Oula Ghannoum, University of Western Sydney, Australia
Despite much diversity in life form and biochemical process, all of the photosynthetic pathways focus upon a single enzyme which is by far the most abundant protein on earth, namely ribulose-1,5-bisphosphate carboxylase/oxygenase, or Rubisco (Figure 2.1a). Localised in the stroma of chloroplasts, this enzyme enables the primary catalytic step in photosynthetic carbon reduction (or PCR cycle) in all green plants and algae. Although Rubisco has been highly conserved throughout evolutionary history, this enzyme is surprisingly inefficient with a slow catalytic turnover (Vcmax), a poor specificity for CO2 as opposed to O2 (Sc/o), and a propensity for catalytic misfiring resulting in the production of catalytic inhibitors. This combination severely restricts photosynthetic performance of C3 plants under current ambient conditions of 20% O2 and 0.039% CO2 (390 μL L-1). Furthermore, Rubisco has a requirement for its own activating enzyme, Rubisco activase, which removes inhibitors from the catalytic sites to allow further catalysis. Accordingly, and in response to CO2 limitation, C4, C3-C4 intermediate, CAM and SAM variants have evolved with metabolic concentrating devices which enhance Rubisco performance (Section 2.2).