1. Role of light in the activation of some enzymes of calvin cycle " Dark phase enzymes "
Dr.Howaida Nounou

3. • fixation of CO2 in to carbohydrate has been described " as the dark phase" of photosynthesis. • the implication being that light play no role in the calvin ( C3) cycle other than to generate the ATP and NADPH required to drive it . • Recent work has, however, shown that this is not the case, for it has become clear that light activates at least five of the calvin cycle enzymes, namely 1- Ru DP carboxylase ( Rubisco) , 2-glyceraldehydes-3-p dehydrogenase, 3-fructose 1.6 diphosphatase, 4-seduoheptulase diphosphatase and 5-ribulose 5- P kinase.

4. • Thus, it's probably now incorrect to speak of the " dark phase" photosynthesis ; the term has, however, been retained so that the continuity with older textbooks may be maintained and possible confusion avoided .

5. • Some enzymes of the calvin cycle are indirectly activated by light.
• It illumination of chlorophyll, leads to the transport of protons ( H+) from the stroma of the chloroplast into the thylakaid lumen across the thylakoid membrane , making the stromal compartment alkaline and is accompanied by a flow of Mg2+ out of the thylakoid compartment into the stroma.

6. Mg++ H+
the stroma be comes alkaline & Mg2+moves out from the thylakoid lumen into the stroma .

7. •. some enzymes present in the stoma e. g (Rubisco),is ( ribulose 1
• some enzymes present in the stoma e.g (Rubisco),is ( ribulose 1.5 bisphosphate carboxylase )
activated by : increase Mg++
: alkaline PH
• also, fructose 1.6 diphosphatase requires Mg2+ and is very dependent upon PH .

8. • these enzymes:. Ribulose 5-P Kinase, sedoheptulose, 1
• these enzymes: Ribulose 5-P Kinase, sedoheptulose, 1.7 diphosphtase and fructose 1.6 diphosphatase
exist in either of 2 forms differing in the oxidation state of cysteine ( cys) residues which are essential to their catalytic activity.
• When their cys residues are oxidized as disulfide bonds ( S-S) , the enzymes are inactive this is normal situation in the dark .

9. Light ( active enzyme )
Reduced state
Dark
on illumination
Dark ( inactive enzyme )
Oxidized state

11. • with illumination , electrons flow from photosystem I
( PSI) to ferrodoxin , which passes electrons to small, soluble , disulfide – containing protein called thioredoxin.
Thiroedoxin donates electrons for the reduction of the disulfide bridges of these light activated enzymes
and is then reactivated in a disulfide – exchange reaction catalysed by thioredoxin reductase .

12. Thioredoxin reducatse
(Fd)
reduced e-
Thioredoxin reducatse

13. the balanced equation for the net reaction of the calvin cycle is :
Then 3 molecules of ATP and two of NADPH are consumed in converting CO2 into a hexose such as glucose or fructose.