• Light-independent reactions
• Responsible for all C atoms "fixed" as biomolecules
• Cannot proceed w/o ATP & NADPH made by light dependent reactions

• Phosphorylation of ribulose-5-phosphate by ATP to make ribulose-1,6-bisphosphate
• Molecules of gaseous CO₂ condensed w/ Ribulose-1,6,bisphosphate

-Enzyme: Rubisco

-Rate limiting step

• Hydrolysis of unstable product to make 2 molecules of 3-phosphoglycerate.

• Stromal enzyme
• 16 subunits (8 large, 8 small) → No cooperativity
• k_cat = 3 sec^-1 very inefficient

• Requires Mg²⁺ & non-substrate CO₂
• Mg²⁺ binds in active site → activating bound ribulose-5-phosphate
• CO₂ forms a carbamate w/ a lysine
• Negatively charged carbamate binds Mg²⁺

• O₂ → substrate for Rubisco
• Catalytic Imperfection
• O₂ & CO₂ compete for active site
• Oxygenase rxn also requires cabamylated lysine-Mg²⁺

       ^{- Photorespiration competes w/ carboxylase rxn}

• Phosphoglycolate not ver useful → salvage pathway yields 1 Ser & 1 CO₂

• 3-phosphoglycerate → Glucose
  
• GAP goes on to make glucose
  

          *GAP isomerizes to DHAP → triose     

             phosphate isomerase.

          *Aldolase condenses 3-C sugars to 

            fructose-1,6-bisphosphate →

            dephosphorylated to fructose

            6-phosphate → readily converted to

            other hexoses.

• Produce 5-C sugar from 3-C and 6-C sugars

                 *Transketoalase

                 *Aldolase

• Beginning w/ three 5-C sugars
  
• Use ATP to phosphorylate again
  
• Use 6 ATP to phosphorylate six 3-C sugars
  
• Use 6 NADPH for reduction phase
  
• Condense two of the six 3-C sugars to make one fructose-6-phosphate 

*hexose pool or regeneration reactions

Making 1 Glucose Molecule

# of cycles?

Cost?

• Two rounds of Calvin Cycle needed
  
• Cost of single glucose → 54 ATP equivalents

• Drives ATP & NADPH synthesis → both substrates for Calvin Cycle.
• Drives changes in pH of stroma that activate rubisco

*Electron transport Chain → pumps H+ out of stroma into lumen *Mg2+ leaves lumen to balance influx of protons

*Stromal pH rises, favoring carbamylation of rubisco active site Lys

• Coordinates electron transport and carbon fixation (stage 1)
• Small protein w/ single disulfide bond → can be reduced/oxidized
• Oxidized → can be reduced by reduced ferrodoxin from PSI
• Reduced → can oxidize disulfide bonds in other proteins.
  

Pentose Phosphate Pathway:

what are the two phases?

• Phase 1 → oxidative generation of NADPH
• Phase 2 → Synthesis of 5C sugars

           *Interconversion of sugars

           *Excess ribose-5-phosphate converted into glycolytic

             intermediates.

• Rate of pentose phosphate and flow of glucose-6-phosphate depends on:-

             *NADH

             *Ribose-5-phosphate

             *ATP

• Increased need for ribose-5-phosphate

             *Most of G-6-P converted into F-6-P and GAP by glycolysis →

               which are then converted to Ribose-5-phosphate

• Balanced need for NADPH and ribose-5-phosphate
• G-6-P converted to Ribulose-5-phosphate (generating 2 NADPH) → converted to Ribose-5-phosphate
  

• Increased need for NADPH
• Complete oxidation of all G-6-P (and equivalents) to CO₂ yields NADPH
  

• Both NADPH & ATP required
  
• Formation of ribose-5-phosphate generates NADPH
• Converstion of ribose-5-phosphate to GAP to Pyruvate generates ATP