Editing Photorespiration (section)

==Substrate specificity of RuBisCO==
[[File:RuBisCO reaction O2.svg|class=skin-invert-image|thumb|350px|Oxygenase activity of RuBisCO]]

The oxidative photosynthetic carbon cycle reaction is [[catalyst|catalyzed]] by [[RuBisCO|RuBP oxygenase]] activity:

:[[Ribulose 1,5-bisphosphate|RuBP]] + {{chem|O|2}} → Phosphoglycolate + [[Glycerate 3-phosphate|3-phosphoglycerate]] + 2 {{chem|H|+}}

During the catalysis by RuBisCO, an 'activated' intermediate is formed (an enediol intermediate) in the RuBisCO active site. This intermediate is able to react with either {{chem|CO|2}} or {{chem|O|2}}. It has been demonstrated that the specific shape of the RuBisCO active site acts to encourage reactions with {{chem|CO|2}}. Although there is a significant "failure" rate (~25% of reactions are oxygenation rather than carboxylation), this represents significant favouring of {{chem|CO|2}}, when the relative abundance of the two gases is taken into account: in the current atmosphere, {{chem|O|2}} is approximately 500 times more abundant, and in solution {{chem|O|2}} is 25 times more abundant than {{chem|CO|2}}.<ref name="Griffiths_2006">{{cite journal | vauthors = Griffiths H | title = Plant biology: designs on Rubisco | journal = Nature | volume = 441 | issue = 7096 | pages = 940–1 | date = June 2006 | pmid = 16791182 | doi = 10.1038/441940a | bibcode = 2006Natur.441..940G | s2cid = 31190084 }}</ref>

The ability of RuBisCO to specify between the two gases is known as its selectivity factor (or Srel), and it varies between species,<ref name="Griffiths_2006"/> with angiosperms more efficient than other plants, but with little variation among the [[vascular plant]]s.<ref name="Ehleringer 1991">{{cite journal | vauthors = Ehleringer JR, Sage RF, Flanagan LB, Pearcy RW | title = Climate change and the evolution of C(4) photosynthesis | journal = Trends in Ecology & Evolution | volume = 6 | issue = 3 | pages = 95–9 | date = March 1991 | pmid = 21232434 | doi = 10.1016/0169-5347(91)90183-x }}</ref>

A suggested explanation of RuBisCO's inability to discriminate completely between {{chem|CO|2}} and {{chem|O|2}} is that it is an evolutionary relic:{{citation needed|date=October 2014}} The early atmosphere in which primitive plants originated contained very little oxygen, the early evolution of [[RuBisCO]] was not influenced by its ability to discriminate between {{chem|O|2}} and {{chem|CO|2}}.<ref name="Ehleringer 1991"/>