Glycolysis Gluconeogenesis Reciprocal Regulation

Main Ideas

• Glycolysis and gluconeogenesis require reciprocal regulation. Glycolysis breaks down glucose to release energy, while gluconeogenesis makes glucose from smaller molecules. Running both pathways at once would waste energy in a futile cycle.
• The body favors glycolysis when glucose is abundant and energy is needed, and gluconeogenesis when glucose is scarce. This balance helps maintain blood glucose, which is especially important for the brain. Gluconeogenesis occurs mainly in the liver, with some contribution from the kidneys.
• Regulation occurs at several levels:
  • Hormonal signals and gene expression change how much of an enzyme is made.
  • Allosteric regulation changes enzyme activity when molecules bind at regulatory sites.
  • Phosphorylation and dephosphorylation can switch enzymes between more- and less-active forms.
  • Sequestration can make an enzyme unavailable for use.

Hormones and Enzyme Abundance

• Insulin signals that blood glucose is high. It promotes glucose use and storage, including glycolysis, and reduces gluconeogenesis.
• Glucagon signals low blood glucose. In the liver, it promotes gluconeogenesis while suppressing glycolysis.
• Changing enzyme abundance changes a pathway’s capacity: more enzyme can raise the pathway’s maximum rate, but does not necessarily change the enzyme’s substrate affinity (Km).

Important Regulatory Steps and Signals

Glucose → Glucose 6-Phosphate

• When glucose 6-phosphate builds up, it can inhibit hexokinase, indicating that glucose is entering the pathway faster than it is being used.
• Hexokinase forms differ between tissues:
  • Muscle hexokinase has a high affinity for glucose, helping muscle take up and retain it.
  • The liver’s form has a lower affinity, allowing the liver to help regulate blood glucose rather than trapping all available glucose.
• In liver cells, a regulatory protein can sequester hexokinase in the nucleus. High fructose 6-phosphate favors sequestration, while high glucose favors release, allowing the enzyme to act.
• Insulin increases the amount of hexokinase described in the lecture and decreases production of a gluconeogenic phosphatase.

Fructose 6-Phosphate ↔ Fructose 1,6-Bisphosphate

This is a major control point, regulated by PFK-1 in glycolysis and fructose 1,6-bisphosphatase (FBPase-1) in gluconeogenesis.

• AMP signals low energy. It activates PFK-1 and inhibits FBPase-1, favoring glycolysis.
• ATP signals sufficient energy and inhibits PFK-1, reducing glycolysis. ATP can act both as a reactant at the enzyme’s active site and as an inhibitor at a separate allosteric site.
• Citrate, which can accumulate when energy-producing metabolism is backed up, inhibits PFK-1 and favors the opposing pathway.
• High H⁺ (lower pH, such as during intense muscle activity) inhibits PFK-1. The lecture presents this as a protective brake against excessive acidification.
• Fructose 2,6-bisphosphate (F2,6BP) strongly favors glycolysis: it activates PFK-1 and inhibits FBPase-1. It also increases PFK-1’s apparent affinity for its substrate.

Control of F2,6BP

• F2,6BP is made and broken down by a bifunctional enzyme with two domains: a kinase (PFK-2) and a phosphatase (FBPase-2).
• In the liver, glucagon signaling activates protein kinase A (PKA). PKA phosphorylates the bifunctional enzyme, turning off its kinase domain and turning on its phosphatase domain. F2,6BP falls, favoring gluconeogenesis.
• With insulin or high glucose, the enzyme is dephosphorylated. Its kinase domain becomes active, F2,6BP rises, and glycolysis is favored.
• The lecture also notes that high fructose 6-phosphate can promote dephosphorylation, supporting F2,6BP production.

Phosphoenolpyruvate → Pyruvate

• Fructose 1,6-bisphosphate activates pyruvate kinase. This feed-forward signal helps move intermediates through glycolysis.
• Alanine inhibits pyruvate kinase; its buildup suggests that the cell’s material needs may already be met.
• ATP signals that energy is sufficient and slows glycolysis. ADP signals lower energy and, as described in the lecture, inhibits energy-consuming gluconeogenesis.
• In the liver, glucagon signaling activates PKA, which phosphorylates and inactivates pyruvate kinase. Dephosphorylation restores its activity. The lecture distinguishes this liver regulation from the allosteric regulation also found in muscle.

Sugars That Enter Glycolysis at Different Points

• Galactose can enter as glucose 6-phosphate.
• Some fructose can enter near the PFK step, but much dietary fructose is processed in the liver and enters glycolysis as three-carbon intermediates downstream of PFK, a key regulatory step.
• Because this route bypasses that checkpoint, excess fructose can continue into pyruvate and acetyl-CoA even when the cell does not need more energy. The lecture says excess can be directed toward fatty acids and triglycerides, and links high fructose consumption with fatty liver and type 2 diabetes.

Speakers and Sources

• Speaker: One instructor or narrator from Sparks Chemistry. No additional speakers or named external sources are featured.