By Temea Turjaka - Biochemistry Student @ Christ Church College, Oxford
Bioengineering of RuBisCO to improve catalysis in photosynthesis
Climate change is a major threat to global food security. For example, projected increases in temperatures, changes in precipitation patterns, extreme weather events, and reductions in water availability may all result in reduced agricultural productivity. Photosynthetic efficiency is being manipulated to sustainably increase the yield of crops.
RuBisCO is the enzyme that catalyses the carbon-fixing reaction in the Calvin Cycle in photosynthesis; it compromises around 50% of chloroplasts, making it the most abundant protein in nature. However, it is an inefficient enzyme as it is needed in large amounts for a single turn of the Calvin Cycle, producing one molecule of glucose. Synthetic biology can be used to overcome RuBisCO’s catalytic imperfections.
There are several existing ideas for synthetic approaches to overcome the evolutionary constraints of the enzyme. The main inefficiency of RuBisCO stems from its bifunctional activity whereby CO2 and O2 compete at the active site. The fixation of O2 leads to the wasteful process of photorespiration that consumes up to 30% of the total ATP (adenosine triphosphate) requirement for CO2 fixation and photorespiration, significantly decreasing crop productivity. A current option that is available and being developed is the direct manipulation of RuBisCO by chloroplast transformation. Chloroplast transformation works by using catalytic screening to identify superior forms of RuBisCO that outperform crop counterparts. Catalytic diversity studies have already made considerable progress in RuBisCO bioengineering by identifying many RuuBisCO forms that are candidates for transplantation into chloroplasts. Another method focuses on creating a ‘neo-RubisCO’, an enzyme that catalyses the same chemistry as RubisCO but in a different protein scaffold that would allow it to discriminate better against O2.
Only the future will tell whether carbon fixation through an alternatively evolved RubisCO or a neo-RubisCO will allow us to rewrite or even overwrite the chapter of RubisCO's evolution.
How Bt crops can improve modern agriculture
Genetic engineering can be used in modern agriculture to create natural pesticides. Bt Crops are named after Bacillus thuringiensis, a bacteria that naturally produces a crystal protein that is toxic to many pests and insects. Bt crops are plants genetically modified to contain the bacterial endospore (or crystal) Bt toxin to be resistant to certain insect pests. Natural pesticides have the advantage of allowing more crops to be grown on less land, without the detrimental effects chemical pesticides have on the ecosystem.
The bacteria Bt produces a crystal protein toxic to many pests. These Cry proteins are highly specific in their mode of action within insect larvae. When the Cry protein reaches the gut of the insect, it is partially degraded, releasing a toxin. This toxin is activated upon binding to the specific receptor on the epithelial cells lining the gut of a larval insect. Once bound, the toxin recruits other Cry toxins to the same cell and together they form holes in the cell membrane, ultimately causing the cell to burst as water enters the punctures. The cumulative effect of perforations being made in the insect’s cells causes the larva to be killed, thus preventing further insect reproduction as the life cycle is stopped in its middle stage.
The targeted mechanism of the Bt Cry toxin makes it an excellent pesticide since it has been shown to be safe for human consumption and improves crop yield. This could potentially contribute to solving world hunger if it is more widely implemented in modern agriculture.
Further reading:
Research paper on ‘Engineering chloroplasts to improve catalysis of RubisCO’ available at: https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/nph.14351
Article on ‘Insecticidal Plants: The Tech and Safety of GM Bt Crops’ available at: http://sitn.hms.harvard.edu/flash/2015/insecticidal-plants/
Scientific journal on ‘A short history of RubisCO: the rise and fall’ available at: https://www.sciencedirect.com/science/article/pii/S095816691730099X