The Dark Reactions

The Calvin-Benson Cycle

How does the NADPH and ATP formed in the light reactions help in the biosynthesis of complex carbon structures? Working with the green algae chlorella, Melvin Calvin and Andy Benson, at the University of California at Berkeley, elucidated the following pathway:

The cycle runs 6 times, each time incorporating a new carbon. Ribulose is a five-carbon sugar and the gylceraldehydes are three-carbon sugars (remember them from glycolysis?). Running through the cycle six times generates:

6(5-carbon sugars) + 6(incorporated carbon dioxides)

Those six carbon dioxides are reduced to glucose by the conversion of NADPH to NADP+. Glucose can now serve as a building block to make polysaccharides, other monosaccharides, fats, amino acids, nucleotides, and all the molecules living things require.

RUBISCO

The key enzyme in the Calvin Cycle is the one that catalyzes the transformation of the 5-carbon sugar ribulose-5-phosphate and the single-carbon carbon dioxide to two 3-carbon 3-phosphoglycerates. This reaction has a very high delta-G of -12.4 kcal/mol. The enzyme is called ribulose-1-5-biphosphote carboxylase or Rubisco . Rubisco accounts for 16% of the protein content of the chloroplast and is likely the most abundant protein on Earth. Why is this protein so abundant? Is it because it is so crucial to all life to have a source of carbon fixation? Or perhaps it is because the enzyme is very inefficient and has only evolved once, so it has not had to sustain any competition from more effective carbon fixers. The question is an interesting one to ponder. In fact, we will discover in the next section that Rubisco is, in fact, very inefficient, and that a mechanism has evolved to deal with this handicap.

Light Regulation of the Calvin Cycle

The energy required for the Calvin Cycle, in the form of ATP and NADPH, comes from the light reactions. It makes sense that the plant or photosynthesizing bacterium would want to tightly regulate the Calvin Cycle with photosynthesis. It would be detrimental and wasteful to try to run this process using ATP generated for other plant metabolism.

The light and dark reactions are linked by several mechanisms:

The pH of the stroma increases as protons are pumped out of it through the membrane assembly of the light reactions. The enzymes of the Calvin Cycle function better at this higher pH.
As the reduction potential of ferredoxin (fd) increases, it reduces a protein called thioredoxin. This reduction breaks a disulphide bridge in thioredoxin. The enzyme now has free cysteines that can go around and compete for the the disulphide bonds in other enzymes . In particular, several enzymes of the Calvin Cycle are activated by the breaking of disulphide bridges. So the activity of the light reactions is communicated to the dark reactions by an enzyme intermediate.
The reactions of the Calvin cycle have to stop when they run out of substrate; as photosynthesis stops, there is no more ATP or NADPH in the stroma for the dark reactions to take place.
The light reactions increase the permeability of the stromal membrane to cofactors such as Mg++ which are required for the Calvin Cycle.

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