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

• 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.

• 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 .

• 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.

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

•. 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 .

• 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 .

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

• 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 .

Thioredoxin reducatse (Fd) reduced e- Thioredoxin reducatse

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.