The Full Form of RUBP is Ribulose 1,5-Bisphosphate.

Ribulose 1,5-bisphosphate is an organic substance that is involved in photosynthesis. It is a colourless anion, a double phosphate ester of the ketopentose called ribulose. Salts of RuBP can be isolated, but its crucial biological function happens in solution

Despite its enormous evolutionary success (it is the carboxylating enzyme of all photosynthetic pathways from microorganisms to higher plants), Rubisco is rather inefficient due to wasteful competitive inhibition by molecular oxygen. Quite critically, the intimate mechanism of O₂ addition is unknown. We show here that isotope effects (¹³C, ¹⁸O, ²⁵Mg, or ²H) and high level computations of redox potential are all consistent with oxygen acting as an oxidant in a redox reaction generating superoxide which then attacks the substrate. Our results explain why the elimination of oxygenation by enzymatic bioengineering is so difficult, because it would require a drastic change in electrostatic and/or redox potential of the substrate, and this would alter carboxylation activity.

Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) is the cornerstone of atmospheric CO₂ fixation by the biosphere. It catalyzes the addition of CO₂ onto enolized ribulose 1,5-bisphosphate (RuBP), producing 3-phosphoglycerate which is then converted to sugars. The major problem of this reaction is competitive O₂ addition, which forms a phosphorylated product (2-phosphoglycolate) that must be recycled by a series of biochemical reactions (photorespiratory metabolism). However, the way the enzyme activates O₂ is still unknown. Here, we used isotope effects (with ²H, ²⁵Mg, and ¹⁸O) to monitor O₂ activation and assess the influence of outer sphere atoms, in two Rubisco forms of contrasted O₂/CO₂ selectivity. Neither the Rubisco form nor the use of solvent D₂O and deuterated RuBP changed the ¹⁶O/¹⁸O isotope effect of O₂ addition, in clear contrast with the ¹²C/¹³C isotope effect of CO₂ addition. Furthermore, substitution of light magnesium (²⁴Mg) by heavy, nuclear magnetic ²⁵Mg had no effect on O₂ addition. Therefore, outer sphere protons have no influence on the reaction and direct radical chemistry (intersystem crossing with triplet O₂) does not seem to be involved in O₂ activation. Computations indicate that the reduction potential of enolized RuBP (near 0.49 V) is compatible with superoxide (O₂^•−) production, must be insensitive to deuteration, and yields a predicted ¹⁶O/¹⁸O isotope effect and energy barrier close to observed values. Overall, O₂ undergoes single electron transfer to form short-lived superoxide, which then recombines to form a peroxide intermediate.