Question

Ls4203_endsem_2017

The question is from Advanced Biochemistry and Cellular Metabolism.

6. What are the functions of Thioredoxin? What is role of Thioredoxin in photosynthesis? What are the activation mechanisms of Thioredoxin? How Dark phase of photosynthesis is regulated by light? (1+1+2+2 = 6 marks)

Answer 1

Thioredoxin is a redox protein present in all organisms. It is involved in redox signalling. Thioredoxin acts as antioxidant by helping in the reduction of other proteins by cysteine-thiol exchange reaction.

Thioredoxin is involved in activation of many enzymes involved in photosynthesis by reduction of the disulphide bonds. The enzymes are active in their reduced state.

Thioredoxin is activated by an iron-sulphur enzyme called Ferredoxin-thioredoxin reductase which inturn is activated by ferredoxin. Ferredoxin is reduced by light in photosynthesis. The reduced ferredoxin then activates the ferredoxin-thioredoxin reductase by reduction. This enzyme in turn reduces and activates thioredoxin.

Dark reactions or Calvin cycle reactions are regulated indirectly by light. Four enzymes of Calvin cycle are regulated by light: Ribulose 5-phosphate kinase, fructose1,6-bisphosphatase, sedoheptulose 1,7-bisphosphatase and glyceraldehyde 3-phosphate dehydrogenase. These enzymes are active in their reduced form. The light dependent activation of the thioredoxin reduces and activates these enzymes. In the dark, thioredoxin is inactive and thus the enzymes are predominant in their inactive oxidised form.