EFB530 Plant Physiology
Photosynthesis-Photorespiratory Carbon Oxidation (PCO) or C2 cycle
Rubisco can utilize O2 as a substrate instead of CO2
BUT the ratio of CO2/O2 in water in equilibrium with air at 25ºC=1/24
Therefore, for every 3 CO2 incorporated, there is 1 O2
Oxygenation reaction:
RuBP + O2 -> 2-phosphoglycolate + 3-PGA + 2H+
See reactions in Table 8.2, pg. 154
Regeneration of RuBP from phosphoglycolate
Chloroplast:
phosphoglycolate + H2O -> glycolate + Pi
Peroxisome:
glycolate + O2 -> glyoxylate + H2O2 (peroxide, converted to H2O and O2 by catalase)
glyoxylate + glutamate -> glycine + alpha-ketoglutarate
Mitochondria:
2 glycine + NAD+ + H2O ->-> serine + CO2 + NADH + H+ + NH3
Peroxisome:
serine + ketoglutarate -> hydroxypyruvate + glutamate
hydroxypyruvate + NADH + H+ -> glycerate + NAD+
Chloroplast:
glycerate + ATP -> PGA + ADP + H+
Energetic cost:
for each O2 fixed: 2 ATP + 2.5 NAD(P)H (also lose 0.5 fixed CO2)
ATP=29 kJ; NAD(P)H=217 kJ
carboxylation uses 3 ATP and 2 NADPH=(3*29)+(2*217)=521 kJ for each CO2
oxygenation use 2 ATP and 2.5 NAD(P)H=600 kJ for each O2
Assuming 1 oxygenation for every 3 carboxylations
600 kJ + (3*521 kJ)=2163 kJ BUT only 2.5 CO2 fixed
2163 / 2.5 = 865 kJ for every CO2 {compared to 521 kJ if no oxygenation}
when sugar is burned in respiration, each C =467 kJ
with no oxygenation, efficiency = 467 / 521= 90%
with oxygenation, efficiency = 467 / 865 = 54%
Carboxylation/oxygenation:
The ratio of carboxylation to oxygenation can be affected by:
1) the ratio of CO2 to O2 in the environment
2) temperature:
Major efforts have been made to modify the properties of Rubisco to eliminate the oxygenation reaction, especially using molecular genetics