Reduction of CO2 into Carbohydrates | Biochemistry

In this article we will discuss about the photosynthetic cycle of reduction of Co₂ into carbohydrates.

The transformation of light energy into chemical energy results in the production of ATP and NADPH (see fig. 3-12). Our present knowledge of the metabolism of carbohydrates will enable us to study what is sometimes called the dark phase of photosynthesis, i.e. the series of reactions permitting the reduction of CO₂ to carbohydrates thanks to ATP and NADPH formed during the “light phase”.

It was observed that under illumination, starch accumulated in plant leaves. As it was easy to envisage, on the one hand, the reduction of CO₂ to formal­dehyde and, on the other hand, the polymerization of formaldehyde (CH₂O) to glucose (C₆H₁₂O₆), it was originally thought that glucose was the product directly formed during the dark phase of photosynthesis.

It is now known that this is not true and that in reality, CO₂ is first incorporated in 3-phosplioglyceric acid. Calvin, using green algae (such as Chlorella) which he exposed to light and radioactive CO₂ (¹⁴C-labeled) for very short periods (of the order of a few seconds), and then dipped the cells in boiling alcohol to denature the enzymes and block all reactions, showed that the first stable compound formed from radioactive CO₂ is 3-phosphoglyceric-acid.

After contact times with ¹⁴CO₂ of the order of 30 seconds, a large number of compounds (trioses-phosphates, pentoses-phosphates, hexoses-phosphates) could be characterized by a radioactive spot following two-dimensional paper chromatography, but after 5 seconds, the only radioactive spot was that of 3-phosphoglyceric acid whose carboxylic group contained the major part of radioactivity.

It was then showed that CO₂ binds, in presence of carboxydismutase (or ribulose-1, 5-bisphosphate carboxylase), to ribulose-1,5-bisphosphate thus forming a ketoacid of 6 carbon atoms, very unstable, which is hydrolysed to 2 molecules of 3-phosphoglyceric acid (see fig. 4-44).

Besides this incorporation of CO₂ in compounds in C₃, the fixation of CO₂ in compounds in C₄ could be observed in some plants (like maize) and it was shown that CO₂ binds to phosphoenol pyruvic acid to give oxaloacetic acid thanks to the action of phosphoenol pyruvate carboxylase, an enzyme catalyzing a reaction very similar to the one catalyzed by phosphoenol pyruvate carboxykinase (see fig. 4-34).

As may be observed in the diagram of figure 4-45, 3-phosphoglyceric acid is transformed into 1, 3-diphosphoglyceric acid by phosphoglycerate kinase in presence of ATP and then converted into glyceraldehyde-3-℗ by glyceral- dehyde-3-℗ dehydrogenase in presence of NADPH; these two reactions studied during glycolysis operate here in a direction opposite to that of carbohydrate catabolism.

Furthermore, one clearly sees the purpose served by ATP and NADPH, indispensable compounds formed — as mentioned previously — during the light phase of photosynthesis.

Two molecules of glyceraldehyde-3-℗ (in equilibrium with dihydroxy- acetone ℗) can give, thanks to aldolase, one molecule of fructose-1, 6-bis-℗ which will be converted by fructose-1, 6-bisphosphatase into fructose-6-℗ (these two reactions were also observed during glycolysis). Or, 12 molecules of triose-℗ will give 6 molecules of fructose-6℗.

From 5 of these fructose-6-℗ molecules, 6 molecules of ribulose-5-℗ will be formed and there is one excess molecule of fnictose-6-℗.

The 6 molecules of ribulose-5-℗ formed will be phosphorylated by a kinase to ribulose-1, 5-bis-℗ and will then again bind CO₂ and thus enter a new turn of the cycle.

One turn of the cycle may be summarized as follows:

6 CO₂ + 6 ribulose-1, 5-bis-℗ + 18 ATP + 12(NADPH + H⁺) → 6 ribulose-1, 5-bis-℗ + fructose-6-℗ + 18 ADP + 18 P_i + 12 NADP⁺

In short, it is observed that 6 CO₂ were reduced to one molecule of fructose- 6-℗ which can be isomerized to glucose-6-℗ and then stored in the form of starch after the transformations we already studied (see fig. 4-22).

In fact, a certain proportion of various intermediates can leave the cycle and enter other important metabolic pathways. For example, ribulose-5-℗ can isomerize to ribose-5-℗ required for the synthesis of nucleic acids. On the other hand, triose-℗ can give a-glycerophosphoric acid (see fig. 4-32) which will be used for the synthesis of lipids.

Even the first compound formed, 3-phosphoglyceric acid, can take another direction: it can be oxidized by the glycolytic pathway followed by the Krebs cycle. Therefore, the output in glucose units cannot be expected to be the theoretical output described above.

It must be noted that photosynthesis has a determining effect on the direc­tion of the glycolytic reactions and the cycle of pentose-phosphates: under illumination, thanks to the production of ATP and NADPH in high concentra­tions, the photosynthetic cycle will permit the reduction of CO₂ to hexose-℗; as just mentioned, this cycle uses the series of inter-conversions of the pentose- phosphates cycle in the direction hexoses-℗ → pentoses-℗, i.e. in the direc­tion opposite to that enabling the pentose-phosphates cycle to oxidize glucose with production of CO₂; moreover, the photosynthetic cycle follows some glycolytic reactions, but in the reverse direction.

On the contrary, in the dark, ATP and NADPH concentrations will fall (there is no light energy for their synthesis and yet their utilization continues in order to bring about the metabolic reactions requiring energy) so that the plant cells will catabolize glucose to CO₂ just like animal cells, i.e. by glycolysis followed by Krebs cycle, or by the pentose-phosphates cycle. This is why, in the dark, contrary to what happens under illumination, plants are found to breathe like animals, absorbing oxygen and eliminating carbon dioxide.