BIOCHEMISTRY - L. Stryer - 1984

VOLUME 2

PART II GENERATION AND STORAGE OF METABOLIC ENERGY

CHAPTER 19. PHOTOSYNTHESIS

19.15. CO₂ Reacts with Ribulose Bisphosphate to Form Two Molecules of Phosphoglycerate

The radiochromatogram obtained after 60 s of illumination proved to be so complex (Fig. 19.19) that it was impossible to distinguish the earliest intermediate in the CO₂ fixation process. However, after illumination for only 5 s, the pattern was considerably simpler: the chromatogram showed only a single distinct radioactive spot, which turned out to be 3-phosphoglycerate.

Class="center">Fig. 19.18. Freeze-fracture electron micrograph of spinach thylakoid membranes, showing the regular lattice of transmembrane particles

Fig. 19.19. Radiochromatograms of illuminated algal Suspensions 5 and 60 s after the Introduction of CO₂ into the system

The formation of 3-phosphoglycerate as the first detectable radioactive intermediate suggested that the CO₂ acceptor is a two-carbon compound. This turned out not to be the case. The actual reaction sequence is much more complex:

A CO₂ molecule condenses with ribulose 1,5-bisphosphate to form a short-lived six-carbon intermediate, which is rapidly hydrolyzed to two molecules of 3-phosphoglycerate. The overall reaction is highly exergonic (∆G⁰ = -12.4 kcal/mol).

The reaction is catalyzed by ribulose 1,5-bisphosphate carboxylase, which is localized on the stroma-facing surface of the thylakoid membranes. METABOLISM/14.html">Chloroplasts are very rich in this enzyme, which accounts for more than 16% of their total protein content. Ribulose 1,5-bisphosphate carboxylase is probably the most abundant protein in the biosphere. It consists of eight large (55 kDa) and eight small (15 kDa) subunits arranged in two layers (Fig. 19.20). The large subunits are catalytically active even in the absence of the small subunits, which play a regulatory role. The enzyme Functions simultaneously as an oxygenase (Section 19.20) and a carboxylase.

Fig. 19.20. Schematic representation of The Structure of ribulose 1,5-bisphosphate carboxylase

19.16. Formation of Fructose 6-Phosphate and Regeneration of Ribulose 1,5-Bisphosphate

The steps in The conversion of 3-phosphoglycerate to fructose 6-phosphate (Fig. 19.21) are similar to those in The Gluconeogenesis pathway (Section 15.13), with the sole exception that chloroplast glyceraldehyde-3-phosphate dehydrogenase is specific for NADPH rather than NADH. As a result of these reactions, CO₂ is brought to the hexose level. It remains only to regenerate ribulose bisphosphate, the CO₂ acceptor in the initial dark stage. The problem reduces to generating a five-carbon sugar from six-carbon and three-carbon sugars. This is accomplished through Reactions Catalyzed by transketolase and aldolase. Transketolase also participates in the Reactions of the Pentose Phosphate Pathway (Section 15.4).

Recall that transketolase, an enzyme that employs thiamine pyrophosphate as a prosthetic group, transfers a two-carbon unit (CH₂OH—CO—) from a ketose to an aldose. Aldolase catalyzes an aldol Condensation between dihydroxyacetone phosphate and an aldehyde. This enzyme is highly specific for dihydroxyacetone phosphate, but reacts with a variety of aldehydes. Specific reactions catalyzed by transketolase and aldolase in The Calvin Cycle include the following:

Fig. 19.21. Pathway for the conversion of 3-phosphoglycerate to fructose 6-phosphate in chloroplasts

Four additional Enzymes are required for the dark reactions of Photosynthesis. One of these, a phosphatase, hydrolyzes sedoheptulose 1,7-bisphosphate to sedoheptulose 7-phosphate. The second, phosphopentose epimerase, converts xylulose 5-phosphate into ribulose 5-phosphate. The third, phosphopentose isomerase, converts ribose 5-phosphate into ribulose 5-phosphate. Recall that the second and third enzymes also participate in the reactions of The pentose phosphate pathway (Section 15.4). Summing up the aforementioned reactions, we obtain

Fructose-6-phosphate + 2 glyceraldehyde-3-phosphate + dihydroxyacetone phosphate → 3 ribulose-5-phosphate.

Finally, the fourth enzyme, phosphoribulokinase, catalyzes the phosphorylation of ribulose-5-phosphate, regenerating ribulose-1,5-bisphosphate, the CO₂ acceptor.

Ribulose-5-phosphate + ATP → Ribulose-1,5-bisphosphate + ADP + H⁺.

This group of reactions is known as the Calvin cycle (Fig. 19.22).

Fig. 19.22. The Calvin cycle. The detailed pathway for the formation of ribulose-5-phosphate from 3-carbon and 6-carbon sugars is not shown in this scheme

19.17. Three ATP and Two NADPH Drive CO₂ to the Hexose Level

What is the energy cost of hexose synthesis? Six turns of the Calvin cycle are required because one carbon atom is reduced in each turn (Fig. 19.22). Twelve ATP are consumed to phosphorylate 12 molecules of 3-phosphoglycerate to 1,3-bisphosphoglycerate, and 12 NADPH are consumed in reducing 12 molecules of 1,3-bisphosphoglycerate to glyceraldehyde-3-phosphate. An additional six ATP are expended to regenerate ribulose-1,5-bisphosphate.

We can now write the balanced equation for the overall reaction of the Calvin cycle:

6CO₂ + 18ATP + 12NADPH + 12H₂O → C₆H₁₂O₆ + 18ADP + 18Pi + 12NADP⁺ + 6H⁺.

Thus, the conversion of CO₂ into a hexose (glucose or fructose) requires three molecules of ATP and two molecules of NADPH. The efficiency of photosynthesis can be evaluated as follows:

1. ∆G⁰ for the reduction of CO₂ to the hexose level is 114 kcal/mol.

2. The reduction of NADP⁺ involves The transfer of two electrons. Consequently, the formation of two NADPH requires the absorption of four photons by Photosystem I. The electrons donated by photosystem I are replenished by Photosystem II, which must absorb an equivalent number of photons. Hence, eight photons are required to generate the necessary NADPH. Concurrently, three ATP molecules are produced, which are needed for the conversion of CO₂ into hexose.

3. The energy content of a mole of photons with a wavelength of 600 nm is 47.6 kcal, and therefore the energy input from eight moles of photons is 381 kcal. Thus, the overall efficiency of photosynthesis under standard conditions is at least 114/381, or 30%.