by sulfide minerals. The biological cycling of Fe is mediated by iron oxidizing and reducing microbes.