Ribulose monophosphate cycle

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. 

The reactions enclosed within the shaded box of Fig. 17-14 do not give the whole story about the coupling mechanism. A phospho group was transferred from ATP in step a and to complete the hydrolysis it must be removed in some future step. This is indicated in a general way in Fig. 17-14 by the reaction steps d, e, and/. Step/represents the action of specific phosphatases that remove phospho groups from the seven-carbon sedoheptulose bisphosphate and from fructose bisphosphate. In either case the resulting ketose monophosphate reacts with an aldose (via transketolase, step g) to regenerate ribulose 5-phosphate, the C02 acceptor. The overall reductive pentose phosphate cycle (Fig. 17-14B) is easy to understand as a reversal of the oxidative pentose phosphate pathway in which the oxidative decarboxylation system of Eq. 17-12 is... 

Figure 21 -8 Major glycolytic pathways of the erythrocyte. Substrates are in uppercase type, and enzymes are in parentheses. EMP, The Embden-Meyerhof pathway HMP hexose monophosphate pathway or pentose shunt RLC, the Rapoport-Luebering cycle ADP, adenosine diphosphate ATP, adenosine triphosphate NAD, nicotinamide-adenine dinudeotide NADH, reduced nicotinamide-adenine dinucleotide NADP, nicotinamide-adenine dinucleotide phosphate NADPH, reduced nicotinamide-adenine dinucleotide phosphate.The step from ribulose-5-phosphate, which is shown as being catalyzed by transketolase and transaldolase, is an abbreviation of this portion of the HMR...

The grand picture, requiring three turns of the cycle, is that 3 equivalents of carbon dioxide (CO2) (i.e., 3 Ci) react with 3 equivalents of the five-carbon carbohydrate derivative, ribulose-l,5-bisphosphate (i.e., 3 C5) to yield three (hypothetical) six-carbon adducts, 2-carboxy-3-ketoribitol-l,5-bisphosphate (3 Ci + 3 C5 = 3 Ce). The three Ce species then undergo what can be written as a retroaldol Chapter 9, Part 1, IV, C, 2 and Part 2, E to yield 6 equivalents of the monophosphate ester of the three-carbon dihydroxy monocarboxylate, 3-phosphoglycerate (PGA) (i.e., 3 Ce = 6 C3). Of the 6 equivalents of PGA produced, five are used to regenerate 3 equivalents of ribulose-l,5-bisphosphate (i.e., 5 C3 = 3 C5), while the sixth is now available to combine with another to make a six-carbon sugar (2 C3 = 1 Ce) such as glucose or for other ends, such as decarboxylation to acetate. 

As shown in Scheme 12.87, D-ribulose-5-phosphate can also be derived from D-glucose-6-phosphate, which, as shown in Scheme 11.13, is generated from dihydroxyacetone monophosphate (i.e., an isomer of 3-phosphoglycerate— from the Calvin cycle). 

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