Assimilation Pathways

One molecule of glyceraldehyde 3-phosphate is the net product of the carbon assimilation pathway. The other live triose phosphate molecules (15 carbons) are rearranged in steps to (S) of Figure 20-10 to form three molecules of ribulose 1,5-bisphosphate (15 carbons). The last step in this conversion requires one ATP per ribulose 1,5-bisphosphate, or a total of three ATP. Thus, in summary, for every molecule of triose phosphate produced by photosynthetic C02 assimilation, six NADPH and nine ATP are required. 

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. 

In hepatocytes and adipocytes, cytosolic NADPH is largely generated by the pentose phosphate pathway (see Fig. 14-21) and by malic enzyme (Fig. 21-9a). The NADP-linked malic enzyme that operates in the carbon-assimilation pathway of C4 plants (see Fig. 20-23) is unrelated in function. The pyruvate produced in the reaction shown in Figure 21-9a reenters the mitochondrion. In hepatocytes and in the mammary gland of lactating animals, the NADPH required for fatty acid biosynthesis is supplied primarily by the pentose phosphate pathway (Fig. 21-9b). 

Glyoxylate is an unconventional cell carbon precursor that requires special enzymes to be used in its biosynthesis a second cycle serves as the glyoxylate assimilation pathway [28]. Glyoxylate is combined with propionyl-CoA to P-... 

Fig. 1. The nitrate assimilation pathway in higher plants. The pathway of nitrate assimilation in the tobacco leaf is illustrated. In some other species an additional cytosolic GS is found in the leaf. The pathway in plant roots is more poorly documented and more variable GS in roots is mostly cytosolic, and some enzymes such as GOGAT are found as isoforms utilising alternate reducing substrates. T, expected nitrate carrier NR, nitrate reductase NiR, nitrite reductase GS, glutamine synthetase GOGAT, glutamate synthase Fd, ferredoxin Gin, glutamine Glu, glutamate.

Ammonium, the primary product of nitrogen fixation, is transported to the host cell cytoplasm where it is assimilated into amides and, in some cases, further converted into ureides before being transported to the shoot. Since the physiological environment within the nodule is apparently different from the other parts of the plant, nodule-specific or nodule-abundant forms of several enzymes of the nitrogen and carbon assimilation pathways have evolved, and are induced to improve the efficiency of nitrogen and carbon metabolism in nodules. 

Raven, J. A. 1990. Predictions of Mn and Fe use efficiencies of phototrophic growth as a function of light availability for growth and C assimilation pathway. New Phytologist 116 1-17. 

Sessi, E., Nardi, S., and Gessa, C. (2000). Effects of low and high molecular weight humic substances from two different soils on nitrogen assimilation pathway in maize seedlings. Humic Subst. Environ. 2(1/4), 39 16. 

MuUioUand and Lomas (this volume. Chapter 7) have reviewed the general uptake and assimilation pathways for photoautotrophs (summarized in Fig. 7.2). Transporter proteins are not enzymes in the strictest sense because they do not catalyze particular chemical transformations, but facilitate entry of specific compounds into... 

Table 3 Autotrophic reduction-oxidation reactions coupled to CO2 assimilation. The most common carbon assimilation pathways for each type of metabolism are listed, with alternative modes of CO2 fixation provided in parentheses. The free energy yield of the reaction is provided for pathways that do not require light.

Many other carboxylation reactions exist (Barton et al., 1991). For example, in methylo-trophic bacteria, formaldehyde and CO2 are combined to produce acetyl-CoA in the serine or hydroxypyruvate pathway. In contrast, the ribulose monophosphate cycle, which is another methylotrophic pathway of formaldehyde fixation, does not involve carboxylation steps. In addition to those described above, commonly found carboxylation reactions include those of pyruvate or phosphoenol pyruvate. In view of several relatively recent discoveries of novel CO2 assimilation pathways (e.g., the hydoxypro-pionate cycle and anaerobic ammonium oxidation) and growing interest in deep-subsurface microbiology, novel pathways of CO2 incorporation may be discovered in the near future. 

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