Native photorespiration reaction in plants may have been optimized through evolutionary natural selection pressure

Biology, whether at the gene, genome, protein, cell and tissue level are shaped by evolution through natural selection. In essence, evolution function like an optimisation tool, but the key question is the level at which evolution and natural selection works. For evolution to have an effect, a phenotype must manifest from changes at the genetic level. More importantly, the phenotype in question must endow a fitness advantage or disadvantage. Casting the above to the problem of pathway evolution, natural selection could progressively finetune the enzymes and even structure of a pathway. This work analyses the photorespiration pathway, especially with regards to its level of optimization by evolution. Photorespiration uses oxygen to enable Rubisco to convert ribulose-1,5-bisphosphate to 2-phosphoglycolate and 3-phosphoglycerate. Overall, the pathway is balanced in NADPH and ATP consumption, but only has a theoretical carbon yield of 50% (i.e., 1 acetyl-CoA molecule would be produced for every 2 glycolate molecule consumed). The highlight in this analysis is that photorespiration does not require additional NADPH or ATP. From the kinetic perspective, such a pathway is more favourable despite its carbon loss. This likely comes about due to the energy intensive nature in which a cell needs to regenerate NADPH or ATP where carbon flux flow must be initiated across suitable enzymes to help replenish consumed NADPH and ATP. Carbon loss or low theoretical carbon yield, although not desirable, would thus be tolerable as more energy and carbon flow may be needed to regenerate NADPH or ATP. Hence, penalty from replenishing NADPH or ATP using carbon flux flow may be the anchor on which natural selection pressure acts to finetune photorespiration pathway to its current state over evolutionary timescales. Overall, native pathway may have been optimized by evolution for cellular needs, and observed deficiency such as low theoretical carbon yield or existence of carbon loss should be viewed holistically in light of other considerations such as energy input needed to replenish reducing equivalents or ATP. The above analysis holds clear implications to our current endeavour in developing new improved pathways to augment existing ones via the tools of synthetic biology and metabolic engineering.