Carbon cycle pathways

Kozarich JW (1988) Enzyme chemistry and evolution in the )S-ketoadipate pathway. In Microbial Metabolism and the Carbon Cycle (Eds SR Hagedorn, RS Hanson, and DA Kunz), pp. 283-302. Harwood Academic Publishers, Chur, Switzerland. 

Diacids. The microbial generation of mahc, fumaric, and succinic acid essentially imphes Krebs cycle pathway engineering of biocatalytic organisms to overproduce oxaloacetate as the primary four-carbon diacid that subsequently undergoes reduction and dehydration processes (Scheme 2.9). The use of these four-carbon diacids as intermediate chemicals and the state of their desirable microbial production is briefly outlined. 

The combined activity of the rubisco oxygenase and the glycolate salvage pathway consumes 02 and produces C02—hence the name photorespiration. This pathway is perhaps better called the oxidative photosynthetic carbon cycle or C2 cycle, names that do not invite comparison with respiration in mitochondria. Unlike mitochondrial respiration, photorespiration does not conserve energy and may actually inhibit net biomass formation as much as 50%. This inefficiency has led to evolutionary adaptations in the carbon-assimilation processes, particularly in plants that have evolved in warm climates. 

In C4 plants, the carbon-assimilation pathway minimizes photorespiration C02 is first fixed in mesophyll cells into a four-carbon compound, which passes into bundle-sheath cells and releases C02 in high concentrations. The released C02 is fixed by rubisco, and the remaining reactions of the Calvin cycle occur as in C3 plants. 

To address the question of the respective importance of pathways in heterogeneous photocatalysis, a molecule susceptible to yield different primary products, depending on the initial attack by an OH radical or a hole, has been used. Six-membered aromatic carbon cycles do not fulfil this condition. For example, from a substituted benzene, the monohydroxycyclohexadienyl radical can be formed either from addition of the hydroxyl radical or from the capture of a hole, followed by the hydration and deprotonation of the resulting radical cation. 

The reactive chemical system. Some scientists have attempted to specify key components of such a system but not the entire reactive system. See, for example, de Duve62,63 and Weber.64,65 Others have suggested complete chemical cycles. Modified versions of the reductive citric acid cycle, a carbon-fixation pathway that is used by several organisms today, have been proposed.66 70... 

In conclusion, such a model is convenient to get an idea of the calcium oxalate concentration, CO2 pressure and conditions for potential precipitation of secondary calcium carbonate through oxalotrophic bacterial activity. It demonstrates that as long as calcium is available and oxalotrophic bacteria are present, transformation of oxalate into carbonate can occur under normal conditions found in soils and surficial sediments. Therefore, an oxalate-carbonate cycle, or at least pathway, must exist at the surface of continents (Verrecchia Dumont, 1996), explaining the absence of calcium oxalate accumulation in soils and the fossil record. 

Fig. 12.8. Simplified sketch showing main relationships inside the coupled calcium and carbon cycles of the oxalate-carbonate pathway in a hypothetical ecosystem. Plants and fungi are oxalate producers. Oxalotrophic bacteria (in the soil or animal guts) use oxalate as carbon, energy and electron sources, leading to CO2 and calcium carbonate production. Calcium carbonate can accumulate inside the soils. Because the carbon of the carbonate originates from organic carbon, its fossilization in the soil constitutes a carbon sink.

Carbon cycles in and out of the environment through many pathways. 

FIGURE 5.2 Schematic illustration of the modern "deep Earth" carbon cycle showing the main Earth carbon reservoirs and the pathways between them. The important fluxes are listed in Table 5.3. The mass of carbon in each reservoir is given in giga-tonnes of carbon (1 Gt = 1015g). Figure adapted from Killops and Killops (2005). 

The bifunctional carbon monoxide dehydrogenase (CODH)/acetyl-CoA synthase enzyme is a key enzyme involved in the Wood-Ljungdahl pathway of carbon fixation that operates in anaerobic bacteria. As such, it is a major player in the global carbon cycle. The CODH component of the enzyme catalyzes the reversible reduction of CO2 to CO (Equation (15)), which is then channeled to the ACS active site where it reacts with CoA and a methyl group provided by the corrinoid iron-sulfur protein (CFeSP) to form acetyl-CoA 30 (Equation (16)). 

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