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.
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.
