ABSTRACT
Improving CO2 fixation Carbon assimilation takes place in all algae, as well as in cyanobacteria, through the metabolic route known as Calvin (or Calvin-Benson-Bassham) cycle. The initial step in this pathway is the fixation of CO2 mediated by ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco). Cyanobacterial and most eukaryotic Rubiscos are composed of eight large and eight small subunits, assembled in a hexadecameric holoenzyme. Sequence comparison has revealed two evolutive branches among the hexadecameric Rubiscos (Tabita et al. 2008). One of them comprises the enzymes of cyanobacteria, green algae, and higher plants, encoding the large subunit in the chloroplast genome and the small subunit in the nucleus. These enzymes are called “green-like” Rubiscos. On the other hand, red and brown algae encode both subunits in the chloroplast, and the corresponding enzymes show a significant divergence (at the sequence level) from the green-like forms, termed “red-like” Rubiscos. In all cases, the catalytic site resides at the contact surface between large subunits. Rubisco catalyzes the carboxylation of ribulose 1,5-bisphosphate (RuBP) to produce two molecules of 3-phosphoglycerate, which are further metabolized through the Calvin cycle. However, Rubisco also promotes the oxygenation of RuBP, thus rendering only one molecule of phosphoglycerate and another of 2-phosphoglycolate. This latter reaction initiates the photorespiratory pathway, which metabolizes 2-phosphoglycolate to recover two thirds of its carbon as 3-phosphoglycerate, while the other third is released as CO2. Therefore, loss of CO2 through photorespiration opposes CO2 fixation through the Calvin cycle, and the net balance between the two outcomes determines the rate of carbon biomass accumulation. Rubisco oxygenase activity is thought to be an unavoidable escape of the catalytic mechanism, while the photorespiratory pathway appears to be just a metabolic solution to avoid the accumulation of 2-phosphoglycolate (which inhibits Calvin cycle enzymes) and to salvage part of its carbon. Despite photorespiration having acquired some secondary functions (such as protection against oxidative stress), the experimental fact is that favouring carbon fixation over photorespiration-by raising the environmental CO2/O2 ratio-substantially increases the growth rate of plants. Therefore, biotechnological suppression or reduction of the oxygenase activity of Rubisco would, in principle, enhance biomass production significantly.
