Rubisco also creates a tiny amount of xylulose-1,5-bisphosphate (XuBP), a powerful inhibitor of Rubisco4, in addition to carboxylating the five-carbon sugar ribulose-1,5-bisphosphate (RuBP)1-3. XuBP and RuBP are both competitive inhibitors of Rubisco that bind at its active site.
XuBP is more efficient at inhibiting Rubisco than RuBP is, but only slightly so2. The inhibition mechanism of XuBP and RuBP is identical; they both block the activity of Rubisco by preventing it from binding substrate molecules. However, XuBP is much less effective at stimulating the release of CO2 from Rubisco than is RuBP3. Thus, although RuBP is a more potent activator of Rubisco than is XuBP, under natural conditions where Rubisco is inhibited by high levels of oxygen, the enzyme will function better if it is activated by low levels of oxygen instead3.
The main reason why XuBP is not used as an inhibitor of Rubisco in nature is because it is very inefficient at blocking the enzyme's activity. When oxygen is present in large amounts, most of the time Rubisco is able to convert some of its RuBP substrates into product molecules, which means that it can still carry out its role in photosynthesis even when XuBP is present4.
Ribulose Bisphosphate Carboxylase Oxygenase is abbreviated as RuBisCO. Ribulose bisphosphate (RuBP), a five-carbon ketose sugar, is used to make it. RuBisCO is the most common enzyme on the planet. As a result, RuBisCO acts as a carboxylase and attaches to plants to carry out more carboxylation. Carboxylation means adding carbon dioxide to organic molecules by using hydrogen ions (hydrogen atoms) from water gas (carbon dioxide and water). This process can form many different compounds such as pyruvate, oxaloacetate, and phosphoenolpyruvate.
As an oxygenase, RuBisCO converts chemical compounds into other chemicals. It does this by combining carbon dioxide with six molecules of ribulose bisphosphate to make one molecule of epoxyribulose phosphate and two molecules of xylulose 5-phosphate. Epoxyribulose phosphate is then converted into glyceraldehyde 3-phosphate and CO2. Xylulose 5-phosphate is converted into dihydroxyacetone phosphate and CO2. There are other enzymes involved in the photosynthesis process but most people are familiar with RuBisCO because it is the most common enzyme involved in this reaction.
You may have heard that plants use carbon dioxide to make carbohydrates. That's true but they also use it as a source of energy during the day when sunlight is available.
Ribulose 1,5-bisphosphate (RuBP) is an organic compound that participates in photosynthesis, most notably as the primary CO2 acceptor in plants. It is a colorless anion that is a double phosphate ester of ribulose, a ketopentose (a keto-containing sugar with five carbon atoms). The phosphate groups are attached to the alpha and beta positions of ribulose respectively.
RuBP is formed from ribulose 5-phosphate by ribulose 1,5-bisphosphate phosphatase. In animals, this reaction produces xylulose 5-phosphate which is used in the biosynthesis of purines and pyrimidines. In plants, it is cleaved to form 3-phosphoglycerate and inorganic phosphate.
RuBP is involved in many important biological processes including photorespiration in plants and mevalonate pathway for sterol biosynthesis. It can also be used as a substrate by enzymes involved in polyketide antibiotic production.
In chemistry, RuBP serves as a starting material for the synthesis of carbohydrates: Ribose 5-phosphate is converted into deoxyribose 5-phosphate by ribose 5-phosphate isomerase, and then deoxyribose 5-phosphate is converted into D-xylulose 5-phosphate by deoxyribose 5-phosphate kinase.