Question

UDP-glucose is the activated precursor for glycogen synthesis, but ultimately ATP is the power behind glycogen synthesis. Prove it by showing the reactions required to convert glucose 6 -phosphate into a unit of glycogen with the concomitant regeneration of UTP.

Short answer

ATP drives glycogen synthesis by regenerating UTP from UDP.

Step-by-step solution

Conversion of Glucose 6-Phosphate to Glucose 1-Phosphate

The process begins with the conversion of glucose 6-phosphate to glucose 1-phosphate. This is catalyzed by the enzyme phosphoglucomutase. The reaction is as follows: \[ \text{Glucose 6-phosphate} \rightarrow \text{Glucose 1-phosphate} \] This step prepares the glucose molecule for attachment to UDP.

Formation of UDP-Glucose

Glucose 1-phosphate is then combined with UTP to form UDP-glucose and pyrophosphate (PP\(_i\)). This reaction is catalyzed by UDP-glucose pyrophosphorylase:\[ \text{Glucose 1-phosphate} + \text{UTP} \rightarrow \text{UDP-glucose} + \text{PP}_i \] This step generates the activated form of glucose needed for glycogen synthesis.

Synthesis of Glycogen

The UDP-glucose is added to the growing glycogen chain by glycogen synthase, releasing UDP. The reaction is:\[ \text{Glycogen}_{n} + \text{UDP-glucose} \rightarrow \text{Glycogen}_{n+1} + \text{UDP} \]This elongates the glycogen chain by one glucose unit and UDP is released.

Regeneration of UTP from UDP

The UDP released in the previous step is converted back to UTP. This process is driven by ATP, where UDP and ATP are converted to UTP and ADP by nucleoside diphosphate kinase:\[ \text{UDP} + \text{ATP} \rightarrow \text{UTP} + \text{ADP} \]This regeneration of UTP illustrates the role of ATP as the ultimate energy source in glycogen synthesis.

Key concepts

UDP-glucose

UDP-glucose plays a crucial role in the synthesis of glycogen, a stored form of glucose in the body. It acts as an 'activated' glucose molecule, meaning that it's more reactive and ready to be added to the growing glycogen chain. This activation is vital because it allows for the efficient synthesis of glycogen, a necessity for energy storage in cells.
To create UDP-glucose, glucose 1-phosphate and UTP (uridine triphosphate) come together in a reaction catalyzed by the enzyme UDP-glucose pyrophosphorylase.
This process not only forms UDP-glucose but also releases pyrophosphate (PP\(_i\)), illustrating the preparatory stage for glycogen synthesis.
The activated UDP-glucose provides the glucose units that are linked together to grow the glycogen chain, making it an indispensable component in this metabolic process.

ATP

ATP, or adenosine triphosphate, is the energy currency of the cell, providing power for many cellular functions. In glycogen synthesis, ATP plays a pivotal role by regenerating UTP from UDP.
The enzyme nucleoside diphosphate kinase catalyzes this regeneration process, where UDP and ATP are converted to UTP and ADP, respectively.
This transformation is crucial because it allows the cycle of glycogen synthesis to continue, ensuring a consistent supply of 'energy-ready' UTP. Thus, while UDP-glucose is vital for building glycogen, ATP is the underlying energy driving force, highlighting its importance in metabolic pathways.

Glucose 6-phosphate

Glucose 6-phosphate is a central molecule in cellular metabolism and serves as the starting point for various biochemical pathways. In the context of glycogen synthesis, it undergoes a transformation to become glucose 1-phosphate, a necessary precursor for creating UDP-glucose.
This conversion sets the stage for the glucose molecules to be activated and added to the growing glycogen chain.

• This step ensures that glucose can be mobilized for storage efficiently.
• It's an important regulatory point, as the direction of glucose 6-phosphate conversion can influence whether glucose is used for energy or stored.

Therefore, glucose 6-phosphate serves as a key junction in managing the body's energy and storage needs.

Phosphoglucomutase

Phosphoglucomutase is the enzyme responsible for converting glucose 6-phosphate to glucose 1-phosphate. This is a critical step in glycogen synthesis, as it prepares glucose molecules for activation.
Enzymes like phosphoglucomutase are catalysts, meaning they speed up biochemical reactions without being consumed.

• This ensures quick and efficient processing of glucose molecules.
• The conversion is reversible, allowing flexibility in metabolism depending on the cell's needs.

Understanding the role of phosphoglucomutase provides insight into how the body effectively manages its energy resources, ensuring glucose is stored as glycogen when needed.