MCB 28 Photosynthesis Calvin Cycle

Turn CO2 into sugar

- Carboxylation
- Reduction
- Regeneration

CO2 is fixed in the presence of RuBP (5 carbon) and is converted 2 molecules of a three-carbon sugar 3PGA

- ATP and NADPH produced by electron transport is consumed
- three-carbon sugar 3PGA is converted into G3P

- Additional ATP is consumed so that some of the G3P can be converted back into RuBP, allowing for the repetition of CO2 fixation

twice

1. Enzyme abstracts a proton from C3 of ribulose-1,5-bisphosphate, Mg2+ ion may help
stabilize the developing negative charge
2. Enediolate intermediate nucleophilically attacks CO2
3. H2O attacks C3
4. The 6-carbon product splits to yield 2 molecules of 3-phosphoglycerate

1. Inactive Rubisco + CO2 ---> carbamate
2. carbamate gets stabilized by Mg2+and becomes active
3. PGA is metabolized
4. Analogues of RuBP can block the active site on the enzyme
5. CA1P binds tightly to Rubisco and makes it inactive

- Rubisco reacts with O2 in a process called oxygenation
- Ribulose bisphosphate, in combination with rubisco, helps generate 2-Phosphoglycolate
and 3-Phosphoglycerate

- energy efficient
- non reducing sugar

- Stroma pH increases from 7.1 (dark) to 7.9 (light), and Rubisco is more active at pH 7.9
- Stroma Mg2+ concentration increases from 1-2mM in dark to 3-5mM in light (rubisco
requires Mg2+)
- CO2 presence activates Rubisco in morning through Rubisco activase
Rubisco seems to be light dependent?

● Rubisco remains inactive until it is activated by a CO2 molecule that carbamylates its
catalytic Lysine to bind Mg2+ that helps complete the activation process
● CA1P inhibits Rubisco
● The binding of CO2 and Mg2+ is reversible from the active site, which helps its
regulation in turning rubisco on and off

Bicarbonate ion can help yield a four-carbon compound (malate) --> into pyruvate
● NAD-malic enzyme that converts malate into pyruvate kicks off CO2, which undergoes the calvin cycle within the chloroplasts
● CO2 concentrates in the bundle sheath cells and helps suppress photorespiration

- water conservation and water-use efficiency, which occurs in succulents such as cacti and is found in arid areas such as the desert

● stomata is only open at night
○ CO2 uptake during the night to conserve water loss
● CO2 is stored in organic acids within vacuoles until they can be decarboxylated during
the day

Dark: Opened Stomata
- CO2 -> HCO3- -> oxaloacetate
- oxaloacetate is converted to malate, which is stored in the vacuole as malic acid

Light: Closed Stomata
- Closed stoma prevents H2O loss and CO2 uptake
- malate from the vacuole is converted back to CO2
- Some of the malate is turned into pyruvate, which is converted into starch within the chloroplast

- Specialized bacterial microcompartment found in some autotrophic bacteria
- Crucial role in carbon fixation pathway
- Found in cyanobacteria and some chemoautotrophic bacteria