ribulose bisphosphate

Examples of ribulose bisphosphate in the following topics:

• Regulation of the Calvin Cycle

  • The carbon dioxide is combined with ribulose 1,5-bisphosphate to form two 3-phosphoglycerate molecules (3-PG).
  • The enzyme that catalyzes this specific reaction is ribulose bisphosphate carboxylase (RuBisCO).
  • RuBisCO is only active during the day as its substrate, ribulose 1,5-bisphosphate, is not generated in the dark.
  • This phase is characterized by the conversion of G3P, which was produced in earlier phase, back to ribulose 1,5-bisphosphate.
  • The following is a brief summary of each enzyme and its role in the regeneration of ribulose 1,5-bisphosphate in the order it appears in this specific phase.

• Study of Photosynthesis

  • C3 carbon fixation is a metabolic pathway that converts carbon dioxide and ribulose bisphosphate into 3-phosphoglycerate.

• Carboxysomes

  • These compartments are thought to concentrate carbon dioxide to overcome the inefficiency of RuBisCo (ribulose bisphosphate carboxylase/oxygenase) - the predominant enzyme in carbon fixation and the rate limiting enzyme in the Calvin cycle.

• The Calvin Cycle

  • In the stroma, in addition to CO2,two other components are present to initiate the light-independent reactions: an enzyme called ribulose bisphosphate carboxylase (RuBisCO) and three molecules of ribulose bisphosphate (RuBP).

• Intermediates Produced During the Calvin Cycle

  • The Calvin cycle can be divided into three major phases which include: Phase 1: carbon fixation; Phase 2: reduction; and Phase 3: regeneration of ribulose .
  • However, additional GAP molecules that are formed will be converted to ribulose-1,5-bisphosphate (RuBP), which is responsible for the conversion of CO2 to 3-PGA in phase 1, via numerous steps.

• The Pentose Phosphate Shunt

  • The first is the oxidative phase in which glucose-6-phosphate is converted to ribulose-5-phosphate.
  • Glucose 6-phosphate + 2 NADP+ + H2O → ribulose-5-phosphate + 2 NADPH + 2 H+ + CO2
  • Depending on the body's state, ribulose-5-phosphate can reversibly isomerize to ribose-5-phosphate.
  • Ribulose-5-phosphate can alternatively undergo a series of isomerizations as well as transaldolations and transketolations that result in the production of other pentose phosphates including fructose-6-phosphate, erythrose-4-phosphate, and glyceraldehyde-3-phosphate (both intermediates in glycolysis).
  • It produces ribulose-5-phosphate, used in the synthesis of nucleotides.

• The Energy-Requiring Steps of Glycolysis

  • A second ATP molecule donates a high-energy phosphate to fructose-6-phosphate, producing fructose-1,6-bisphosphate.
  • The newly-added high-energy phosphates further destabilize fructose-1,6-bisphosphate.
  • The fourth step in glycolysis employs an enzyme, aldolase, to cleave 1,6-bisphosphate into two three-carbon isomers: dihydroxyacetone-phosphate and glyceraldehyde-3-phosphate.

• Control of Catabolic Pathways

  • The enzyme's activity is increased when fructose-1,6-bisphosphate levels increase.
  • (Recall that fructose-1,6-bisphosphate is an intermediate in the first half of glycolysis. ) The regulation of pyruvate kinase involves phosphorylation, resulting in a less-active enzyme.

• Insulin Secretion and Regulation of Glucagon

  • Once bound, the glucagon activates Fructose 2,6-bisphosphate which is an important regulatory molecule in glycolysis/gluconeogenesis.

• Adrenergic Neurons and Receptors

  • The PLC cleaves phosphatidylinositol 4,5-bisphosphate (PIP2), which in turn causes an increase in inositol triphosphate (IP3) and diacylglycerol (DAG).