Ribulose

Definition

Ribulose is a five-carbon sugar that is an important intermediate in the Calvin cycle of photosynthesis. It serves as a key substrate in the conversion of carbon dioxide into organic compounds within plant cells.

5 Must Know Facts For Your Next Test

1. Ribulose is the primary substrate for the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), which catalyzes the first step of the Calvin cycle.
2. The stereochemistry of ribulose is classified as a D-aldose, meaning it has a hydroxyl group on the right-most carbon when drawn in the Fischer projection.
3. Ribulose can exist in both the linear and cyclic (furanose) forms, with the cyclic form being the predominant structure in biological systems.
4. In addition to its role in photosynthesis, ribulose is also an important intermediate in the pentose phosphate pathway, which generates NADPH and pentose sugars for biosynthesis.
5. The configuration of ribulose, with a ketone group at the second carbon and a terminal aldehyde group, makes it a unique monosaccharide compared to other aldoses and ketoses.

Review Questions

• Explain the role of ribulose in the Calvin cycle of photosynthesis.
  • Ribulose is the primary substrate for the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), which catalyzes the first step of the Calvin cycle. In this step, RuBisCO adds a molecule of carbon dioxide to ribulose-1,5-bisphosphate, producing two molecules of 3-phosphoglycerate. This reaction is the key carbon-fixing step that converts inorganic carbon into organic compounds, allowing plants to produce glucose and other essential biomolecules.
• Describe the stereochemical configuration of ribulose and how it relates to its classification as an aldose.
  • Ribulose is classified as a D-aldose, meaning it has a hydroxyl group on the right-most carbon when drawn in the Fischer projection. This stereochemical configuration is important because it allows ribulose to participate in various metabolic pathways, such as the Calvin cycle and the pentose phosphate pathway, where its specific structure and reactivity are crucial. The presence of the aldehyde group at the terminal carbon, combined with the ketone group at the second carbon, gives ribulose a unique set of chemical properties that distinguish it from other monosaccharides.
• Analyze the significance of ribulose's ability to exist in both linear and cyclic (furanose) forms, and explain how this impacts its biological functions.
  • The ability of ribulose to exist in both linear and cyclic (furanose) forms is an important characteristic that influences its reactivity and biological functions. The cyclic furanose form is the predominant structure in biological systems, as it allows ribulose to participate in enzymatic reactions more efficiently. This cyclic structure also enhances the stability of ribulose and helps protect it from unwanted side reactions. The interconversion between the linear and cyclic forms is a dynamic process that is regulated by various enzymes and environmental factors, ensuring that ribulose is available in the appropriate form to support key metabolic pathways, such as the Calvin cycle and the pentose phosphate pathway. This flexibility in structure is a crucial aspect of ribulose's role as a versatile and important biomolecule in plant and microbial metabolism.

Related terms

Aldose: An aldose is a monosaccharide with an aldehyde group at the end of the carbon chain.

Ketose: A ketose is a monosaccharide with a ketone group at the second carbon atom.

Pentose: A pentose is a monosaccharide with five carbon atoms in the backbone.