RuBP Full Form

RuBP (ribulose 1,5-bisphosphate) is an organic compound that plays a role in photosynthesis, most notably as the primary CO2 acceptor in plants. It’s a colourless anion that’s a double phosphate ester of ribulose, a ketone-containing sugar with five carbon atoms. RuBP salts can be separated, but its biological activity is performed in solution. RuBP is found in all domains of life, including Archaea, Bacteria and Eukarya, as well as in plants.

History

Andrew Benson developed RuBP in 1951 while working in Melvin Calvin’s laboratory at UC Berkeley. Calvin, who was not named as a co-author since he was away from the lab at the time of discovery, controversially omitted the whole molecular name from the title of the first report, referring to it just as “ribulose.” The molecule was originally called ribulose diphosphate (RDP or RuDP), but the prefix di- was replaced with bis- to underline the nonadjacency of the two phosphate groups.

Role in photosynthesis and the Calvin-Benson Cycle

The reaction between RuBP and carbon dioxide is catalysed by the enzyme ribulose-1,5-bisphosphate carboxylase-oxygenase (rubisco). 3-keto-2-carboxyarabinitol 1,5-bisphosphate, also known as 2′-carboxy-3-keto-D-arabinitol 1,5-bisphosphate, is a highly unstable six-carbon intermediate (CKABP). This six-carbon -ketoacid intermediate hydrates to produce a gem-diol, which is another six-carbon intermediate. This intermediate is then broken down into two molecules of 3-phosphoglycerate (3-PGA), which is turned to glucose and utilised in a variety of metabolic processes.

RuBP is a result of ATP phosphorylation of ribulose-5-phosphate (generated by glyceraldehyde 3-phosphate) in the Calvin-Benson cycle.

Interactions with Rubisco

RuBP is an enzyme inhibitor for the rubisco enzyme, which controls the net activity of carbon fixation. The ability to activate via carbamylation with CO2 and Mg2+ is hindered when RuBP binds to an active site of rubisco. Rubisco activase works by removing RuBP and other inhibitory bound molecules from the active site, allowing carbamylation to resume.

Role in Photorespiration

Rubisco also catalyses the reaction of RuBP with oxygen (O2) in a process known as photorespiration, which is more common at high temperatures. RuBP interacts with O2 during photorespiration to form 3-PGA and phosphoglycolic acid. The photorespiratory route, like the Calvin-Benson Cycle, has been criticised for its enzymatic inefficiency, albeit this definition of rubisco’s enzymatic kinetics has been disputed. Photorespiration rates in C4 plants are reduced due to increased RuBP carboxylation and lower rubisco oxygenation caused by higher CO2 concentration in the bundle sheath. Photorespiration is also limited in CAM photosynthesis due to kinetic delays in enzyme activation, which are caused by the carbon dioxide/oxygen ratio.

Measurement

The conversion of and RuBP into glyceraldehyde 3-phosphate allows RuBP to be detected isotopically. An enzymatic optical test can then be used to quantify G3P. Given the abundance of RuBP in biological materials, differentiating specific substrate reservoirs, such as RuBP internal to a chloroplast vs external, is much more difficult. Subtractive inference, or measuring a system’s total RuBP, removing a reservoir (e.g., by centrifugation), re-measuring the total RuBP, and utilising the difference to infer the concentration in the given repository, is one way to resolve problem.

RuBisCO

Ribulose-1,5-bisphosphate carboxylase-oxygenase, also known as RuBisCo, rubisco, RuBPCase, or RuBPco, is an enzyme involved in the first major phase of carbon fixation, in which atmospheric carbon dioxide is transformed to energy-rich molecules like glucose by plants and other photosynthetic organisms. It catalyses the carboxylation of ribulose-1,5-bisphosphate in chemical terms (also known as RuBP). It’s the most common enzyme on the planet.

Conclusion

The cornerstone of atmospheric  carbohydrate fixation by the biosphere is ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco). It catalyses the addition of CO2 to enolized ribulose 1,5-bisphosphate (RuBP), resulting in the formation of 3-phosphoglycerate, which is then transformed to sugars. This reaction’s main issue is competitive Oxygen addition, which produces a phosphorylated product (2-phosphoglycolate) that must be recycled through a series of biological events (photorespiratory metabolism). However, the mechanism by which the enzyme activates Oxygen is uncertain.