VL 10 (Elke Dittmann) Flashcards
What is the Calvin Cycle and who discovered it?
- its the dark reaction of photosynthesis
- CO2 in its fully oxidized state is reduced to a hexose
- utilizes NADPH as reductant
CO2 + H2O + Energy –> O2 + C6H12O6
- discovered by Calvin in 1961 NP
- single cell green algea Chlorella = model organism
- CO2 is taken up
- With the help of ATP and NADPH from the light reactions it is stepwise reduced
- There are 13 reactions forming a cycle
- By utilizing various enzymes CO2 is finally reduced to glucose
Name the 3 consecutive stages of the calvin cylce
- Fixation of CO2 by RubisCO to form 2 molecules of 3-phophoglycerte (3PGA)
- Reduction of 3 PGA to form hexose sugars
- Regenertion of ribulose 1,5-bisphophate
6 Rounds of calvin cycle are requiered to generate 1 hexose utilizing 18 molecules ATP and 12 molecules NADPH
6 CO2 + 18 ATP + 12 NADPH + 12 H2O –> C6H12O6 + 18 ADP + 18 Pi + 12 NADP+ + 6 H+
What is RubisCO ?
RubisCO = Ribulose 1,5 bisphosphate carboxylase/oxygenase
- CO2 condenses with rubulose 1,5 bisphosphate to form an unstable 6-Carbon compund
- This reaction is highly exergnoic and catalyzed by ribulose 1,5-bisphosphate carboxylase/ Oxygenase (rubisco)
- RubisCO consits 8 large and 8 small subunits and is located in the stromal side of thylakoids in chloroplasts of plants
- Each L subunit contains a catalytic site and a regulatory site. S subunits enhances the activity
- RubisCO is considered as the most abundant enzyme on earth
Steps:
1. Carboxylation / Oxygenation
2. Generation of Hexose phosphates from 3PGA
3. Regeneration of Ribulose-1,5-bisphosphate
Explain the activation of RubisCO
- requieres a divalent cation (usually Mg2+) for its catalytic activity
- CO2 molecules (other than the substrate) reacts with lysine side chain to form a carbamate and form a negatively charged adduct
- adduct is stabilized by a positvyl charged Mg2+
- Formation of the carbamate is facilitated by the enzyme Rubisco activase in plants (but can also form spontanously)
What is the catalytic mechanism of Rubisco ?
- Ribulose 1,5-bisphosphate binds to Mg2+ through its keto group and adjacent hydroxyl group
- Rubulose 1,5 bisphosphate + Mg2+ Complex is deprotonated ans froms an enolate intermediate
- This reactive species couples with CO2 to form a new C-C bond
- This new product is coordinated to Mg2+ through 3 groups including the new carboxylate
- An H2O molecule is added to the beta-ketoacid to form the C6 intermediate that cleaves to form 2 molecules 3PGA
Name the RubisCO carboxylation reaction
- Ribulose 1,5 bisphosphate (H+ raus)
- Enediolate intermediate (CO2 rein)
- 2-Carboxy-3-keto D-arabinitol 1,5-bisphosphate (H2O rein)
- Hydrate intermediate (H+ rein 2 H+ raus)
- 2 x 3-Phosphoglycerate
All steps of the Rubisco reaction take place while coordinated to the Mg2+-ion
1 Reaction Step
Name the RubisCO oxygenation reaction
- Ribulose 1,5-bisphosphate (H+ raus)
- Enediolate intermdeiate (O2 rein)
- Hydroperoxide intermediate (H2O rein H2= + H+ raus)
- 1 x Phosphoglycerate + 1x 3-Phosphoglycerate
The reactive intermediate at the Mg2+ sometimes reacts with O2 instead of
CO2 to produce phosphoglycolate (2PG) and 3-phosphoglycerate (3PGA)
–> Under atmospheric conditions, rate of carboxylation is four time higher than oxygenation
–> 2PG is not very versatile, salvage pathways recover part of ist sceleton.
1 Reactions Step
Generation of Hexose phosphates from 3PGA
- 3PGA→ 3 forms of hexose phosphate:
Glucose-1-P, Glucose-6-P, Fructose-6-P
–> all 3 are interconvertable - conversion steps analog to gluconeogenesis
Exception: Glyceraldehyde-3-P dehydrogenase (GAPDH) in chloroplasts is specific for NADPH, not NADH (cytosolic)
2 reaction step
Regeneration of Ribulose 1,5-bisphosphate
- C6 + C3→C5 (transketolase, aldolase (same in glycolysis))
- Responsable Enzymes are and transketolase and aldolase
- Transketolase requires co-factor thiamine pyrophosphate (TPP) to transfer C2 from ketose→aldose
-
Aldolase
–> aldol-condensation: dihydroxyacetone phosphate + aldehyde
–> highly specific for dihydroxyacetone phosphate, accepts wide range of aldehydes
Formation of 5-C-Sugars
Transketolase
* transform C6 and C3 sugars into C4 and C5 sugar
Aldolase
* combines the C4 with C3 sugars to yield a C7 sugar
Transketolase
* combines C7 with another C3 sugar to yield 2 C5 sugars
Regneration of Ribulose 1,5 bisphosphate
- Phosphopentose isomerase and epimerase convert ribose 5-P and Xylulose 5-P into ribulose 5-P.
- Ribulose 5-P is converted into ribulose 1,5 bisphosphate by the enzyme phosphoribulokinase
Stochiometry of calvin reactions
- 12 ATP expended in phosphorylating
- 12 3PGA→1,3-bisphosphoglycerate
- 12 NADPH → glyceraldehyde 3-P
- 6 ATP→ribulose-1,5-bisphosphate
➔ For each CO2 incorporated in a hexose, 3 ATP + 2 NADPH required
➔ to generate 1 molecule of hexose, 6 rounds of Calvin cyclus are required
