chapter 23 Flashcards
what does the calvin cycle do
synthesizes hexoses from carbon dioxide and water
what are the three stages of the calvin cycle
- fixation of CO2
- reduction of 3-phosphoglycerate
- regeneration of ribulose 1,5-bisphosphate
what is the rate limiting step of the calvin cycle
the fixation of CO2
fixation of CO2 is catalyzed by
ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco)
what is the most abundant enzyme in the biosphere
rubisco
what is essential for the mechanism of rubisco
magnesium! Mg2+
how does carbon dioxide fixation work
Mg2+ binds to a carbamate formed by the reaction of CO2 with lysine on the large subunit of rubisco
Mg2+ assists in the binding of ribulose 1,5-bisphosphate and catalysis to form a hydrated intermediate that yields two molecules of 3-phosphoglycerate
what happens to rubisco in the absence of CO2
rubisco binds 1,5-bisphosphate so tightly that the enzyme is inactive
rubisco activase uses…
ATP to induce structural alterations in rubisco that allow release of the substrate and formation of the carbamate, thus restoring rubisco activity
if no ATP production…
no light reaction
what happens in the second stage of the calvin cycle
3-phosphoglycerate (generated by rubisco) is converted into hexose monophosphate using ATP and NADPH synthesized during the light reactions
what pathway is the second stage of the calvin cycle similar to
gluconeogenic pathway
what happens in the third stage of the calvin cycle
regeneration of ribulose 1,5- bisphosphate
what is the main challenge of the calvin cycle
formation of five carbon sugar from six-carbon and three-carbon sugars
what is another pathway for rubisco
rubisco can react with O2 instead o f CO2 to generate 3-phosphoglycerate and phosphoglycolate
