Dark Reactions of Photosynthesis Fix CO2

Dark Reactions of Photosynthesis Fix CO₂

The actual storage form of the carbohydrates produced from carbon dioxide by photosynthesis is not glucose but disaccharides (e.g., sucrose in sugarcane and sugar beets) and polysaccharides (starch and cellulose). However, it is customary and convenient to write the carbohydrate product as glucose, and we shall follow this time-honored practice.

Carbon dioxide fixation takes place in the stroma. The equation for the overall reaction, like all equations for photosynthetic processes, is deceptively simple.

6CO₂ + 12NADPH + 18ATP - - (Enzymes)- > C₆H₁₂O₆ + 12NADP⁺ + 18ADP + 18P_i

The actual reaction pathway has some features in common with glycolysis and some in common with the pentose phosphate pathway.

The net reaction of six molecules of carbon dioxide to produce one mol-ecule of glucose requires the carboxylation of 6 molecules of a five-carbon key intermediate, ribulose-1,5-bisphosphate, to form 6 molecules of an unstable six-carbon intermediate, which then splits to give 12 molecules of 3-phospho-glycerate. Of these, 2 molecules of 3-phosphoglycerate react in turn, ultimately producing glucose. The remaining 10 molecules of 3-phosphoglycerate are used to regenerate the 6 molecules of ribulose-1,5-bisphosphate. The overall reaction pathway is cyclic and is called the Calvin cycle (Figure 22.15) after the scientist who first investigated it, Melvin Calvin, winner of the 1961 Nobel Prize in chemistry.

What is the Calvin cycle?

The Þrst reaction of the Calvin cycle is the condensation of ribulose-1,5-bisphosphate with carbon dioxide to form a six-carbon intermediate, 2-carboxy-3-ketoribitol-1,5-bisphosphate, which quickly hydrolyzes to give two molecules of 3-phosphoglycerate (Figure 22.16). The reaction is catalyzed by the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco).This enzyme is locatedon the stromal side of the thylakoid membrane and is probably one of the most abundant proteins in nature, as it accounts for about 15% of the total protein in chloroplasts. The molecular weight of ribulose-1,5-bisphosphate carboxylase/ oxygenase is about 560,000, and it consists of eight large subunits (molecular weight 55,000) and eight small subunits (molecular weight 15,000) (Figure 22.17). The sequence of the large subunit is encoded by a chloroplast gene, and that of the small subunit is encoded by a nuclear gene. 

The endosymbiotic theory for the development of eukaryotes is consistent with the idea of independent genetic material in organelles. The large subunit (chloroplast gene) is catalytic, whereas the small subunit (nuclear gene) plays a regulatory role, an observation that is consistent with an endosymbiotic origin for organelles such as chloroplasts.

The incorporation of CO₂ into 3-phosphoglycerate represents the actual fixation process; the remaining reactions are those of carbohydrates. The next two reactions lead to the reduction of 3-phosphoglycerate to form glyc-eraldehyde-3-phosphate. The reduction takes place in the same fashion as in gluconeogenesis, except for one unique feature (Figure 22.15): the reactions in chloroplasts require NADPH rather than NADH for the reduction of 1,3-bisphosphoglycerate to glyceraldehyde-3-phosphate. When glyceraldehyde-3-phosphate is formed, it can have two alternative fates: one is the produc-tion of six-carbon sugars, and the other is the regeneration of ribulose-1,5-bisphosphate. Table 22.1 summarizes the reactions that take place and indi-cates their stoichiometry.

The formation of glucose from glyceraldehyde-3-phosphate takes place in the same manner as in gluconeogenesis (Figure 22.15 and reactions 4 through 8 in Table 22.1). The conversion of glyceraldehyde-3-phosphate to dihydroxyacetone phosphate takes place easily. Dihydroxyacetone phosphate in turn reacts with glyceraldehyde-3-phosphate, in a series of reactions we have already seen, to give rise to fructose-6-phosphate and ultimately to glucose. Because we have already seen these reactions, we shall not discuss them again.

How is starting material regenerated in the Calvin cycle?

This process is readily divided into four steps: preparation, reshuffling, isomerization, and phosphorylation. The preparation begins with conversion of some of the glyceraldehyde-3-phosphate to dihydroxyacetone phosphate (catalyzed by triose phosphate isomerase). This reaction also functions in the production of six-carbon sugars. Portions of both the glyceraldehyde-3-phosphate and the dihydroxyacetone phosphate are then condensed to form fructose-1,6-bisphosphate (catalyzed by aldolase). Fructose-1,6-bisphosphate is hydrolyzedto fructose-6-phosphate (catalyzed by fructose-1,6-bisphosphatase). (See Figure 22.15. Reactions 4 through 6 in Table 22.1 are involved here.) With a supply of glyceraldehyde-3-phosphate, dihydroxyacetone phosphate, and fructose-6-phosphate now available, the reshufßing can begin.

Most of the reactions of the reshuffling process are the same as ones we have already seen as part of the pentose phosphate pathway. Consequently, we shall concentrate just on the main outline of the process later because the results are summarized in Figure 22.15 and Table 22.1. Reactions catalyzed in turn by transketolase, aldolase, and sedoheptulose bisphosphatase (Reactions 9 through 12 in Table 22.1) are the reactions of rearrangement of carbon skeletons in the reshuffling phase of the Calvin cycle.

The isomerization step (reactions 13 and 14 in Table 22.1) involves the conversion of both ribose-5-phosphate and xylulose-5-phosphate to ribulose-5-phosphate. Ribose-5-phosphate isomerase catalyzes the conversion of ribose-5-phosphate to ribulose-5-phosphate, and xylulose-5-phosphate epimerase catalyzes the conversion of xylulose-5-phosphate to ribulose-5-phosphate (Figure 22.15). The reverse of both these reactions takes place in the pentose phosphate path-way, catalyzed by the same enzymes.

In the final step (reaction 15 in Table 22.1), ribulose-1,5-bisphosphate is regenerated by the phosphorylation of ribulose-5-phosphate. This reaction requires ATP and is catalyzed by the enzyme phosphoribulose kinase. The reac-tions leading to the regeneration of ribulose-1,5-bisphosphate summarized in Table 22.1 give a net equation obtained by adding all the reactions.

Taking these points into consideration, we arrive at the net equation for the path of carbon in photosynthesis.

6CO₂ + 18ATP + 12NADPH + 12H⁺ + 12H₂O - - > Glucose + 12NADP⁺ + 18ADP + 18P_i

The efÞciency of energy use in photosynthesis can be calculated fairly easily. The ∆G° for the reduction of CO₂to glucose is +478 kJ (+114 kcal) for each mole ofCO₂ (see Question 37), and the energy of light of 600-nm wavelength is 1593 kJ mol⁻¹ (381 kcal mol⁻¹). We shall not explain in detail here how this Þgure for the energy of the light is obtained, but it comes ultimately from the equation E = hv. Light of wavelength 680 nm or 700 nm has lower energy than light at 600 nm. Thus, the efÞciency of photosynthesis is at least (478/1593) - > 100, or 30%.

Summary

In the dark reactions of photosynthesis, the fixation of carbon dioxide takes place when the key intermediate ribulose-1,5-bisphosphate reacts with carbon dioxide to produce two molecules of 3-phosphoglycerate. This reaction is catalyzed by the enzyme ribulose-1,5-bisphosphate carbox-ylase/oxygenase (rubisco), one of the most abundant proteins in nature.

The remainder of the dark reaction is the regeneration of ribulose-1,5-bisphosphate in the Calvin cycle.