Given population trends, rising carbon emissions, and the constraints on fossil carbon use, crop yields must increase ≥50% by 2050 to feed the world and replace fossil carbon with green feedstocks and fuels. Around 25% of the carbon fixed by photosynthesis is lost via respiration that powers protein turnover and other maintenance processes. Reducing respiratory carbon loss by increasing the efficiency and working life of enzymes could increase the carbon available to make biomass and so improve yields. Until recently, boosting carbon gain (photosynthesis) was the main strategy for yield improvement because molecular targets could be defined for carbon gain, which they could not for carbon loss. Advances in proteomics have now identified carbon-loss targets, namely fast-turnover (= high respiratory cost) enzymes. The parameter catalytic-cycles-till-replacement (CCR) (= in vivo reaction rate/protein turnover rate) pinpoints enzymes whose working lives are particularly short, making them prime targets for improvement. The thiamin synthesis enzyme THI4 has the lowest possible CCR – one! – because it is a suicide enzyme. So, can we convert this inefficient energy hog into an efficient true catalyst? Yes. Plant THI4s are suicide enzymes because they use an active-site cysteine residue as sulfur donor and are inactivated after a single reaction cycle. In contrast, some THI4s from anaerobic prokaryotes use sulfide as sulfur donor and are efficient catalysts. Screening such THI4s for protein expression and functional complementation identified some that can function to some extent in aerobic conditions like those in plants. However, the low sulfide levels in plants may limit the activity of such THI4s. Directed evolution, e.g., with the OrthoRep or EvolvR systems, could equip these microbial THI4s to function efficiently in aerobic, low-sulfide conditions. Replacing a plant THI4 with an energy-efficient one is predicted to increase energy storage in biomass by 3-4%.
Coauthors: Jorge Garcia Garcia – University of Florida;Andrew Hanson – University of Florida