Team:Purdue/Oxygen Sequestration to Inhibit Photorespiration in C3 Plants

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<p> Seems like a good review/introduction to the Calvin cycle:
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Revision as of 03:53, 29 May 2010

Oxygen Sequestration to Inhibit Photorespiration in C3 Plants

Notes from 5/28

Courtesy of John Mason
The Problem:

Photorespiration is an unwanted chemical process that occurs in C3 plants that wastes ATP and organic nitrogen. While many species have evolved to avoid photorespiration (C4 and CAM plants), attempts to transfer these methods to C3 plants have been unsuccessful due to the drastic physiological differences between species. Photorespiration occurs when O2 concentrations increase relative to CO2. In relatively dry conditions, the stomata of plants close to preserve moisture. Unfortunately, these stomata are used for the exchange of atmospheric CO2 with photosynthetically-derived O2. When the stomata close, the light-independent reactions of photosynthesis, notably the Calvin cycle, continue. The Calvin cycle's purpose is to fix inorganic carbon (CO2) to generate a three-carbon sugar (G3P). The key enzyme in the carbon-fixation process is RuBisCO. Normally, RuBisCO directly fixes the CO2 to RuBP (an intermediate of the Calvin cycle) to produce G3P. However, RuBisCO also has a significant affinity for O2, and can oxygenate RuBP instead. This generates less G3P and produces a toxic intermediate. Eliminating this toxic intermediate requires 1 ATP and causes the formation of NH3, which diffuses out of the plant.

The Solution:

Hemoproteins (such as hemoglobin and leghemoglobin) are capable of binding O2 with various affinities. By combining a hemoprotein with a sequence that codes for protein import into the stroma of the chloroplast (a sequence found in the RuBisCO protein), excess oxygen produced during photosynthesis can be sequestered. In theory, this would reduce oxygen concentrations and inhibit photorespiration. A transcription factor for hypoxia/heat-shock (AtHsfA2, or Arabidopsis thaliana heat-shock transcription factor) could increase production of the hemoprotein during times that photorespiration is likely to occur in excess.

Check out:

This video seems like a good review/introduction to the Calvin cycle.