Let us make an in-depth study of the photorespiration and glycolate metabolism. After reading this article you will learn about 1. Factors Affecting Photorespiration and 2. Significance of Photorespiration.
Photorespiration is a special type of respiration shown by many green plants when they are exposed to light. The normal dark respiration (i.e., usual mitochondrial respiration) as a rule is independent of light, its rate being the same in light as well as in dark.
The amount of CO2 released during this process is also equal under both these conditions. But, many workers especially Krotkov et al in Canada while working on gaseous exchange of whole green leaves repeatedly noticed that respiratory CO2 evolution was higher in light than in darkness and that there was a post-illumination burst of CO2 output particularly at higher O2 concentrations.
Krotkov in 1963 coined the term photorespiration to differentiate between these apparently separate forms of CO2 evolution. Both these types of respiration are also different in sensitivity towards O2, temperature and metabolic inhibitors and in the specific activity of respiratory substrates following photosynthesis in 14CO2.
Photorespiration is closely related to CO2 compensation point and usually occurs only in those plants which have comparatively high CO2 compensation point such as tomato, wheat, oats, green alga Chlorella etc. (C3-plants). It is insignificant or rather absent in plants which have very low CO2 compensation point such as maize, sugarcane etc. (C4-plants).
i. Photorespiration occurs only in chlorophyllous tissues of plants.
ii. Process of photorespiration is accomplished in 3 different cell organelles viz., chloroplasts, peroxisomes and mitochondria.
iii. Glycolate (glycolic acid) is the chief metabolite of photorespiration and also its substrate. Other important metabolites are the amino acids glycine and serine.
iv. Like usual mitochondrial respiration, the photorespiration is also an oxidative process where oxidation of glycolate occurs with subsequent release of CO2 (post illumination burst of CO2).
Various steps of the glycolate metabolism (Fig. 11.26) i.e., synthesis of glycolate and its oxidation with subsequent release of CO2 (photorespiration) are as follows:
(i) Glycolate is synthesized as a side product from some intermediates of photosynthesis in chloroplasts. It is probably derived from C1 and C2 of the ketose sugar phosphates of the carbon reduction cycle (i.e., Calvin cycle).
It is now known that O2 competes with CO2 for the enzyme Ribulose bisphosphate carboxylase (RuBP—Carboxylase). When this enzyme reacts with O2 instead of CO2, it is called as RuBP-oxygenase. In the latter case, one molecule of phosphoglyceric acid (PGA) and one molecule of phosphoglycolic acid are formed from ribulose bisphosphate as shown below:
Phosphoglyceric acid again enters into the Calvin cycle while phosphoglycolic acid is dephosphorylated in the presence of the enzyme Phosphatase to form glycolate.
(ii) From chloroplasts, the glycolate migrates into peroxisome where it is oxidised (photo-respired) to glyoxylate in the presence of the enzyme glycolic acid oxidase.
The hydrogen peroxide formed is removed by the enzyme catalase.
H2O2 → H2O + ½O2
(iii) Glyoxylate is now converted into an amino acid glycine. This is a transamination reaction which takes place at the expense of L-Glutamate and in the presence of the enzyme L- Glutamate glyoxylate transaminase.
(iv) The glycine formed in peroxisomes migrates into mitochondrion where 2 molecules of glycine react to form one molecule of another amino acid serine with liberation of CO2 (post-illumination burst of co, & photorespiration) and also nh3. This reaction is catalysed by the enzyme serine hydroxymethyl transferase.
(v) The serine passes back to the peroxisome where it is transaminated to hydroxypyruvate in the presence of Serine: Glyoxylate amino transferase.
(vi) Hydroxypyruvate is now reduced in peroxisome by the NAD+—requiring hydroxypyruvate reductase to form glyceric acid.
(vii) The glyceric acid (glycerate) now diffuse into the chloroplast where it is phosphorylated to 3-phosphoglyceric acid (PGA) in the presence of the enzyme glycerate kinase. PGA is well known intermediate of the Calvin cycle.
Thus, starting from intermediates of Calvin cycle with the synthesis of glycolate, serine is formed which again is converted into intermediates of calvin cycle thus completing the glycolate cycle. And because glycolate and some other metabolites of this cycle e.g., glyoxylate and glycine are 2-C compounds, the glycolate metabolism or glycolate cycle is also called as C2—cycle. It is also known as photosynthetic carbon oxidation cycle (PCO-cycle)
Factors Affecting Photorespiration:
As described earlier, photorespiration is closely associated with CO2 compensation point. Besides the particular plant species (C3 or C4 plant), the factors which influence CO2 compensation point also affect rate of photorespiration. Thus, rate of photorespiration is higher when O2 concentration is higher and CO2 concentration is low. Higher temperatures also have a favourable effect on this process. On the other hand inhibitors of glycolic acid oxidase such as a-hydroxysuiphonates inhibit the process of photorespiration.
Significance of Photorespiration:
Positive function of photorespiration in plants is not yet known. It rather seems to be a doubly wasteful process. Firstly, it has been estimated that during photosynthesis by algae and C3 plants, up to 50% of the CO2 fixed may have to pass through photo-respiratory process (glycolate pathway) to form carbohydrates such as sucrose thereby resulting in considerable decrease of photosynthetic productivity.
Secondly, unlike usual mitochondrial respiration neither reduced co-enzymes are generated in photorespiration nor the oxidation of glycolate is coupled with the formation of ATP molecules. Moreover, there is consumption of reduced coenzyme and ATP in the glycolate cycle (See reactions No. vi and vii). According to Hatch & Slack (1970) photorespiration is a metabolic adjunct to the Calvin cycle (i.e., it has been added to the Calvin cycle but essentially is not its part).
However, the knowledge about the process of photorespiration is of great importance to Agriculturists. By manipulating the different atmospheric conditions, use of inhibitors of glycolic acid oxidase such as a-hydroxysulphonates, and through genetic control, the process of photorespiration can be regulated and consequently the photosynthetic productivity can be increased. But, sustained efforts by plant physiologists in these directions have not been successful.
Scientists are now thinking to prevent photorespiration and to increase photosynthetic productivity on other lines such as to develop a mechanism for concentrating CO2 in the photosynthetic cells. Although, positive roles of photorespiration and glycolate metabolism are yet to be explored, but it is now clear that glycolate metabolism undoubtedly serves a scavenger function. Two turns of this cycle produce two molecules of phosphoglycolate by oxygenation which contain 2 + 2 i.e., 4 carbon atoms.
One of these four C-atoms is lost as CO2 (see reactions no. (iv), 2 mols of glycine being derived from two mols of phosphoglycolate) and the remaining 3-C atoms are cycled back to chloroplast as glycerate. Thus, glycolate pathway recovers 75% of the carbon which would otherwise be lost as 2-phosphoglycolate from the Calvin-cycle.