HL Photosynthesis

Calvin experiment

Photorespiration: enzyme specificity vs plant evolution

Rubisco: victim of its own success

RuBisCo is the planet's most abundant protein and it really is an amazing enzyme but it is not perfect. When photosynthesis first appeared on Earth, the atmosphere had a a high partial pressure of carbon dioxide and very little oxygen. It was great for photosynthesis! RuBisCO (RuBP, ribulose bi-phosphate carboxylase) is the first enzyme of the Calvin Cycle and is able to "fix" carbon dioxide gas into a 3 carbon molecule, G3P (3-phosphoglycerate). However, the active site of RuBisCo, interacts with RuBP and forms an "activated intermediate" that can bind to both carbon dioxide and oxygen. The specific shape of the RuBisCO's active site promotes reactions with carbon dioxide instead of oxygen. However, when partial pressure of oxygen becomes much higher than partial pressure of carbon dioxide, the rate of RuBisCo interaction with oxygen increases significantly. This reaction leads to the oxygenation of RuBP instead of the carboxylation of RuBP, which is the first step of the Calvin cycle. Interestingly, data suggests that the specificity of RuBisCo decreases with temperature. Plants exposed to higher temperatures have higher a rate of photorespiration and therefore, a decreased efficiency in photosynthesis.

The oxydation of RuBP leads to the formation of a non-desired compound called 2-Phosphoglycolat which acts as an inhibitor of the enzymes of the Calvin cycle. Photosynthetic organisms have developed a metabolic pathway to transform this non-desired compound into the desired G3P which is used in the Calvin cycle. This metabolic pathway receives the name of photorespiration.

A recent research conducted by the University of Illinois and U.S. Department of Agriculture and published in the journal Science on January 5th, 2019, has demonstrated that crops can be engineered with a "photorespiratory shortcut" that increases their biomass about 40% more than normal plants. Over 1700 tobacco plants were tested with different genetic-constructs (with algae and bacterial DNA) designed to promote alternative pathways to eliminate the toxic products of photorespiration. So, what could have happened by chance at some point in evolution, has now been encouraged by scientists using other organisms' genes to design alternative pathways from plants to deal with their auto-generated toxic metabolites. The goal now is to transfer this new engineered metabolic pathways to other plants that produce more useful outcome for humans, such as rice or wheat. Tobacco is great for experimentation, but not for human health! You are encouraged to read more about this research here or here.