UNDERSTANDING CELLULAR RESPIRATION AND PHOTORESPIRATION

Photorespiration  1


Photorespiration (also known as the oxidative photosynthetic carbon cycle, or C2 photosynthesis) is a process in plant metabolism which attempts to reduce the consequences of a wasteful oxygenation reaction by the enzyme RuBisCO. The desired reaction is the addition of carbon dioxide to RuBP (carboxylation), a key step in the Calvin–Benson cycle, however approximately 25% of reactions by RuBisCO instead add oxygen to RuBP (oxygenation), producing a product that cannot be used within the Calvin–Benson cycle. This process reduces efficiency of photosynthesis, potentially reducing photosynthetic output by 25% in C₃ plants.^[1] Photorespiration involves a complex network of enzyme reactions that exchange metabolites between chloroplasts, leaf peroxisomesand mitochondria.
The oxygenation reaction of RuBisCO is a wasteful process because 3-Phosphoglycerate is created at a reduced rate and higher metabolic cost compared with RuBP carboxylase activity. While photorespiratory carbon cycling results in the formation of G3P eventually, there is still a net loss of carbon (around 25% of carbon fixed by photosynthesis is re-released as CO₂)^[2] and nitrogen, asammonia. Ammonia must be detoxified at a substantial cost to the cell. Photorespiration also incurs a direct cost of one ATP and one NAD(P)H.

http://ww2.methuen.k12.ma.us/mnmelan/understanding_cellular_respirati.htm


Schematic representation of the photorespiratory pathway (in black) and the three circumvent pathways designed to overcome the photorespiratory losses. 1 The reactions of bypass 1 (in red) are entirely realized into the chloroplast and comprise the transformation of glycolate to glycerate, introducing glycolate dehydrogenase, glycine decarboxylase and tartronate semialdehyde reductase similar to the E. coli glycolate catabolic pathway (Kebeish et al., 2007). Bypass 2 (in green) follows the E. coli glyoxylate catabolic pathway in the peroxisomes by introducing glycine decarboxylase and hydroxypyruvate isomerase (Carvalho et al., 2011). Bypass 3 (in blue) oxidizes glycolate to CO2 in the chloroplast, using exogenous (glycolate oxidase and catalase from the peroxisomes, and malate synthase from the glyoxysomes) and endogenous (malic enzyme and pyruvate dehydrogenase) enzymes (Maier et al., 2012). In all three bypasses release of ammonia in the mitochondrion is abolished; 75% of the glycolate redirected toward bypasses 1 and 2 is returned to the Calvin-Benson cycle as 3-PGA; bypasses 1 and 3 dislocate CO2 released from the mitochondrion to the chloroplast. Reactions stoichiometry is not taken into account; the numbers of carbon atoms of each metabolic compound are in italic; 3-PGA, 3-phosphoglycerate; RuBP, ribulose 1,5-bisphosphate.


1) http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4054791/

Photorespiration 1

The enzyme, rubisco, not only initiates carbon fixation in the Calvin cycle; it also combines with oxygen to initiate photorespiration. As its name suggests (rubsiCO) the enzyme is both a carboxylase and an oxygenase. The active site of rubisco cannot distinguish the two similar substrates: O=C=O and O=O. As we shall see, the two reactions catalyzed by the same enzyme are diametrically oppposed to each other.

Each reaction pathway undoes the other, and both reactions can operate in a cell simultaneously depending upon the environmental conditions. As both substrates combine with the active site of rubisco, they are competitive inhibitors of each other's reactions. One might recall our earlier discussions about competitive inhibition. The relative concentration of the two substrates and the differential affinity of the enzyme for each substrate will determine which of the reactions (Calvin cycle or Photorespiration) predominate. Fortunately for plants (and for us indirectly!) rubisco has an affinity for carbon dioxide that is 80 times higher than its affinity for oxygen. However, the relatively low ratio of CO2 to O2 of mesophyll fluids in contact with air (0.04) means that, in a typical plant, the Calvin cycle only occurs about three times faster than photorespiration. Temperature also influences the relative rates of photorespiration and the Calvin cycle. Because increased temperature more efficiently removes carbon-dioxide from solution than it does oxygen, high temperatures favor photorespiration.



The photorespiration pathway is an enzymatic one that is not coupled to any electron transfer system. It does not generate ATP. It does use oxygen and it does produce carbon dioxide, and it uses a sugar-phosphate as its primary fuel.

1) http://plantphys.info/plant_physiology/photoresp.shtml

http://edoc.ub.uni-muenchen.de/7577/1/Saschenbrecker_Sandra.pdf

Unfortunately, RuBisCO is not absolutely specific for CO2. Instead, it can also utilize O2 as substrate and thus catalyze the oxygenation of RuBP, resulting in 3-phosphoglycerate and 2-phosphoglycolate (Fig. 12 B). Since 2-phosphoglycolate is a metabolically useless compound, it usually enters the glycolate pathway, which includes sequential reactions in the chloroplast stroma, the peroxisomes and in the mitochondrial matrix. In this pathway, two molecules of 2-phosphclycolate are converted to one molecule of CO2 and one molecule of 3-phosphoglycerate. The latter can re-enter the Calvin cycle and therefore some fixed carbon is salvaged. But nevertheless, a considerable proportion of it becomes released as CO2 and, even worse, significant amounts of cellular energy are consumed by the whole process. Hence, the RuBisCO-catalyzed oxygenation reaction is a pretty expensive process for the cell – it results in no fixation of carbon and causes both energetic and metabolic losses. Since this pathway consumes oxygen and produces carbon dioxide, similar to mitochondrial respiration, it is also called photorespiration .Photorespiration principally reduces the photosynthetic potential of plants and consequently limits their growth rate.