Reductive pentose cycle

Flow of Electrons from H20 to NADP+ Drives Proton Transport into the Thylakoid Lumen Protons Return to the Stroma through an ATP-Synthase Carbon Fixation Utilizes the Reductive Pentose Cycle Ribulose-Bisphosphate Carboxylase-Oxygenase Photorespiration and the C-4 Cycle... 

The reductive pentose cycle, or Calvin cycle. The number of arrows drawn at each step in the diagram indicates the number of molecules proceeding through that step for every three molecules of C02 that enter the cycle. The entry of three molecules of C02 results in the formation of one molecule of glyceraldehyde-3-phosphate (box on right), and requires the oxidation of six molecules of NADPH to NADP+ and the breakdown of nine molecules of ATP to ADP. 

Carbon Fixation Utilizes the Reductive Pentose Cycle... 

Other enzymes in the stroma can convert glycerate-3-phosphate back to ribulose-1,5-bisphosphate. The reactions of this reductive pentose cycle, or Calvin cycle, are shown in figure 15.25. 3-Phosphoglycerate is first phosphorylated to glycerate-l,3-bisphosphate at the expense of ATP and then reduced to glyceraldehyde-3-phosphate by NADPH. (Note that the nucleotide specificity in the reductive step differs from that of the cytosolic glyceraldehyde-3-phos-... 

The C-4 pathway requires the cooperation of two types of cells. Mesophyll cells (left) take up C02 from the air and export malate to the bundle sheath cells (right). The bundle sheath cells return pyruvate to the mesophyll cells and fix the C02 using ribulose bisphosphate carboxylase and the reductive pentose cycle. 

Many other sugars are involved in primary metabolic pathways such as the oxidative and reductive pentose cycle, glycolysis, and photosynthesis. 

There is increasing evidence that CAM is controlled to a great extent at the enzyme level. As outlined earlier (see Chap. 3), the main metabolic sequence linking malate to the reductive pentose cycle is ... 

The pathway of carbon dioxide fixation and assimilation to the level of sugar consists of a cyclic series of reactions sometimes referred to as the reductive pentose cycle (in contrast to the dissimilatory oxidative pentose cycle), the carbon reduction cycle or the Calvin cycle. ... 

Chloroplasts are the organelles which carry on the total synthesis of carbohydrates from CO2 through the reductive pentose cycle driven by the assimilatory power formed in the light phase of photosynthesis and made up of NADPH and ATP, but the oxidative pentose cycle can also operate inside the chloroplasts and regenerate NADPH in the dark.< - >... 

Some plants, such as corn and sugar cane, have evolved an auxiliary C4-dicarboxylic acid cycle< > that cooperates with the reductive pentose cycle in the photosynthetic assimilation of CO2. In plants with this cycle (sometimes referred to as the Hatch and Slack cycle), chloroplasts in the mesophyll cells near the surface on the leaf contain three C4-pathway specific enzymes pyruvate, phosphate-dikinase that directly converts pyruvate into phosphoenolpyruvate (PEP) with ATP, PEP carboxylase that catalyzes the carboxyla-tion of PEP to oxaloacetate, and malate dehydrogenase that finally reduces oxaloacetate to malate with NADPH. The purpose of these steps is apparently to incorporate CO2 and NADPH into malate in order to translocate them to the vascular bundle sheath cells, where they are again released by the action of a NADP-dependent malic enzyme. The malic enzyme is located in the bundle sheath chloroplasts together with the en mes of the Calvin cycle. CO2 is then reduced to carbohydrates while pyruvate is presumably transported back to the mesophyll cells. Besides the malate-type C4-plants, there is a second and larger group of species (aspartate type) that contains little malic enzyme and utilizes aspartate as the COj carrier. 

Amon and coworkers< > have postulated a new cyclic pathway, named reductive carboxylic acid cycle, that provides another mechanism independent of the reductive pentose cycle for the assimilation of CO2 in bacterial photosynthesis. This cycle generates acetyl-CoA from two molecules of CO2 and is driven by reduced ferredoxin, reduced nucleotides, and ATP, according to the overall simplified equation ... 

Phosphoribulokinase, the other unique enzyme of the reductive pentose cycle together with RuDP carboxylase, is also located exclusively in the chloroplasts and is likewise subject to activation by light, but apparently by a diflerent and more direct mechanism. Kinetic studies by Gibbs and collaborators have shown that Ru5P kinase is activated 2-to 4-fold by illumination of intact chloroplast preparations, with a half-time of less than 15 seconds. The photoactivated state of the enzyme decays in the dark with a half-time of about 8 minutes. Since dark incubation of broken chloroplasts with dithiothreitol causes... 

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