Oxygen cycle coupling

FIGURE 18.3 The flow of energy in the biosphere is coupled primarily to the carbon and oxygen cycles. 

Hence, the sources and sinks of dissolved inorganic nitrogen and dissolved oxygen in the ocean are intimately linked and their global cycles coupled. 

Schidlowski M. and Junge C. E. (1981) Coupling among the terrestrial sulfur, carbon and oxygen cycles numerical... 

The formation of reduced carbon can account for the qualitative features of the oxygen cycle. The fate of the reduced carbon in sediments is, however, coupled with the sulfur cycle, as described by Berner (1984) and Garrels and Lerman (1981). Some of the reduced carbon may be oxidized by bacteria under anoxic conditions with sulfate acting as the oxidizing agent. The carbon is fully oxidized to carbonate and the sulfur is reduced in several stages to a sulfide or to iron pyrite. The "stripped-down", balanced, chemist s redox equation is ... 

CONTROL OF OXYGEN ON COUPLED P AND C CYCLING IN CONTINENTAL MARGINS A CASE STUDY FROM THE ANOXIC SAANICH INLET... 

Krebs Cycle and Fatty Acid Oxidation. A possible role of Krebs cycle intermediates in supporting fatty acid oxidation is now apparent. Complete oxidation to CO2 requires oxalacetate to introduce acetyl CoA into the citric acid cycle. But even the formation of acetoacetate requires the continued generation of ATP to support the activation of fatty acids. The transfer of electrons from fatty acid to oxygen is coupled with phosphate esterification, so that fatty acid oxidation has the theoretical capacity to be self-supporting. In the crude systems that contain all of the essential factors for fatty acid oxidation, fatty acid activation must compete with other reactions for the available ATP, and maximum rates of oxidation occur only when additional ATP is generated through operation of the Krebs cycle. 

This impressive reaction is catalyzed by stearoyl-CoA desaturase, a 53-kD enzyme containing a nonheme iron center. NADH and oxygen (Og) are required, as are two other proteins cytochrome 65 reductase (a 43-kD flavo-protein) and cytochrome 65 (16.7 kD). All three proteins are associated with the endoplasmic reticulum membrane. Cytochrome reductase transfers a pair of electrons from NADH through FAD to cytochrome (Figure 25.14). Oxidation of reduced cytochrome be, is coupled to reduction of nonheme Fe to Fe in the desaturase. The Fe accepts a pair of electrons (one at a time in a cycle) from cytochrome b and creates a cis double bond at the 9,10-posi-tion of the stearoyl-CoA substrate. Og is the terminal electron acceptor in this fatty acyl desaturation cycle. Note that two water molecules are made, which means that four electrons are transferred overall. Two of these come through the reaction sequence from NADH, and two come from the fatty acyl substrate that is being dehydrogenated. 

The laccases, classed as polyphenol oxidases, catalyze the oxidation of diphenols, polyamines, as well as some inorganic ions, coupled to the four-electron reduction of oxygen to water see Fig. 12.4 for the proposed catalytic cycle. Due to this broad specificity, and the recognition that this specificity can be extended by the use of redox mediators [27], laccases show promise in a range of applications [28], from biosensors [29-32], biobleaching [27, 33-35] or biodegradation [36], to biocatalytic fuel cells [1-3, 18, 26, 37-42]. 

All the ATP comes from oxidative phosphorylation coupled to the metabolism of acetyl-CoA by the TCA cycle. No oxygen, no p oxidation. 

It is postulated that the mechanism of the silane-mediated reaction involves silane oxidative addition to nickel(O) followed by diene hydrometallation to afford the nickel -jr-allyl complex A-16. Insertion of the appendant aldehyde provides the nickel alkoxide B-12, which upon oxygen-silicon reductive elimination affords the silyl protected product 71c along with nickel(O). Silane oxidative addition to nickel(O) closes the catalytic cycle. In contrast, the Bu 2Al(acac)-mediated reaction is believed to involve a pathway initiated by oxidative coupling of the diene and... 

Van Cappellen, P. Wang Y. 1996. Cycling of iron and manganese in surface sediments a general theory for the coupled transport and reaction of carbon, oxygen, nitrogen, sulfur, iron and manganese. American Journal Sciences, 296, 197-243. 

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