Template:Redirect-distinguish Template:Chembox Ribulose 1,5-bisphosphate (RuBP) is an organic substance that is involved in photosynthesis, notably as the principal [[carbon fixation|Template:Chem2 acceptor]] in plants.<ref name="ppm">Template:Cite book</ref>Template:Rp It is a colourless anion, a double phosphate ester of the ketopentose (ketone-containing sugar with five carbon atoms) called ribulose. Salts of RuBP can be isolated, but its crucial biological function happens in solution.<ref name="lehninger2000">Template:Cite book</ref> RuBP occurs not only in plants but in all domains of life, including Archaea, Bacteria, and Eukarya.<ref name="tabita1999">Template:Cite journal</ref>
HistoryEdit
RuBP was originally discovered by Andrew Benson in 1951 while working in the lab of Melvin Calvin at UC Berkeley.<ref name="sharkey2018">Template:Cite journal</ref><ref name="benson1951">Template:Cite journal</ref> Calvin, who had been away from the lab at the time of discovery and was not listed as a co-author, controversially removed the full molecule name from the title of the initial paper, identifying it solely as "ribulose".<ref name="sharkey2018" /><ref name="benson2005">Template:Cite book</ref> At the time, the molecule was known as ribulose diphosphate (RDP or RuDP) but the prefix di- was changed to bis- to emphasize the nonadjacency of the two phosphate groups.<ref name="sharkey2018" /><ref name="benson1951" /><ref name="wildman2002">Template:Cite journal</ref>
Role in photosynthesis and the Calvin-Benson CycleEdit
Template:See also The enzyme ribulose-1,5-bisphosphate carboxylase-oxygenase (rubisco) catalyzes the reaction between RuBP and carbon dioxide. The product is the highly unstable six-carbon intermediate known as 3-keto-2-carboxyarabinitol 1,5-bisphosphate, or 2'-carboxy-3-keto-D-arabinitol 1,5-bisphosphate (CKABP).<ref name="lorimer1986">Template:Cite journal</ref> This six-carbon β-ketoacid intermediate hydrates into another six-carbon intermediate in the form of a gem-diol.<ref name="mauser2001">Template:Cite journal</ref> This intermediate then cleaves into two molecules of 3-phosphoglycerate (3-PGA) which is used in a number of metabolic pathways and is converted into glucose.<ref name="kaiser">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="hatch1970">Template:Cite journal</ref>
In the Calvin-Benson cycle, RuBP is a product of the phosphorylation of ribulose-5-phosphate (produced by glyceraldehyde 3-phosphate) by ATP.<ref name="hatch1970" /><ref name="bartee">Template:Cite book</ref>
Interactions with rubiscoEdit
RuBP acts as an enzyme inhibitor for the enzyme rubisco, which regulates the net activity of carbon fixation.<ref name="jordan1983">Template:Cite journal</ref><ref name="spreitzer2002">Template:Cite journal</ref><ref name="taylor1997">Template:Cite journal</ref> When RuBP is bound to an active site of rubisco, the ability to activate via carbamylation with Template:Chem2 and Template:Chem2 is blocked. The functionality of rubisco activase involves removing RuBP and other inhibitory bonded molecules to re-enable carbamylation on the active site.<ref name="ppm" />Template:Rp
Role in photorespirationEdit
Template:See also Rubisco also catalyzes RuBP with oxygen (Template:Chem) in an interaction called photorespiration, a process that is more prevalent at high temperatures.<ref name="baker1996">Template:Cite book</ref><ref name="keys1977">Template:Cite journal</ref> During photorespiration RuBP combines with Template:Chem to become 3-PGA and phosphoglycolic acid.<ref name="sharkey1988">Template:Cite journal</ref><ref name="kebeish2007">Template:Cite journal</ref><ref name="leegood1995">Template:Cite journal</ref> Like the Calvin-Benson Cycle, the photorespiratory pathway has been noted for its enzymatic inefficiency<ref name="kebeish2007" /><ref name="leegood1995" /> although this characterization of the enzymatic kinetics of rubisco has been contested.<ref name="bathellier2018">Template:Cite journal</ref> Due to enhanced RuBP carboxylation and decreased rubisco oxygenation stemming from the increased concentration of Template:Chem2 in the bundle sheath, rates of photorespiration are decreased in [[C4 carbon fixation|Template:Chem2 plants]].<ref name="ppm" />Template:Rp Similarly, photorespiration is limited in CAM photosynthesis due to kinetic delays in enzyme activation, again stemming from the ratio of carbon dioxide to oxygen.<ref name="Niewiadomska2008">Template:Cite book</ref>
MeasurementEdit
RuBP can be measured isotopically via the conversion of Template:Chem2 and RuBP into glyceraldehyde 3-phosphate.<ref name="latzko1972">Template:Cite book</ref> G3P can then be measured using an enzymatic optical assay.<ref name="latzko1972" /><ref name="latzko1969">Template:Cite journal</ref>Template:Efn Given the abundance of RuBP in biological samples, an added difficulty is distinguishing particular reservoirs of the substrate, such as the RuBP internal to a chloroplast vs external. One approach to resolving this is by subtractive inference, or measuring the total RuBP of a system, removing a reservoir (e.g. by centrifugation), re-measuring the total RuBP, and using the difference to infer the concentration in the given repository.<ref name="sicher1979">Template:Cite journal</ref>