Glucose Regulation

Feedback Inhibition vs Feed-Forward Activation

• Downstream product suppresses an upstream enzyme

• Prevents over-accumulation of intermediates

•  reduce overall product output by decreasing enzyme activity

• Metabolites promote pathway activity

• Primes the pathway it response

• increase product output by enhancing enzyme activity

Metabolic gatekeepers: Regulation happens at committed steps

• Regulatory steps in glycolysis/gluconeogenesis is at the commitment steps

  • Cell makes an energetic investment that commits intermediates to a pathway

  • Downstream intermediates can't signal buildup at these point, modulating Q isn't possible

 

Glycolysis

• PFK1 and PK are major control points

Gluconeogenesis

• Fructose-1,6-bisphosphatase and pyruvate carboxylase/PEP carboxykinase are critical checkpoints

Cell prevents futile cycles, maintains energy efficiency and ensures metabolic flu aligns with needs

Futile cycle

• Metabolism when 2 opposing pathways run simultaneously so a metabolite is converted back and for the without net productive outcome, consumes energy in the process

• Avoided by irreversible commitment steps that are independently regulated to ensure pathways are never fully active simultaneously

Phosphofuctokinase-1, key regulation point in glycolysis

• Hexose phosphate pool has many uses, PFK-1 is unique to glycolysis = commitment step

• Regulation of PFK-1 has a selective effect on glycolysis

  • ATP and citrate inhibit (lots of ATP means the body doesn't need production of energy), signals success in catabolism

  • ADP and AMP activates, signals need for energy/ATP

  • Feedforward activation

    • Glycolysis/TCA will generate ATP and remove AMP and ADP by recharging them back into ATP and inhibit PFK-1

• ATP/ADP bind at a distinct allosteric site to modulate PFK-1 activity (ATP inhibit ADP activates)

• Feedback inhibition of PFK-1 supports homeostasis

  • Self-limiting, product X inhibits it's own pathway to limit accumulation, encourages homeostatis, stable physiological state

End product regulator

Final metabolite shuts off its own synthesis

Allosteric modulation

Product binds to an allosteric site, changes enzyme conformation

Reversible

Inhibition is lifted when product levels fall, turning flux to cell needs

Biological importance

• Conserves energy and resources, keeps metabolite concentration within optimal ranges

• Prevent futile cycles or harmful buildup

Frc1,6, bisphosphatase-1 is a commitment step in gluconeogenesis

• Hexose phosphate pool increase, F6P is processed by PFK-2 to form Fructose 2,6-bisphosphate

  • Signal hexose phosphate pool build up is too high and need to catabolize pool than build up

  • Feedback inhibitor of FBPase-1

 

F2,6BP is a reciprocal regulation of glycolysis

• Made during gluconeogenesis to inhibit that pathway by inhibiting FBPase-1 and activates PFK-1

  • Inhibiting F6P production and activate removing F6P

• Loss of F2,6BP stimulates refilling hexose phosphate pool

PFK-1 binds substrate F6P with positive cooperativity

• Activity vs [S] graph is sigmoidal, indicates cooperativity, substrate binding to one active site influences binding at another site

• PFK-1 is a tetramer that binds for F6P cooperatively, shift equilibrium toward high activity R site than low activity T site

• EC50 substrate concentration to produce 1/2 max enzyme activity. Slope of rate vs [S] is steepest near EC50

  • Ideal attribute for regulation

• [ATP] is low, EC50 decreases

  • ATP stabilizes the T state

Heterotopic modulation

• ATP binding at site influences F6P binding

Homotropic modulation

• F6P binds at one activate site and cooperatively promotes F6P binding at another site

Sites within PFK-1

Active site binding for ATP is near the F6P site to perform phosphoryl transfer

F6P is at a subunit interface to enable cooperatively in favor of R state

Allosteric site for ATP is at subunit interface to influence R vs T balance. ADP and AMP bind to same site but favors R state

Metabolic Regulation of PC and PEPCK in gluconeogenesis

• Key control points in the 3C bypass

• PC is activated by acetyl-CoA

  • TCA cycle has sufficient intermediates and promotes conversion of pyruvate to OAA

• Both PC and PEPCK are inhibited by high ADP levels

  • Signals energy insufficiency, suppresses gluconeogenesis and favors glycolysis

 Metabolic and hormonal regulation of tissue-specific pyruvate kinase isozymes

• Pyruvate kinase isozymes (2 enzymes same function different structures) different in tissue distribution and responsiveness

  • In the liver, glucagon activates PKA and phosphorylates pyruvate kinase, inactivates and prevents the liver from using glucose as fuel

• Inhibition

  • ATP, acetyl-CoA and long-chain fatty acids are allosteric inhibitors of pyruvate kinase, signaling energy abundance and suppresses glycolysis

  • Pyruvate turns to alanine by transamination inhibits pyruvate kinase to prevent excess pyruvate accumulation

• Activation

  • F1,6BP activates pyruvate kinase by feed forward activation

Pancreatic endocrine cells

• Alpha cells secrete glucagon to raise blood glucose

• Beta cells secrete insulin to lower blood glucose

  • Secrete serotonin to amplify insulin's response

Epinephrine

• Activate gluconeogenesis for Cori cycle and inhibit glycolysis in the liver

• Direct muscle and liver to inhibit glycogen synthesis and break down of glycogen (glycogenolysis)

• Causes muscle to undergo glycolysis to produce ATP

• GET ATP INTO MUSCLE FOR RAPID ENERGY EXPENDITURE

• Glucose generation and export in liver, muscle glycolysis

Insulin

• Induces glucose clearance in time of plenty

  • After meal [Glc] increase and sensed by beta cell which secrete insulin

  • Insulin is sensed by multiple tissue which responds by removing Glc from blood

• Glc clearance

  • Into muscle, liver and other tissues

• Beta-cells sense lower [Glucose] and ceases insulin secretion

Feed-forward activation

• High [Glc] activates insulin secretion to cause Glc clearance

  • Promote enzyme acitivty

• Low [Glc] activates glucagon secretion to reduce Glc clearance

  • Promote enzyme activity

Glucagon, time of need in 2 ways, high energy demand signaled by epinephrine or low glucose availability by glucagon 

• Low [Glc] induces alpha cells to secrete glucagon and direct liver to export Glc

• Glucagon triggers reciprocal regulation of opposing pathways to boost blood glucose levels

  • Glycogenesis inhibited, glycogenolysis activated

  • Glycolysis inhibited, gluconeogenesis is activated

    • Raise blood glucose levels instead of creating ATP

• Without epinephrine, muscle release little lactate, gluconeogenesis still proceeds using precursors like amino acids

• Regulated by feedback inhibition

  • Controls the production of X

    • Increasing glucose inhibits pathway to stop producing glucose

    • Decreased glucose enables the production pathway

  • High blood glucose inhibits glucagon and suppress gluconeogeneiss

    • Decrease enzyme activity

  • Low blood glucose stimulate glucagon release, activating gluconeogenesis to restore glucose levels

    • Decrease enzyme activity