The reductive pentose phosphate cycle

The reductive pentose phosphate cycle is the only fundamental carboxylating mechanism in plants. In C3 plants the entire process of photosynthesis (the light reactions and the RPP cycle) occurs within chloroplasts. The enzymes catalysing steps in the RPP cycle are water-soluble and are located in the soluble portion (chloroplast stroma or extract). 

Elucidation of the pathway was chiefly the work of Calvin, Benson, Bassham and co-workers, although there were important contributions by others. In their experiments they used green algae, Chlorella and Scenedesmm, but since that time their results have been confirmed many times in a wide variety of higher plants. 

The crux of the pathway (Fig. 1) is the carboxylation of ribulose 1,5-bisphos-phate (Rbu-1,5-P2) at the C-2 carbon, giving rise to a short-lived six-carbon intermediate which is cleaved to produce two molecules of 3-phosphoglycerate (3-PGA) (Eqn. 2). This reaction is catalysed by ribulose-1,5-bisphosphate carboxylase oxygenase (rubisco), one of the most abundant proteins on earth. 

Intermediates formed from G3P are utilized (Fig. 1) via a series of isomerizations, condensations and rearrangements resulting in the conversion of five molecules of triose phosphate to three of pentose phosphate, eventually ribulose 5-phosphate (Rbu-5-P). Phosphorylation of Rbu-5-P with ATP regenerates the original carbon acceptor Rbu-1,5-P2, thus completing the cycle. 

The RPP cycle displays four features which are necessary for its role as a fundamental carboxylating system [2]. 

---

Wolosiuk, R., Ballicora, M., Hagelin, K. (1993) The reductive pentose phosphate cycle for photosynthetic C02 assimilation enzyme modulation. FAS EB J. 7, 622-637. 

Figure 17-14 (A) The reductive carboxylation system used in reductive pentose phosphate pathway (Calvin-Benson cycle). The essential reactions of this system are enclosed within the dashed box. Typical subsequent reactions follow. The phosphatase action completes the phosphorylation-dephosphorylation cycle. (B) The reductive pentose phosphate cycle arranged to show the combining of three C02 molecules to form one molecule of triose phosphate. Abbreviations are RCS, reductive carboxylation system (from above) A, aldolase, Pase, specific phosphatase and TK, transketolase.

Fig. 1. The reductive pentose phosphate cycle (RPP). The solid lines indicate reactions of the RPP cycle. The number of lines per arrow indicates the number of times each reaction occurs for one complete turn of the cycle in which three molecules of COj are converted to one molecule of G3P. Each reaction of the cycle occurs at least once. The double dashed lines indicate the principal reactions removing intermediate compounds of the cycle for biosynthesis. Abbreviations RuBP, ribulose 1,5-bis-phosphate PGA, 3-phosphoglycerate DPGA, 1,3-diphosphoglycerate, FBP, fructose 1,6-bisphos-phate F6P, fructose 6-phosphate SBP, sedoheptulose 1,7-bisphosphate S7P, sedoheptulose 7-phosphate Xu5P, xylulose 5-phosphate R5P, ribose 5-phosphate Ru5P, ribulose 5-phosphate TPP, thiamine pyrophosphate. From Ref. 1.

In summary, current evidence [39-41] is thus consistent with the view that the ferredoxin/thioredoxin system functions in photosynthetically diverse types of plants as a master switch to restrict the activity of degradatory enzymes and activate biosynthetic enzymes in the light. It is significant that enzymes controlled by the ferredoxin/thioredoxin system (FBPase, SBPase, NADP-G3PDH, and PRK) function in the regenerative phase of the reductive pentose phosphate cycle that is needed to sustain its continued operation - i.e, to regenerate the carbon dioxide acceptor, Rbu-1,5-P2, from newly formed 3-PGA. It seems likely that one of these thioredoxin-linked enzymes limits the regeneration of Rbu-1,5-P2. 

The incorporation of COz into carbohydrate by eukaryotic photosynthesizing organisms, a process that occurs within chloroplast stroma, is often referred to as the Calvin cycle. Because the reactions of the Calvin cycle can occur without light if sufficient ATP and NADPH are supplied, they have often been called the dark reactions. The name dark reactions is somewhat misleading, however. The Calvin cycle reactions typically occur only when the plant is illuminated, because ATP and NADPH are produced by the light reactions. Therefore light-independent reactions is a more appropriate term. Because of the types of reactions that occur in the Calvin cycle, it is also referred to as the reductive pentose phosphate cycle (RPP cycle) and the photo synthetic carbon reduction cycle (PCR cycle). 

According to Osmond and Allaway (1973), PEP is generated during the day at the expense of 3-PGA from the RudP-C reaction. However, 3-PGA is preferably consumed in order to maintain action of the reductive pentose phosphate cycle, thus allowing only limited production of PEP. The limited supply of its substrate would inhibit PEP-C. 

From experiments of Bruinsma (1958) and others (see Wolf, 1960) it is well known that 30% CO2 prevents deacidification in the light. Nishida (1977) found inhibition of deacidification in 5% CO2. Wolf (1960) and Beevers et al. (1966) interpret this as an indication that j5-carboxylation can proceed in the light as well as dark, provided that there is sufficient CO2 to saturate the demand of the reductive pentose phosphate cycle for this substrate (see also Chap. 4.2.1.3). 

---