8.8Dark Reaction : Carbon Dioxide Fixation

Carbon dioxide fixation reactions in photosynthesis are divided into three reaction groups (Column Fig. 8-2). The first is a group of reactions in which ribulose 1,5-bisphosphate incorporates carbon dioxide (CO2) to produce two molecules of phosphoglycerate (top of Fig. 8-6) and is catalyzed by ribulose 1,5-bisphosphate carboxylase/oxygenase (a.k.a. RuBisCO*10). The second is a group of reactions that use ATP and NADPH to produce and release sugar phosphates from the cycle, consequently synthesizing starch and sucrose. The third is a pathway in which various sugar phosphates are connected by equilibrium reactions, regenerating ribulose 5-phosphate - a precursor of ribulose 1,5-phosphate. These reactions are known as the Calvin cycle or the reductive pentose phosphate cycle.

Two reactions catalyzed by RuBisCO

Fig. 8-6. Two reactions catalyzed by RuBisCO

The carbon dioxide fixation reaction (top) and the reaction with oxygen (bottom). It should be noted that two molecules of phosphoglycerate are generated in the above reaction.

The sum of dark reactions after removing the substances regenerated in the reactions outlined above is expressed as follows:

Equation 8-5
              CO2 + 3ATP + 2NADPH + 2H+
              (CH2O) + H2O + 2NADP+ + 3ADP + 2H3PO4

(CH2O) corresponds to a sugar. This equation indicates that three ATP molecules and two NADPH molecules are needed to fix one molecule of CO2.
A distinct characteristic of photosynthetic carbon dioxide fixation reactions is the unique nature of RuBisCO. As its full name suggests, RuBisCO involves carboxylase and oxygenase activity; when the oxygen concentration is high and the carbon dioxide concentration is low, it incorporates O2 instead of CO2 and wastefully uses ribulose 1,5-bisphosphate (bottom of Fig. 8-6). The reactivity of RuBisCO to CO2 is more than 100 times higher than its reactivity to O2; however, since the oxygen concentration in the atmosphere is 500 times higher than that of CO2, the oxygenase activity in chloroplasts cannot be ignored. A byproduct generated in the oxygenase reaction - phosphoglycolate - is converted back to phosphoglycerate using ATP and NADPH; this pathway is called photorespiration, since CO2 is released during the process. Additionally, since the rate of reactions catalyzed by RuBisCO is much slower than that of other metabolic enzymes, the amount of RuBisCO required is several hundred times more than that of other enzymes that catalyze the reactions seen before and after those catalyzed by RuBisCO. As a result, RuBisCO proteins account for over half the water-soluble proteins found in chloroplasts, making them the most abundant proteins on earth. These intriguing characteristics of RuBisCO reflect the high CO2 concentration and absence of O2 in the atmosphere of primitive earth when phototrophs first appeared.

RuBisCO is an abbreviation of the enzyme name ribulose 1,5-bis phosphate carboxylase/oxygenase.


Regulation by Coupling and Light

Since ATP synthesis reactions involving kinase (phosphotransferase) are coupled with metabolic reactions at a ratio of 1:1, the ATP synthesis efficiency is 100%. On the other hand, ATP synthesis reactions involving F-ATP synthase are indirectly coupled with electron transport via H+ transport, making the story a little more complex. As an example, the appearance of chloroplasts differs between daytime and nighttime. During the day, when light is available, chloroplasts are in a state of high energy with the thylakoid membrane maintaining an H+ gradient by electron transport, and are ready to supply ATP when it is used. However, at nighttime, it is difficult to maintain this high-energy state. Although the thylakoid membrane has low permeability for H+ and is therefore suited to maintaining the concentration gradient, it cannot stop H+ from gradually leaking through the membrane during the night. Generally, enzymatic reactions are reversible, and when the concentration gradient of H+ is decreased, the gradient is restored by breaking down ATP, which is a waste of energy. In the F-ATP synthase of chloroplasts, therefore, an activity regulation mechanism involving thioredoxin (an oxidation-reduction protein) has evolved. When thioredoxin with cyctein residue is reduced by photosynthetic electron transport, the cyctein residue of a protein in the stalk of ATP synthase is reduced, thereby activating the enzyme; in dark places, on the other hand, the activity of the enzyme is inhibited through the oxidation of the cyctein residue, thereby reducing the wasteful use of energy.

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