PHOTORESPIRATION

Introduction to Photorespiration

Earlier it was supposed that the rate of respiration in light was almost equal to the respiration in darkness. Recently it has been observed that light affects respiration and the rate of respiration in light about 3 to 5 times more than the respiration in darkness. This led to the discovery of photorespiration. It is also known as C2 cycle. The existence of photorespiration was first demonstrated by Decker in the year 1955 and 1959. He and his associates were the first to use the term photorespiration.

Photorespiration is a process which involves loss of fixed carbon as CO2 in plants in the presence of light. It is initiated in chloroplasts. This process does not produce ATP or NADPH and is a wasteful process. Photorespiration occurs usually when there is the high concentration of oxygen. Under such circumstances, RuBisCO, the enzyme that catalyses the carboxylation of RuBP during the first step of Calvin cycle, functions as an oxygenase. Some O2 does bind to RuBisCO and hence CO2 fixation is decreased. The RuBP binds with O2 to form one molecule of PGA (3C compound) and phosphoglycolate (2C compound) in the pathway of photorespiration. There is neither the synthesis sugar nor of ATP. Rather, it results in the release of CO2 with the utilization of ATP. It leads to a 25 percent loss of the fixed CO2. 

The photorespiration has been reported in green cells of different plants like Nicotiana, Phaseolus, Pisum, Petunia, Gossypium, Capsicum, Antirrhinum, Oryza, Glycine, Helianthus, Chlorella and Nitella etc. It has rarely been reported in tropical grasses. Photorespiration always requires light and its rate is maximum between 25°C and 35°C. It also depends on oxygen concentration. Photorespiration is quite different from that of normal or ground or dark respiration. Photorespiration results from the oxygenase reaction catalyzed by ribulose-1,5-bisphosphate carboxylase/oxygenase. In this reaction glycollate-2-phosphate is produced and subsequently metabolized in the photorespiratory pathway to form the Calvin cycle intermediate glycerate-3-phosphate. During this metabolic process, CO2 and NH3 are produced and ATP and reducing equivalents are consumed, thus making photorespiration a wasteful process. Many plants, called C4 plants, in warm areas use a different strategy to fix carbon dioxide. This strategy involves first fixing the carbon dioxide in specially arranged mesophyll cells using a separate cycle and then transferring the carbon dioxide to cells where the normal Calvin cycle happens. 

The function of this arrangement is to isolate the cells where the Calvin cycle takes place from the high concentration of oxygen that interfere with the enzyme involved in carbon fixation.

Site of Photorespiration

Previously it was believed that the site of photorespiration is chloroplast but the discovery of peroxisomes, containing enzymes of glycolate metabolism, suggested that there is a correlation between the peroxisome and photorespiration. Hence, peroxisome may be the site of photorespiration. In 1969, Khaki and Tolbert showed that peroxisome is not the actual site of C02 evolution during photorespiration. The peroxisome simply provides a substrate for C02 evolution. In 1971, Tolbert proposed that C02 evolution during photorespiration takes place in mitochondria. These findings that have been put forth specify a close relationship among chloroplast, peroxisome and mitochondria. Presently, most of the physiologists believe that chloroplast, peroxisome and mitochondria—all the three cell-organelles participate in photorespiration. Thus, they jointly form the site of photorespiration.

Biochemistry

Chloroplast, mitochondria and peroxisome all three are involved in the photorespiration. These three cell organelles work in close association with each other. When photosynthesis occurs in chloroplast glycolate which is the early product produced is used as a primary substrate for photorespiration. Actually glycolic acid is produced as a result of oxidation of ribulose diphosphate when the concentration of C02 in the external atmosphere is less than 1 per cent. At first ribulose diphosphate is oxidized into 3-phosphoglyceric acid (PGA) and 2-phosphogIycolic acid in presence of enzyme RuDP carboxylase (also known as III oxygenase). 

Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase) which is present at a great concentration in the stroma of the chloroplasts accounts up to 30% of total nitrogen in a typical C3 leaf and is a bifunctional enzyme, catalyzing both the carboxylation and the oxygenation of ribulose-1,5-bisphosphate (RuBP). The two reactions involve the competition of molecular CO2 with O2 for the enediol form of RuBP which is first generated at the active site of the enzyme. The partitioning of RuBP between carboxylation and oxygenation is dependent on the kinetic parameters of Rubisco.  The carboxylation of RuBP results solely in the formation of glycerate-3-P. While five-sixths of glycerate-3-P molecules thus formed are used for regeneration of RuBP, the remaining one-sixth is either exported from the chloroplasts as triose phosphate for synthesis of sucrose and other products in the cytosolic compartment or metabolized to form starch within the chloroplasts. Oxygenation of RuBP leads to the production of one molecule of glycerate- 3-P and one molecule of glycolate-2-P (a two carbon compound).

Glycolate-2-P, which is formed under ambient conditions at very high rates, is salvaged in the photorespiratory pathway ( C2cycle). In the course of this pathway two molecules of glycolate- 2-P are metabolized to form one molecule of glycerate-3-P (PGA) and CO2 and these carbon compounds are used immediately for the regeneration of RuBP via the reductive pentose phosphate pathway without the net synthesis of triose phosphate. C3cycle and the C2 cycle operate, therefore, in perfect synchronization. Low intercellular concentrations of CO2 as may occur, for example, under water stress (e.g. whenever stomata are closed) can result in even higher ratios. Conversely, doubling the CO2 concentration increases the ratio of Rubisco carboxylation/oxygenation activities by approximately two-fold, reducing photorespiration by about 50%. 

The Photo-respiratory Pathway

The glycolate pathway converts two molecules of 2-phosphoglycolate to a molecule of serine (three carbons) and a molecule of CO2.

• In the chloroplast, a phosphatase converts 2-phosphoglycolate to glycolate, which is exported to the peroxisome. There, glycolate is oxidized by molecular oxygen, and the resulting aldehyde (glyoxylate) undergoes transamination to glycine.

• The hydrogen peroxide formed as a side product of glycolate oxidation is rendered harmless by peroxidases in the peroxisome.

• Glycine passes from the peroxisome to the mitochondrial matrix, where it undergoes oxidative decarboxylation by the glycine decarboxylase complex, an enzyme similar in structure and mechanism to two mitochondrial complexes we have already encountered: the pyruvate dehydrogenase complex and the –ketoglutarate dehydrogenase complex.

• The glycine decarboxylase complex oxidizes glycine to CO2 and NH3, with the concomitant reduction of NAD to NADH and transfer of the remaining carbon from glycine to the cofactor tetrahydrofolate.

 

• The one-carbon unit carried on tetrahydrofolate is then transferred to a second glycine by serine hydroxymethyltransferase, producing serine. The net reaction catalyzed by the glycine decarboxylase complex and serine hydroxymethyltransferase is 

 

• The serine is converted to hydroxypyruvate, to glycerate, and finally to 3-phosphoglycerate, which is used to regenerate ribulose 1,5-bisphosphate, completing the long, expensive cycle .

• In bright sunlight, the flux through the glycolate salvage pathway can be very high, producing about five times more CO2 than is typically produced by all the oxidations of the citric acid cycle.

• To generate this large flux, mitochondria contain prodigious amounts of the glycine decarboxylase complex: the four proteins of the complex make up half of all the protein in the mitochondrial matrix in the leaves of pea and spinach plants.

• In nonphotosynthetic parts of a plant, such as potato tubers, mitochondria have very low concentrations of the glycine decarboxylase complex.

• The combined activity of the rubisco oxygenase and the glycolate salvage pathway consumes O2 and produces CO2 hence the name photorespiration.

• This pathway is perhaps better called the oxidative photosynthetic carbon cycle or C2 cycle, names that do not invite comparison with respiration in mitochondria.

• Unlike mitochondrial respiration, ―photorespiration‖ does not conserve energy and may actually inhibit net biomass formation as much as 50%.

• This inefficiency has led to evolutionary adaptations in the carbon-assimilation processes, particularly in plants that have evolved in warm climates.

Photorespiration and C4 Plants

C4 plants occur largely in tropical regions because they grow faster under hot and sunny conditions. On a hot bright day, when photosynthesis has depleted the level of CO2 at the chloroplast and raised that of O2, the rate of photorespiration reaches the rate of photosynthesis. Nevertheless, C4 plants have a unique leaf anatomy as there are two types of photosynthetic cells.

 Bundle-sheath cells

 Mesophyll cells

 

In C4 plants many steps occur in the photosynthesis by which these plants maintain the CO2 concentration by which they can reduce the effect of photorespiration. The following steps occur as follows: 

 Phosphoenol-pyruvate (PEP) converts into Oxaloacetate.PEP carboxylase adds CO2 to PEP to produce Oxaloacetate.this occurs in the mesophyll cells.

 The formation of  malate from Oxaloacetate is catalysed by malate dehydrogenate reductase. NADPH is used during this step.

 Malate is transported from mesophyll cell into the bundle sheath cell.

 Conversion of malate to pyruvate ,it is catalyzed by malic enzyme. In this process two by products formed. They are CO2, NADPH. Both are used in the Calvin Cycle that occurs within the bundle sheath cell.

 The CO2 formed in this step considered to be concentrated.

 Pyruvate leaves the bundle sheath cell and enters the mesophyll.

 In mesophyll cells pyruvate is converted to PEP by Pyruvate-phosphate dikinase.

 Process is repeated to concentrate more CO2.

 Photorespiration is negligible in C4 plants because the concentration of carbon dioxide is always high in the bundle sheath cells.C4 plants “concentrate” CO2.

 

CAM Plants & Photorespiration:-

CAM is an acronym for crassulacean acid metabolism.these are mainly desert plants some examples of this families are Succulent plants (family Crassulacea),family Cactaceae, family Lilaceae,family Orchidaceae and many others in 25 families.

Water-storing plants like cacti open stomata at night and close them during the day. Helps conserve water but prevents CO2 from entering during the day. Stomata open at night and CO2 is taken in and incorporated into C4 compounds using PEP carboxylase. These compounds are stored in vacuoles until morning when stomata close and CO2 is released to enter the Calvin cycle.

 

 

CAM Plants – Night: CAM plants open their stomata at night. If these plants, living in the xeric conditions opened their stomata during the daytime, would lose large amounts of H2O through osmosis and then evaporation.

PEP carboxylase fixes carbon at night in the mesophyll cells through C4.Stomata are open at night Minimizes water loss and allows the entry of CO2 and Calvin Cycle occurs during the daytime. PEP to oxaloacetate to Malate is catalyzesed by PEP carboxylase and malate dehydrogenase fixes CO2 at night in the mesophyll cells. Malate is stored in vacuoles and at morning it enters in to Calvin Cycle occurs. By this method CAM plants restores the CO2 .

Significance of Photorespiration

The Biological Function of Photorespiration is unknown. Following are some of the significance of photorespiration:

• The C2 oxidative photosynthetic carbon cycle recovers 75% of the carbon originally lost from the Calvin cycle as 2-phosphoglycolate.

• Another possible explanation is that photorespiration is important, especially under conditions of high light intensity and low intercellular CO2 concentration (e.g., when stomata are closed because of water stress), to dissipate excess ATP and reducing power from the light reactions, thus preventing damage to the photosynthetic apparatus.

• Photorespiration could serve as an energy sink preventing the overreduction of the photosynthetic electron transport chain and photoinhibition, especially under stress conditions that lead to reduced rates of photosynthetic CO2 assimilation.

• photorespiration provides metabolites for other metabolic processes, e.g. glycine for the synthesis of glutathione, which is also involved in stress protection.

• There is evidence from work with transgenic plants that photorespiration protects C3 plants from photooxidation and photoinhibition.

• Photorespiration helps in classifying the plants into two groups: (a) Plants with photorespiration, (b) Plants without photorespiration. The plants of both the groups have different characteristics.