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The Chemistry of Molecular Oxygen

Metabolism can be either aerobic (requiring oxygen) or anaerobic (occurring in the absence of oxygen). Anaerobic metabolism is the older process: Earth's atmosphere has contained molecular oxygen for less than half the planet's existence. For organisms like yeast that can operate in either mode, aerobic metabolism is generally the more efficient process, yielding tenfold more energy from the metabolism of a molecule of glucose than do anaerobic processes. But the efficiency that comes from the use of molecular oxygen as an electron acceptor carries a price. Molecular oxygen is easily transformed into toxic compounds. For example, hydrogen peroxide, H ₂O ₂, is used as a disinfectant, as is ozone, O ₃. Furthermore, molecular oxygen can also oxidize metal ions, and that can cause problems. Iron‐containing enzymes and proteins use reduced iron, Fe(II) or Fe(I), and don't function if the iron atoms are oxidized to the stable Fe(III) form. Organisms must have means of preventing the oxidation of their iron atoms.

The third problem caused by the use of molecular oxygen as an electron acceptor is the fact that it really isn't very soluble in water. (If it were more soluble, people couldn't drown!) Multicellular organisms have evolved various oxygen transporters to solve the twofold problem of keeping oxygen tied up and less toxic as well as being able to deliver O ₂ rapidly enough and in sufficient quantity to support metabolism. All animals (other than insects) with more than one kind of cell have evolved specialized proteins to carry oxygen to their tissues. The protein responsible for carrying oxygen in the blood of most terrestrial animals is hemoglobin. Within the tissues, especially muscle tissue, a related oxygen carrier, myoglobin, keeps molecular oxygen available for its final reduction to water as the end product of catabolism (nutrient utilization).

Mars

Mars is similar to Earth in that it has about a twenty‐four‐hour day, an atmosphere, polar ice caps (carbon dioxide), and winter and summer seasons. Unlike Earth, Mars does not have a dipolar magnetic field. To date, no evidence of life has been discovered on the planet.

The northern hemisphere is marked by shield volcanoes and volcanic cones. Craters are widely scattered. These volcanic mountains are three times as high as the tallest volcanic mountains on Earth. Olympus Mons is the largest volcanic structure discovered so far in the solar system. The tremendous size of the volcanoes suggests that the magma sources supplied the volcanic vent for a very long time and that the lithosphere is very strong. Horst and graben fault basins have also been identified. The volcanic activity is thought to be fairly recent, since many of the cones have not been pitted by meteoric impacts (an application of relative time principles). Craters are also rare on the volcano slopes, suggesting the most recent layers of volcanic flows are less than 100 million years old. In contrast, the southern hemisphere is studded with thousands of impact craters and is not as volcanically active.

The Martian surface, weathered and composed of clay and sulfate materials, is shaped by winds that form sand dunes. Meandering braided structures look as though they were formed by running water or gigantic floods. Flooding could occur if the frozen surface were suddenly melted by magmatic activity or a change in climate.

The Electron Transport Chain

Electrons flow through the electron transport chain to molecular oxygen; during this flow, protons are moved across the inner membrane from the matrix to the intermembrane space. This model for ATP synthesis is called the chemiosmotic mechanism, or Mitchell hypothesis. Peter Mitchell, a British biochemist, essentially by himself and in the face of contrary opinion, proposed that the mechanism for ATP synthesis involved the coupling between chemical energy (ATP) and osmotic potential (a higher concentration of protons in the intermembrane space than in the matrix). The inner membrane of the mitochondrion is tightly packed with cytochromes and proteins capable of undergoing redox changes. There are four major protein‐membrane complexes.

Complex I and Complex II

Complex I and Complex II direct electrons to coenzyme Q. Complex I, also called NADH‐coenzyme Q reductase, accepts electrons from NADH. The NADH releases a proton and two electrons. The electrons flow through a flavoprotein containing FMN and an iron‐sulfur protein. First, the flavin coenzyme (flavin mononucleotide) and then the iron‐sulfur center undergo cycles of reduction and then oxidation, transferring their electrons to a quinone molecule, coenzyme Q(see Figure 1). Complex I is capable of transferring protons from the matrix to the intermembrane space while undergoing these redox cycles. One possible source of the protons is the release of a proton from NADH as it is oxidized to NAD, although this is not the only explanation. Apparently, conformational changes in the proteins of Complex I also are involved in the mechanism of proton translocation during electron transport. 





                       Figure 1

Complex II, also known as succinate‐coenzyme Q reductase, accepts electrons from succinate formed during the TCA cycle. Electrons flow from succinate to FAD (the flavin‐adenine dinucleotide) coenzyme, through an iron‐sulfur protein and a cytochrome b ₅₅₀ protein (the number refers to the wavelength where the protein absorbs), and to coenzyme Q. No protons are translocated by Complex II. Because translocated protons are the source of the energy for ATP synthesis, this means that the oxidation of a molecule of FADH ₂ inherently leads to less ATP synthesized than does the oxidation of a molecule of NADH. This experimental observation also fits with the difference in the standard reduction potentials of the two molecules. The reduction potential of FAD is ‐0.22 V, as opposed to ‐0.32 V for NAD.

Coenzyme Q is capable of accepting either one or two electrons to form either a semiquinone or hydroquinone form. Figure  2 shows the quinone, semiquinone, and hydroquinone forms of the coenzyme. Coenzyme Q is not bound to a protein; instead it is a mobile electron carrier and can float within the inner membrane, where it can transfer electrons from Complex I and Complex II to Complex III.

     
   

                               Figure 2

Complex III is also known as coenzyme Q‐cytochrome c reductase. It accepts electrons from reduced coenzyme Q, moves them within the complex through two cytochromes b, an iron‐sulfur protein, and cytochrome c ₁. Electron flow through Complex II transfers proton(s) through the membrane into the intermembrane space. Again, this supplies energy for ATP synthesis. Complex III transfers its electrons to the heme group of a small, mobile electron transport protein, cytochrome c.

Cytochrome c transfers its electrons to the final electron transport component, Complex IV, or cytochrome oxidase. Cytochrome oxidase transfers electrons through a copper‐containing protein, cytochrome a, and cytochrome a ₃, and finally to molecular oxygen. The overall pathway for electron transport is therefore:

  
 
or:

The number n is a fudge factor to account for the fact that the exact stoichiometry of proton transfer isn't really known. The important point is that more proton transfer occurs from NADH oxidation than from FADH ₂ oxidation.