Botany 4400/5400

Lecture 15

15 February 2006

Reading: Chapter 8, Taiz and Zeiger's Plant Physiology

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II. Metabolism

C. Photosynthesis

4. Photorespiration

It is believed that the Calvin cycle evolved early in the history of life on Earth, before the appearance of 2-photosystem, oxygen-evolving photosynthesis in the first cyanobacteria. At that time, the atmosphere of the Earth contained much more carbon dioxide than it does now and very little oxygen.

The present atmosphere of the Earth includes 0.035% CO2 (and rising) and 21% O2. RUBISCO "prefers" to react with CO2 but also reacts with O2. At the very low ratio of CO2 to O2 in the present atmosphere, RUBISCO reacts with O2 about 25% of the time. In otherwords, RUBISCO is competitively inhibited by the O2 that is produced by photosynthetic electron transport. This competitive inhibition reduces the efficiency of photosynthesis by about 40%.

The reaction of RUBISCO with O2 produces a 2 carbon compound called 2-phosphoglycerate (2-PG) that cannot be directly used for RUBP synthesis. To avoid depletion of RUBP from the Calvin cycle, the 2 carbon compound must be processed into 3PGA.

The reaction of RUBISCO with oxygen and metabolic processing of the resulting 2-PG is called "photorespiration". It is called this because it only occurs in the light (mitochondrial respiration continues in darkness) and because it consumes oxygen and produces carbon dioxide, just like mitochondrial respiration. We will consider the two parts of photorespiration separately.

a. Oxygenation

The first step of photorespiration is oxygenation. It is catalyzed by RUBISCO and converts RUBP to one molecule fo 3PGA and one molecule of 2PG, which is a 2 carbon compound with one phosphate group.

The oxygenation reaction is influenced by environmental factors. In normal air at 25°C, a well watered plant fixes oxygen once for every 3 carbon dioxide fixations. If the temperature increases or the plant is short of water, however, the occurrence of oxygenation increases.

i. Temperature

Increasing temperature increases oxygenation in two ways:

The first of these is that RUBISCO becomes less specific for CO2 as the temperature rises. Apparently increasing temperature reduces the specificity of the active site of the enzyme such that it performs oxygenation more frequently.

 
The second way in which temperature affects oxygenation is by its effect on the relative solubilities of O2 and CO2. Both O2 and CO2 must dissolve in the water of a leaf cell before they are available to react with RUBISCO. The solubility of any gas in water decreases as temperature increases so that there is less O2 and CO2 dissolved in a cells when they are at higher temperature. The solubility of O2 decreases with temperature less than the solubility of CO2, however. This means that as temperature rises, the ratio of CO2 to O2dissolved in the cell solution decreases. This decreasing ratio favors oxygenation. For example, at 25°C, the saturated concentration of CO2 in water is 11.68 µM while the saturated concentration of O2 in water is 264.6 µM. At 35°C, the saturated concentration of O2 in water is 9.1 µM while the saturated concentration of O2 in water is 228.2 µM. Thus the ratio of CO2 to O2 dissolved in leaf cells decreases by 9% with an increase of 10°C.

 ii. Lack of water

As a plant becomes short of water, it closes its stomata. Inside a leaf with closed stomata, O2 is produced by Photosystem II and builds up. CO2 is consumed by carboxylation and drops to low levels. Thus, the ratio of CO2 to O2 inside a leaf becomes very low when stomata are closed, favoring oxygenation.

iii. High CO2 abolishes oxygenation

Increasing CO2 concentration to 1% abolishes oxygenation. At this concentration, CO2 outcompetes O2 for the active site of RUBISCO. This effect is significant in two ways:


First, the concentration of CO2 in the Earth's atmosphere is rising because of deforestation and the burning of fossil fuels. It has risen by 30% since 1850. As CO2 rises, photorespiration and its metabolic costs to plants decrease.

Second, some plants and algae have evolved ways to avoid photorespiration by increasing the concentration of CO2 in the vicinity of RUBISCO. We will discuss this adaptation in the next lecture.

 
b. Processing of 2 phosphoglycolate

3 PGA is converted back to RUBP by the Calvin cycle during its normal operation. 2 PG cannot be converted to RUBP by the Calvin cycle and is instead processed to 3 PGA by an elaborate series of steps that involve 3 different organelles: the chloroplast, the mitochondrion, and the peroxisome. A diagram of this is shown in your text as Figure 8.7.

The important steps of processing that occur in each organelle are:

i. Chloroplast steps

The original oxygenation reaction by RUBISCO occurs in the chloroplast. This is where O2 is consumed by photorespiration. The 2PG that is formed is dephosphorylated and exported as glycolate to the peroxisome.

The final steps of processing also occur in the chloroplast. Glycerate is converted to 3PGA at the expense of one ATP. This ATP is part of the energetic cost of photorespiration.

ii. Peroxisome steps

In the peroxisome, glycolate from the chloroplast is converted to glyoxylate. This reaction consumes O2 and generates hydrogen peroxide. The O2 is released again, however, as the hydrogen peroxide is converted to water by the enzyme catalase.

Glyoxylate is converted to the amino acid glycine with donation of an amino group that ultimately comes from the amino acid serine. These are the "trans amination" reactions of photorespiration. They shift amino groups from one molecule to another.

 iii. Mitochondrion steps

The mitochondrion is where 2 carbon amino acids are converted into 3 carbon amino acids. 2 glycine molecules, 2 C each, are combined to make one 3 carbon serine. In the process, 1 NH3 is released and so is 1 CO2. This is the CO2 produced by photorespiration. The enzyme that catalyzes this reaction is glycine decarboxylase. Serine is exported to the peroxisome where its amino group is removed, converting it to glycerate. The glycerate is then exported to the chloroplast where it is converted to 3PGA at the expense of one ATP. It should be noted that transport of photorespiration intermediates between organelles requires a special membrane transporter for every different molecule and organelle.

Overall, 1 3PGA is recovered / 2PG processed, thus 3/4 of the carbon lost by oxygenation is recovered. The NH3 is released and so is 1 CO2 released by photorespiration must be reassimilated, with some expense of ATP and reducing power. Also, the phosphorylation of glycerate back to 3 PGA costs one ATp per reaction.

c. The costs of photorespiration

In normal air at 25°C, photorespiration decreases the efficiency of CO2 assimilation by 40%. This decrease in efficiency results from the use of ATP to phosphorylate glycerate and other costs associated with glycolate processing.

 
d. The benefits of photorespiration

Photorespiration is either a necessary evil of plant metabolism or it may have some adaptive function that is not apparent. Some have proposed that photorespiration allows plant leaves to use up excess light energy and reduce photooxidative damage when the plant is water-stressed and the stomata are closed. There are many plants, however, that do no photorespiration and are not especially sensitive to drought. C4 plants are an example of these.