Photorespiration and RuBisCO

The enzyme at the centre of the fixation of carbon in photosynthesis (which is the ultimate source of our food) is RuBisCO (Ribulose Bisphosphate Carboxylase Oxygenase). It is sometimes called the ‘lazy enzyme’ because it works so slowly. This is unjust since the process is so difficult that RuBisCO is the only enzyme that can do it. So it is like the situation of a strong man who is the only one who can carry very heavy rocks and who is called lazy because it takes him so long to move the rocks. The name already indicates that this enzyme catalyses two different reactions with the same substrate, ribulose bisphosphate. So it might also be claimed that it does not concentrate well. Instead of spending all its time on the useful process of carboxylation it apparently wastes a lot of time on the hobby of oxygenation. Carboxylation produces two molecules of phosphoglycerate (PGA) by combining a molecule of carbon dioxide with one of ribulose bisphosphate. In the oxygenation reaction only one molecule of PGA is produced together with one of phosphoglycolate. At least superficially the latter substance is not only useless but actual causes the cell a lot of trouble getting rid of it. I have not seen a definitive explanation about what purpose this process, photorespiration, might serve. One idea is that it might act as a safety valve. A too large input of energy to the photosynthesis system might damage it and photorespiration can be used to dissipate this energy in a relatively harmless way when necessary. The relative rates of the two reactions are influenced by the concentration of carbon dioxide and for normal atmospheric concentrations the relative rate of photorespiration is quite high.

The discussion up to now has been appropriate for what is called C_3 photosynthesis. The name comes from the fact that the first molecule to be produced is the three-carbon molecule PGA. There is an alternative called C_4 photosynthesis. This also contributes to food production, even to the food I eat. My generic breakfast is cornflakes and the most economically important plant which uses C_4 photosynthesis is maize. In this process carbon dioxide is used to make malate. Malate is not directly useful but is an intermediate. It is transported to an internal compartment (bundle-sheath cells) where it releases carbon dioxide. The result is an increased concentration of carbon dioxide and RuBisCO, which is waiting there, is pushed towards doing more carboxylation at the expense of oxygenation. Since the C_4 process leads to greater crop yields under certain conditions there is a project to genetically engineer rice so as to produce a C_4 variant. There is third type of photosynthesis , CAM photosynthesis for crassulacean acid metabolism. It is used, for instance by pineapples. There are similarities with C_4 with the difference that while in C_4 carbon dioxide is harvested in one place and released in another in CAM it is harvested at one time (during the night) and released at another (during the day).


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