Question

What are four possible metabolic fates of glucose- 6 phosphate?

Short answer

Glycolysis, Pentose Phosphate Pathway, Glycogenesis, Dephosphorylation and Gluconeogenesis.

Step-by-step solution

Identify Pathway 1: Glycolysis

One possible fate of glucose-6-phosphate (G6P) is to enter glycolysis. In glycolysis, G6P is converted to fructose-6-phosphate, and through a series of steps, ultimately leads to the production of pyruvate, ATP, and NADH.

Identify Pathway 2: Pentose Phosphate Pathway

Another fate of G6P is to enter the pentose phosphate pathway. In this pathway, G6P is converted to ribulose-5-phosphate and produces NADPH and ribose-5-phosphate, which are crucial for biosynthesis and antioxidant defenses.

Identify Pathway 3: Glycogenesis

G6P can also be funneled into glycogenesis. In this anabolic pathway, G6P is converted to glucose-1-phosphate and then to UDP-glucose, which is used to synthesize glycogen for storage in the liver and muscle.

Identify Pathway 4: Dephosphorylation and Gluconeogenesis

A less direct fate of G6P is its dephosphorylation by glucose-6-phosphatase in the liver, generating free glucose that can be released into the bloodstream. Additionally, G6P can enter gluconeogenesis to generate glucose from non-carbohydrate substrates.

Key concepts

Glycolysis

Glycolysis is one of the primary metabolic pathways of glucose-6-phosphate (G6P). In this process, G6P is converted into fructose-6-phosphate, which then undergoes a series of reactions to produce pyruvate. This process generates ATP, the energy currency of the cell, and NADH, a crucial electron carrier. Glycolysis occurs in the cytoplasm of cells and does not require oxygen, making it an anaerobic process. Key points to remember include:

• First step: G6P is converted to fructose-6-phosphate
• Final products: pyruvate, ATP, and NADH
• Location: Cytoplasm
• Does not require oxygen (anaerobic)

This pathway is essential for providing quick energy to the cell, especially when oxygen levels are low.

Pentose Phosphate Pathway

The pentose phosphate pathway is another critical fate of G6P. Unlike glycolysis, this pathway does not focus on energy production; instead, it generates NADPH and ribose-5-phosphate. NADPH is essential for anabolic reactions, such as fatty acid synthesis and maintaining antioxidant defenses, while ribose-5-phosphate is a precursor for nucleotide synthesis. Some important details include:

• Initial conversion: G6P to ribulose-5-phosphate
• Primary products: NADPH and ribose-5-phosphate
• Key roles: Biosynthesis and antioxidant defense

This pathway operates in the cytoplasm and is crucial for cellular growth and repair.

Glycogenesis

When the body needs to store excess glucose, G6P can enter glycogenesis. In this anabolic pathway, G6P is first converted to glucose-1-phosphate, which is then converted to UDP-glucose. UDP-glucose is the building block for glycogen, which is stored in the liver and muscles for later use. Key facts about glycogenesis include:

• Initial step: G6P to glucose-1-phosphate
• Intermediate: UDP-glucose
• Outcome: Glycogen storage
• Storage sites: Liver and muscle tissues

This pathway ensures that the body has a reserve of glucose available for energy during periods of fasting or increased physical activity.

Gluconeogenesis

Gluconeogenesis involves the formation of glucose from non-carbohydrate substrates. G6P can be dephosphorylated by glucose-6-phosphatase to produce free glucose, which can be released into the bloodstream. Alternatively, G6P can enter gluconeogenesis, especially in the liver, to generate glucose from precursors such as lactate, glycerol, and amino acids. Important aspects of gluconeogenesis are:

• Dephosphorylation: G6P to free glucose
• Sources: Non-carbohydrate substrates (lactate, glycerol, amino acids)
• Primary location: Liver

This pathway is vital during fasting, providing glucose to maintain blood sugar levels and supply energy to the brain and other tissues.