0.21 Bis2a 06.1 red/ox in metabolism v1.2  (Page 2/5)

In metabolism, reduced compounds (organic or inorganic) can be used as electron donors, as long as the organism has the tools (enzymes) to use them. In other words whether a compound can be used as an electron source (or acceptor) is determined by whether that organism has the appropriate machinery, enzymes, to utilize the compound as a substrate. This is the basis of metabolic diversity. Some organisms to use all sorts of compounds as electron donors and electron acceptors. This allows them to live in environments others can not. While other organisms, such as us humans, are very limited as to what we can use: either NADH or FADH ₂ for electron donors and only molecular oxygen (O ₂ as a terminal electron acceptor.

In biological systems, oxidation-reduction reactions are catalyzed by enzymes that transfer electrons from the donor (the reduced compound or source of electrons) to the acceptor (the oxidized compound or electron acceptor). These enzymes can be single proteins to large, multiprotein complexes. Often times, the removal of electrons is simultaneously coupled to the removal of a proton, or a hydrogen atom (one proton and one or two electrons). Many of these enzymes are called dehydrogenases , because they remove a hydrogen atom (a proton and one or two electrons)

The removed electrons and/or the associated proton are not "carried" on the protein per se but are carried on either a cofactor (sometimes referred to as a coenzyme) or a prosthetic group intimately coupled to the enzyme. The two most common coenzymes of oxidation-reduction reactions are nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD) . Their respective reduced coenzymes are NADH and FADH ₂ , which have a very low reduction potential and can be used to synthesize ATP or do aid in other forms of cellular work.

How are red/ox reactions used to generate usable energy for the cell

We have already discussed how some compounds can store a lot of energy while other compounds are relatively poor in energy stores. We know this intuitively, methane is explosive, methanol will catch on fire, and carbon dioxide is relatively inert. We also discussed that a distinguishing feature of "energy" rich compounds tend to more reduced than their "energy" poor relative. The difference between methane, methanol and carbon dioxide is the oxidation state of the carbon, -4 in methane, -2 in methanol and +4 in carbon dioxide. Methane is the most reduced state while carbon dioxide is the most oxidized state.

The same can be true for all sorts of compounds. In particular, for studying metabolism, we are interested in three types of molecules. The first are those compounds that can provided energy for the cell, such as glucose or methane, or even Fe ²⁺ . These compounds are referred to as the electron donor and it is during their chemical modification by the cell that energy in the form of ATP can be generated.

The second type of molecules are those that cells can use as a final resting place for the electrons extracted from the electron source. These compounds are referred to as the terminal electron acceptor . These can be inorganic molecules or ions, such as molecular oxygen (O ₂ ) or nitrate (NO ₃ ^- . They can also be organic molecules such as lactate or acetate.