Photorespiration & functioning of Rubisco enzyme
- 1. PHOTORESPIRATION & FUNCTIONING OF RUBISCO ENZYME PRESENTED BY DAVID DAIMARY M.SC 2ND SEMESTER M.C COLLEGE SUPERVISED BY MR. TRIDIP BORUAH ASSISTANT PROFESSOR M.C. COLLEGE, BARPETA
- 2. PHOTORESPIRATION PHOTORESPIRATION IS A METABOLIC PATHWAY IN PLANTS, WHERE PLANTS RELEASES CARBON DIOXIDE, CONSUMES OXYGEN AND DOES NOT PRODUCES ANY BIOCHEMICAL ENERGY OR FOOD. 1 • THIS PROCESS IS ALSO KNOWN AS C2 CYCLE. • IT IS A LIGHT DEPENDENT PROCESS AND CATALYZED BY RUBISCO ENZYME.
- 3. 2 • IN MOST PLANTS INITIAL FIXATION OF CO2 OCCURS THROUGH RUBISCO AND RESULT IN A THREE CARBON COMPOUND, 3 - PHOSPHOGLYCERATE. • IN SOME CASES C3 PLANTS CLOSE STOMATA PORES TO PREVENT EXCESS WATER LOSS. • AS THE STOMATA REMAIN CLOSED BUT DUE TO CALVIN CYCLE CO2 LEVEL DROPS DOWN. • AT THE SAME TIME, O2 LEVEL RISES DUE TO LIGHT REACTION. • HERE THE WASTEFUL ACTIVITY (OXYGENASE ACTIVITY) OF RUBISCO OCCURS.
- 4. 3 • RIBULOSE 1,5-BISPHOSPHATE CARBOXYLASE OXYGENASE HAS BOTH CO2 AND O2 FIXING PROPERTIES. • BUT WHEN O2 CONCENTRATION INCREASES, IT ADDS O2 TO RUBP. • IN NORMAL CONDITION RUBISCO ACCEPTS CO2.
- 5. 4 THE PHOTORESPIRATION REACTION OCCURS IN THREE ORGANELLES CHLOROPLAST PEROXISOME MITOCHONDRIA
- 6. 4
- 7. CHLOROPLAST 5 • Under high concentration of oxygen, the enzyme RuBisCo acts as oxygenase so that instead of CO2 , oxygen gets attached to its binding site and yield one molecule of phosphoglycolate (a C2 compound ) and one molecule of PGA ( a C3 compound ). PGA enters the Calvin cycle ( C3 cycle ). • The phosphoglycolate undergoes dephosphorylation by the enzyme phosphoglycolate phosphatase to yield glycolate (C2 compound). • Glycolate then leaves the chloroplast and enters the peroxisome via cytosol.
- 8. PEROXISOME 6 • On reaching peroxisome is oxidized to glycoxylate (C2 compound) by the enzyme glycolate oxidase. In this oxidation H2O2 is produced which is toxic to the cell. Hence it is decomposed to H2O and O2 by the enzyme catalase , a constituent of peroxisome • Glyoxylate is then converted to glycine by peroxisomal transaminase and glutamateglyoxylate amino transferase. Then glycine leaves peroxisome and enters mitochondria via cytosol.
- 9. MITOCHONDRIA 7 • In mitochondria, 2 molecules of glycine are converted to one molecule each of serine, NH3 and CO2 in two-step process. Thus this step is the source of evolution of CO2 in photorespiration and it is catalyzed by the enzyme glycine decarboxylase complex. This step also involves utilization of NAD+ as an oxidant and the resultant NADH is reoxidized by the mitochondrial electron transport system with the generation of ATP.
- 10. PEROXISOME 8 • In the peroxisome, serine is further metabolized to hydroxypyruvate by the enzyme serine- glyoxylate aminotransferase. • Hydroxypyruvate is reduced to glycerate by NADH-hydroxypruvate reductase . • On reaching chloroplast, glycerate is phosphorylated to phosphoglycerate (PGA) by the enzyme glycerate kinase with the utilization of one molecule of ATP. CHLOROPLAST • PGA may enter C3 cycle(Calvin cycle) and ultimately converted to carbohydrate. • Thus the cycle is completed.
- 11. 9 SIGNIFICANCE • WASTEFUL PROCESS- 25% OF CARBON IS LOST BY OXYGENATION. • ENEGRY CONSUMING PROCESS. • PROTECT CHLOROPLAST AGAINST PHOTO OXIDATION. • DOES NOT PRODUCE ENERGY DISADVANTAGES • DECREASE EFFICIENCY OF PHOTOSYNTHESIS.
- 12. 10 REFERENCE 1.Oxidative damage to plants by Pravaiz Ahmed (Book) 2. LIFE SCIENCES I by Pranav Kumar.
- 13. THANK YOU