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.