C-3 Pathway / Calvin Cycle / CBB / Dark Reaction

C-3 pathway / Calvin cycle/  reductive pentose phosphate cycle / C-3 cycle / carbon assimilation cycle / dark reaction / Calvin–Benson–Bassham (CBB) cycle

Calvin cycle is a series of biochemical redox reaction.It though it is called dark reaction occurs during the day time and is strictly light dependent mechanism.It was Melvin Calvin and Andy Benson who first elucidated this cycle, and also won a Nobel prize in 1961. It is called C-3 cycle because the first stable product formed is a 3 carbon compound : glyceraldehyde-3-phosphate (3-PGA) which can enter several other biosynthetic pathway for each molecule of 3-PGA 3 CO2 are utilized. It takes place in the stroma of chloroplast in eukaryotic photosynthetic organisms and in the lumen in photosynthetic prokaryotic organisms.

The Calvin cycle is divided into 3 parts

  1. Carboxylation
  2. Reduction
  3. Regenration

img_20170114_203955

Calvin cycle with all the products formed (along with the intermediates) and enzymes used.

CARBOXYLATION REACTION

The first reaction in which RuBP (ribulose-1,5-biphosphate), a 5 carbon compound undergoes a carboxylation reaction in the presence of the enzyme RuBiCso (Ribulose-1,5-bisphosphate carboxylase/oxidase). It forms a transient unstable intermediate with the enzyme . The intermediate break down into 6 molecules of 3 carbon: glyceraldehyde-3-phosphate (3-PGA) first stable compound of C3 cycle

  • RuBP: It is called the pace maker of the cycle. it is made up of 16 subunits. 8 large subunits and 8 small subunits that will be joined by chaperones. Large subunits are encoded by the chloroplast genome and the small subunits are encoded by the nuclear genome (RbCS)
  • It is the most abundant compound on Earth.
  • Its activity is regulated by CO2 , O2 , Mg2+ ions and works best at an optimal pH of 8.
  • Rubisco is only active when the ε-amino acid group of lysine reacts with CO2 molecule to form a carbamate to which Mg2+ ions bound.
  • The CO2 forms the carboxylic acid group of the PGA molecule.

REDUCTION REACTIONS

In these reactions the 3-PGA molecule is converted to 3-PGAl. This takes place in 2 steps with an intermediate: 1,3-biphospogycerate.

  • The reaction which converts 6 molecules of  3-PGA to the intermediate ( 1,3-biphospogycerate) is catalyzed by phosphogylcerate kinase and this reaction needs 6ATP molecule.
  • The reaction which converts 6 molecules of 1,3-biphospogycerate to  3-PGAl. (glyceraldehyde-3-phosphate) is catalyzed by the enzyme glyceraldehyde-3-phosphate dehydrogenase and this reaction needs 6 NADP molecules.

REGENRATION REACTION

This is the last step of Calvin cycle in which 3 molecules of RuBP are re-generated from the 5 molecules of glyceraldehyde-3-phosphate and this reaction is catalyzed by Phospho Rubilo kinase (PRK). Regeneration of RuBP is an active process and regeneration of 1RuBP requires 1 ATP so for 3 molecules of RuBP 3ATP are requires.

The regenration is a set of 5-6 reactions

BIOENERGETICS OF CALVIN CYCLE

The final molecule of Calvin cycle is Triose Phosphate. It can enter various other biosynthetic cycles.

To synthesize 1 triose phosphate molecule 9ATP are required and 6 NADPH . (6 atp and 6 NADPH in the reduction reaction and 3ATP in the regenation reaction.)

To make one glucose molecule i.e. 6 carbon 2 cycles are needed i.e. 18ATP+12NADPH 

 

3CO2 + 6NADPH + 9ATP ⇒ G3P + 6NADP+  + 9ADP

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