Glycolysis Regulation: Covalent vs Allosteric Control, Hormonal and Energetic Regulation

Covalent vs Allosteric Regulation

• Two major forms of enzymatic regulation discussed

  • Covalent regulation: involves chemical modification of the enzyme itself (typically phosphorylation or dephosphorylation). Often reversible, but can be effectively irreversible without specific enzymes. Common players: kinases (e.g., Protein Kinase A, PKA) and phosphatases.

  • Allosteric regulation: regulation by binding of another molecule at a site separate from the active site; depends on metabolite concentrations (energy status) rather than covalent changes.

• Covalent example highlighted:

  • Protein Kinase A (PKA) adds a phosphate to target enzymes (phosphorylation). In glycolysis/glycogen metabolism context, glucagon triggers cAMP, activating PKA, which participates in glycogenolysis (glycogen breakdown).

  • Phosphorylation can activate or inhibit enzymes (e.g., activating glycogen phosphorylase to promote glycogenolysis). Phosphatases remove phosphate groups to reverse this covalent regulation.

• Allosteric example highlighted:

  • Energy status molecules (ATP, ADP, AMP) regulate enzymes by binding and changing activity without covalent modification.

  • High ATP indicates high energy; ATP allosterically inhibits enzymes in glycolysis to slow pathway flux.

  • Low energy (high ADP/AMP) allosterically activates enzymes to generate more ATP.

  • Other allosteric signals include citrate and fructose-2,6-bisphosphate that inform the pathway about current metabolic state.

• Summary viewpoint:

  • Covalent regulation often reflects hormonal signaling (e.g., insulin/glucagon) via phosphorylation/dephosphorylation cycles.

  • Allosteric regulation reflects immediate cellular energy state and metabolite levels, providing rapid, reversible control.

Regulation of Glycolysis: Key Enzymes and Tissue Specificity

• First commitment step enzymes:

  • Hexokinase: traps glucose in most tissues; high affinity for glucose. Phosphorylates glucose to glucose-6-phosphate (G6P).

  • Glucokinase (Hexokinase IV): liver, pancreas, kidneys; lower affinity; higher Km; induced by insulin; not inhibited by its product G6P.

• Liver-specific bypass option:

  • Glucose-6-phosphatase enzyme in liver can remove phosphate from G6P to release free glucose back into the blood, bypassing glucokinase. This allows the liver to export glucose when insulin:glucagon ratios are low (fasting state).

• Tissue differences in handling glucose:

  • Liver can export glucose via G6Pase; other tissues (e.g., muscle, adipose) cannot bypass hexokinase once glucose is phosphorylated and become committed to metabolism or storage.

• Regulatory implications:

  • Glucokinase is induced by insulin; hexokinase is not (insulin has a regulatory effect but hexokinase is constitutively active due to high glucose affinity).

  • Glucokinase is not inhibited by its product; glucokinase activity rises with higher blood glucose and insulin levels.

  • Hexokinase is inhibited by its product, G6P, providing feedback to prevent excessive glucose phosphorylation in tissues that cannot export glucose.

• Kinetics and glucose sensing in the liver:

  • At euglycemia, hexokinase in other tissues is already near maximal activity; glucokinase in liver responds to elevated blood glucose and insulin to continue processing glucose via glycolysis or storage.

  • GLUT-2 transporters on liver have low affinity (high Km) and become effective at higher glucose concentrations (up to around 10 mM), aligning with glucokinase activity and hepatic glucose handling.

• Carbohydrate fate decisions:

  • Once glucose is phosphorylated, the downstream enzymes (PFK-1, PK, PDH) become the regulatory bottlenecks for glycolysis and subsequent energy production.

Allosteric Regulation and Energetics in Glycolysis

• Key rate-limiting enzymes and their regulation:

  • Phosphofructokinase-1 (PFK-1): main control point in glycolysis.

  • Pyruvate kinase (PK): catalyzes a later step, also a control point.

  • Pyruvate dehydrogenase (PDH) complex: links glycolysis to the mitochondria by converting pyruvate to acetyl-CoA.

• Allosteric activators and inhibitors (energetics-driven):

  • PFK-1:

  • Activated by low energy status: AMP or ADP (signaling “make ATP now”).

  • Inhibited by high energy status: ATP.

  • Activated by fructose-2,6-bisphosphate (F-2,6-BP) as a feed-forward signal.

  • Citrate acts as a negative feedback signal indicating sufficient downstream flux through the citric acid cycle.

  • Pyruvate kinase (PK):

  • Activated by fructose-1,6-bisphosphate (F-1,6-BP) as feed-forward activation.

  • Inhibited by high ATP (low energy state reduces PK activity).

  • Insulin promotes PK expression/activity; glucagon inhibits PK.

  • Pyruvate dehydrogenase (PDH) complex:

  • Activated by low energy state (low NADH/NAD+ ratio; low acetyl-CoA signaling a need for more ATP).

  • Inhibited by high energy state (high NADH, high acetyl-CoA).

  • PDH is subject to covalent regulation: phosphorylation inhibits the complex; dephosphorylation activates it.

  • Covalent control example: phosphorylation of PDH inhibits the complex; dephosphorylation (via phosphatases) activates it. Hormonal control: glucagon promotes phosphorylation (inhibition); insulin (and cellular Ca2+) promotes dephosphorylation (activation).

• Metabolite-driven regulation within glycolysis:

  • Fructose-2,6-bisphosphate (F-2,6-BP) is a potent allosteric activator of PFK-1; produced by phosphofructokinase-2 (PFK-2) when insulin signaling is high.

  • PFK-2 is activated (phosphorylated) by insulin signaling, increasing F-2,6-BP; glucagon signaling leads to PFK-2 dephosphorylation and reduced F-2,6-BP.

  • The product of glycolysis, ATP, subsequently inhibits PFK-1 and PK as energy needs are met.

• Energetic state signals and the “traffic jam” concept:

  • A high flux through glycolysis requires coordination among several enzymes; intermediates (e.g., fructose-6-phosphate, fructose-1,6-bisphosphate) provide feed-forward signals to downstream steps (e.g., PK) when upstream steps are active but downstream capacity is limited.

• Transcriptional regulation by hormones:

  • Insulin promotes the expression of PFK-1 and PK, increasing glycolytic capacity when blood glucose is high.

  • Glucagon inhibits these glycolytic enzymes as part of promoting gluconeogenesis and glucose release.

Pyruvate Dehydrogenase Complex: Gatekeeper to Mitochondrial Oxidation

• Role of PDH:

  • Converts pyruvate to acetyl-CoA, linking glycolysis to the citric acid cycle and oxidative phosphorylation.

• Regulation modes:

  • Energetics-based allosteric regulation:

  • Low energy state (low NADH, low ATP) favors PDH activity to generate acetyl-CoA and drive the citric acid cycle.

  • High energy state (high NADH, high ATP) inhibits PDH to prevent excessive carbohydrate oxidation.

  • Pyruvate levels increase can provide feed-forward activation to PDH.

  • Covalent regulation (phosphorylation): PDH kinase phosphorylates PDH to inactivate it; PDH phosphatase dephosphorylates PDH to activate it.

  • Hormonal control: Glucagon promotes phosphorylation (inactivation) of PDH; insulin promotes dephosphorylation (activation).

• Practical implication:

  • When there is ample oxygen and energy demand, PDH is active to feed acetyl-CoA into the citric acid cycle for ATP production.

  • In low oxygen or fasting states, alternate routes (e.g., lactate production via lactate dehydrogenase) help regenerate NAD+ to sustain glycolysis and allow tissues (e.g., RBCs) that lack mitochondria to function.

Lactate Production, Red Blood Cells, and the Cori Cycle

• Anaerobic glycolysis context:

  • If oxygen delivery cannot meet mitochondrial oxidative capacity, pyruvate is reduced to lactate by lactate dehydrogenase (LDH), regenerating NAD+ for continued glycolysis.

  • Reaction: $ext{Pyruvate} + ext{NADH} <br>ightleftharpoons ext{Lactate} + ext{NAD}^+$

• Red blood cells (RBCs) dependence on LDH:

  • RBCs lack mitochondria; they rely on glycolysis and LDH to generate ATP and regenerate NAD+.

  • RBCs export lactate to the blood for processing by liver or other tissues.

• Cori cycle concept:

  • Lactate released by tissues (e.g., RBCs, muscle under anaerobic conditions) is taken up by the liver and converted back to glucose through gluconeogenesis, providing a continuous substrate for energy metabolism during fasting or between meals.

• Physiological relevance:

  • During fasting or high-intensity exercise, lactate becomes an important substrate for maintaining blood glucose and energy homeostasis, with the liver acting as a glucose reservoir.

Oxygen Availability and the Balance Between Glycolysis and Oxidative Metabolism

• Aerobic vs. anaerobic glycolysis context:

  • Glycolysis can proceed under aerobic conditions, but the rate is modulated by the capacity of mitochondria to oxidize pyruvate via PDH and the citric acid cycle.

  • When oxidative capacity is exceeded, glycolysis upregulates, producing more pyruvate, which may be shunted to lactate to regenerate NAD+.

• Practical physiology during exercise:

  • Initial exercise triggers increased ventilation and cardiovascular delivery of oxygen to tissues (oxygen debt).

  • As oxygen delivery matches tissue demand, mitochondria can oxidize more substrates (fatty acids, glucose) and glycolysis can be downregulated in favor of lipid oxidation to spare glucose.

• Summary idea:

  • Glycolysis is regulated both by energy state and by hormonal signals; the decision to oxidize or store glucose vs export glucose is coordinated to meet immediate energy needs while maintaining glucose homeostasis.

Putting It All Together: Metabolic State and Pathway Flow

• Hormonal state drives covalent control:

  • High insulin (fed state, high blood glucose): activate glycolysis pathway through transcriptional upregulation (PFK-1, PK), activation of PFK-2 and production of F-2,6-BP, and activation of PDH to push carbons toward energy production.

  • High glucagon (fasting state, low blood glucose): promote covalent phosphorylation events that inhibit glycolysis (e.g., PDH phosphorylation → inactivation) and promote gluconeogenesis and glycogenolysis through pathways like PKA signaling.

• Energetic state drives allosteric control:

  • Low energy (high AMP/ADP): allosterically activate PFK-1 and PK to accelerate glycolysis and ATP production.

  • High energy (high ATP): allosterically inhibit PFK-1 and PK; citrate accumulation provides feedback to slow glycolysis and possibly push carbon flux toward storage or gluconeogenesis.

• Substrate-level integration:

  • Glucose entry and phosphorylation via hexokinase/glucokinase determine how glucose is committed.

  • G6P fate differs by tissue: in liver, G6P can be dephosphorylated to release glucose; in other tissues, G6P commits glucose to glycolysis or glycogen synthesis.

• Practical takeaway: the glycolytic flux is a balance of covalent and allosteric controls responding to hormonal signals and cellular energy needs, ensuring glucose utilization aligns with organismal energy balance and substrate availability.

Key Equations (for quick reference)

• Glucose phosphorylation (first committed step):
  $ext{Glucose} + ext{ATP} <br>ightarrow ext{Glucose-6-phosphate} + ext{ADP}$

• Glycolysis commitment toward fructose-1,6-bisphosphate: (PFK-1 step)
  $ext{Fructose-6-phosphate} + ext{ATP} <br>ightarrow ext{Fructose-1,6-bisphosphate} + ext{ADP}$

• Pyruvate kinase step (PEP to Pyruvate):
  $ext{Phosphoenolpyruvate} + ext{ADP} <br>ightarrow ext{Pyruvate} + ext{ATP}$

• Pyruvate dehydrogenase (link to mitochondria):
  $ext{Pyruvate} + ext{NAD}^+ + ext{CoA} <br>ightarrow ext{Acetyl-CoA} + ext{NADH} + ext{CO}_2$

• Pyruvate dehydrogenase regulation (phosphorylation):

  • Active PDH: dephosphorylated

  • Inactive PDH: phosphorylated (PDH-P)

• Pyruvate dehydrogenase covalent regulation (hormone-driven):

  • 
    Glucagon promotes PDH phosphorylation (inactivation); insulin promotes dephosphorylation (activation).

• Lactate production under anaerobic conditions (Cori cycle component):
  $ext{Pyruvate} + ext{NADH} <br>ightleftharpoons ext{Lactate} + ext{NAD}^+$

• Fructose-2,6-bisphosphate allosteric activation of PFK-1 (via PFK-2):

  • Insulin increases PFK-2 activity via phosphorylation to generate
    $ext{Fructose-2,6-bisphosphate} <br>ightarrow ext{activates} ext{PFK-1}$

• Glucokinase vs hexokinase difference (conceptual):

  • Glucokinase in liver has Km ≈ 10 mM and is induced by insulin; not inhibited by its product G6P.

  • Hexokinase has high affinity (lower Km) and is inhibited by G6P.

• Glucose-6-phosphatase (liver-specific):
  $ext{Glucose-6-phosphate} <br>ightarrow ext{Glucose} + ext{P}_i$

Covalent vs Allosteric Regulation

• Two major forms of enzymatic regulation discussed:

  • Covalent regulation: involves chemical modification of the enzyme itself (typically phosphorylation or dephosphorylation). Often reversible, but can be effectively irreversible without specific enzymes. Common players: kinases (e.g., Protein Kinase A, PKA) and phosphatases.

    • Thought Process: Think of covalent regulation as flipping a more 'permanent' (though reversible) switch on an enzyme by chemically modifying it, often directed by hormonal signals that affect the whole body.

  • Allosteric regulation: regulation by binding of another molecule at a site separate from the active site; depends on metabolite concentrations (energy status) rather than covalent changes.

    • Thought Process: Think of allosteric regulation as a 'dimmer switch' that rapidly adjusts enzyme activity based on the immediate concentrations of other molecules, reflecting the cell's local energy needs.

• Covalent example highlighted:

  • Protein Kinase A (PKA) adds a phosphate to target enzymes (phosphorylation). In glycolysis/glycogen metabolism context, glucagon triggers cAMP, activating PKA, which participates in glycogenolysis (glycogen breakdown).

  • Phosphorylation can activate or inhibit enzymes (e.g., activating glycogen phosphorylase to promote glycogenolysis). Phosphatases remove phosphate groups to reverse this covalent regulation.

• Allosteric example highlighted:

  • Energy status molecules (ATP, ADP, AMP) regulate enzymes by binding and changing activity without covalent modification.

  • High ATP indicates high energy; ATP allosterically inhibits enzymes in glycolysis to slow pathway flux.

  • Low energy (high ADP/AMP) allosterically activates enzymes to generate more ATP.

  • Other allosteric signals include citrate and fructose-2,6-bisphosphate that inform the pathway about current metabolic state.

• Summary viewpoint:

  • Covalent regulation often reflects hormonal signaling (e.g., insulin/glucagon) via phosphorylation/dephosphorylation cycles.

  • Allosteric regulation reflects immediate cellular energy state and metabolite levels, providing rapid, reversible control.

Regulation of Glycolysis: Key Enzymes and Tissue Specificity

• First commitment step enzymes:

  • Hexokinase: traps glucose in most tissues; high affinity for glucose. Phosphorylates glucose to glucose-6-phosphate (G6P).

    • Thought Process: Hexokinase is like a 'greedy sponge' found everywhere, grabbing glucose fast even when there's not much, and keeping it for the cell's own use.

  • Glucokinase (Hexokinase IV): liver, pancreas, kidneys; lower affinity; higher Km; induced by insulin; not inhibited by its product G6P.

    • Thought Process: Glucokinase is like a 'smart meter' in the liver (and pancreas), only really active when there's an abundance of glucose (like after a meal), allowing the liver to manage large amounts of glucose for the whole body or storage, without being immediately shut off.

• Liver-specific bypass option:

  • Glucose-6-phosphatase enzyme in liver can remove phosphate from G6P to release free glucose back into the blood, bypassing glucokinase. This allows the liver to export glucose when insulin:glucagon ratios are low (fasting state).

• Tissue differences in handling glucose:

  • Liver can export glucose via G6Pase; other tissues (e.g., muscle, adipose) cannot bypass hexokinase once glucose is phosphorylated and become committed to metabolism or storage.

• Regulatory implications:

  • Glucokinase is induced by insulin; hexokinase is not (insulin has a regulatory effect but hexokinase is constitutively active due to high glucose affinity).

  • Glucokinase is not inhibited by its product; glucokinase activity rises with higher blood glucose and insulin levels.

  • Hexokinase is inhibited by its product, G6P, providing feedback to prevent excessive glucose phosphorylation in tissues that cannot export glucose.

• Kinetics and glucose sensing in the liver:

  • At euglycemia, hexokinase in other tissues is already near maximal activity; glucokinase in liver responds to elevated blood glucose and insulin to continue processing glucose via glycolysis or storage.

  • GLUT-2 transporters on liver have low affinity (high Km) and become effective at higher glucose concentrations (up to around $10 \text{ mM}$), aligning with glucokinase activity and hepatic glucose handling.

• Carbohydrate fate decisions:

  • Once glucose is phosphorylated, the downstream enzymes (PFK-1, PK, PDH) become the regulatory bottlenecks for glycolysis and subsequent energy production.

Allosteric Regulation and Energetics in Glycolysis

• Key rate-limiting enzymes and their regulation:

  • Phosphofructokinase-1 (PFK-1): main control point in glycolysis.

  • Pyruvate kinase (PK): catalyzes a later step, also a control point.

  • Pyruvate dehydrogenase (PDH) complex: links glycolysis to the mitochondria by converting pyruvate to acetyl-CoA.

• Allosteric activators and inhibitors (energetics-driven):

  • PFK-1:

    • Simple Pathway - The Glycolysis Gas Pedal: If the cell needs energy, it presses the gas pedal: AMP or ADP (low energy) activate PFK-1. Fructose-2,6-bisphosphate (F-2,6-BP) is a special 'boost' signal. If the cell has enough energy, it lifts off the gas: ATP and Citrate (high energy/ample resources) inhibit PFK-1.

    • Activated by low energy status: AMP or ADP (signaling “make ATP now”).

    • Inhibited by high energy status: ATP.

    • Activated by fructose-2,6-bisphosphate (F-2,6-BP) as a feed-forward signal.

    • Citrate acts as a negative feedback signal indicating sufficient downstream flux through the citric acid cycle.

  • Pyruvate kinase (PK):

    • Simple Pathway - Finishing Line Control: PK speeds up when there's an influx from upstream (F-1,6-BP) but slows down if the overall energy levels are high (ATP inhibits) or if hormones signal to conserve glucose (glucagon inhibits).

    • Activated by fructose-1,6-bisphosphate (F-1,6-BP) as feed-forward activation.

    • Inhibited by high ATP (low energy state reduces PK activity).

    • Insulin promotes PK expression/activity; glucagon inhibits PK.

  • Pyruvate dehydrogenase (PDH) complex:

    • Simple Pathway - Gatekeeper to the Mitochondria: PDH opens the gate when more fuel is needed for the powerhouse (low NADH/NAD+, low acetyl-CoA activate). It closes the gate when enough fuel is already present, signaling to save glucose or direct it elsewhere (high NADH, high acetyl-CoA inhibit).

    • Activated by low energy state (low NADH/NAD+ ratio; low acetyl-CoA signaling a need for more ATP).

    • Inhibited by high energy state (high NADH, high acetyl-CoA).

    • PDH is subject to covalent regulation: phosphorylation inhibits the complex; dephosphorylation activates it.

    • Covalent control example: phosphorylation of PDH inhibits the complex; dephosphorylation (via phosphatases) activates it. Hormonal control: glucagon promotes phosphorylation (inhibition); insulin (and cellular $\text{Ca}^{2+}$) promotes dephosphorylation (activation).

• Metabolite-driven regulation within glycolysis:

  • Fructose-2,6-bisphosphate (F-2,6-BP) is a potent allosteric activator of PFK-1; produced by phosphofructokinase-2 (PFK-2) when insulin signaling is high.

  • PFK-2 is activated (phosphorylated) by insulin signaling, increasing F-2,6-BP; glucagon signaling leads to PFK-2 dephosphorylation and reduced F-2,6-BP.

  • The product of glycolysis, ATP, subsequently inhibits PFK-1 and PK as energy needs are met.

• Energetic state signals and the “traffic jam” concept:

  • A high flux through glycolysis requires coordination among several enzymes; intermediates (e.g., fructose-6-phosphate, fructose-1,6-bisphosphate) provide feed-forward signals to downstream steps (e.g., PK) when upstream steps are active but downstream capacity is limited.

• Transcriptional regulation by hormones:

  • Insulin promotes the expression of PFK-1 and PK, increasing glycolytic capacity when blood glucose is high.

  • Glucagon inhibits these glycolytic enzymes as part of promoting gluconeogenesis and glucose release.

Pyruvate Dehydrogenase Complex: Gatekeeper to Mitochondrial Oxidation

• Role of PDH:

  • Converts pyruvate to acetyl-CoA, linking glycolysis to the citric acid cycle and oxidative phosphorylation.

• Regulation modes:

  • Energetics-based allosteric regulation:

    • Low energy state (low NADH, low ATP) favors PDH activity to generate acetyl-CoA and drive the citric acid cycle.

    • High energy state (high NADH, high ATP) inhibits PDH to prevent excessive carbohydrate oxidation.

    • Pyruvate levels increase can provide feed-forward activation to PDH.

  • Covalent regulation (phosphorylation): PDH kinase phosphorylates PDH to inactivate it; PDH phosphatase dephosphorylates PDH to activate it.

  • Hormonal control: Glucagon promotes phosphorylation (inactivation) of PDH; insulin promotes dephosphorylation (activation).

    • Thought Process: Insulin says 'Burn it!', activating PDH to move glycolysis products into the mitochondria. Glucagon says 'Save it!', inhibiting PDH because we're in a fasting state and need to conserve glucose or make new glucose.

• Practical implication:

  • When there is ample oxygen and energy demand, PDH is active to feed acetyl-CoA into the citric acid cycle for ATP production.

  • In low oxygen or fasting states, alternate routes (e.g., lactate production via lactate dehydrogenase) help regenerate NAD+ to sustain glycolysis and allow tissues (e.g., RBCs) that lack mitochondria to function.

Lactate Production, Red Blood Cells, and the Cori Cycle

• Anaerobic glycolysis context:

  • If oxygen delivery cannot meet mitochondrial oxidative capacity, pyruvate is reduced to lactate by lactate dehydrogenase (LDH), regenerating NAD+ for continued glycolysis.

  • Reaction: $\text{Pyruvate} + \text{NADH} \rightleftharpoons \text{Lactate} + \text{NAD}^+$

• Red blood cells (RBCs) dependence on LDH:

  • RBCs lack mitochondria; they rely on glycolysis and LDH to generate ATP and regenerate NAD+.

  • RBCs export lactate to the blood for processing by liver or other tissues.

• Cori cycle concept:

  • Lactate released by tissues (e.g., RBCs, muscle under anaerobic conditions) is taken up by the liver and converted back to glucose through gluconeogenesis, providing a continuous substrate for energy metabolism during fasting or between meals.

• Physiological relevance:

  • During fasting or high-intensity exercise, lactate becomes an important substrate for maintaining blood glucose and energy homeostasis, with the liver acting as a glucose reservoir.

Oxygen Availability and the Balance Between Glycolysis and Oxidative Metabolism

• Aerobic vs. anaerobic glycolysis context:

  • Glycolysis can proceed under aerobic conditions, but the rate is modulated by the capacity of mitochondria to oxidize pyruvate via PDH and the citric acid cycle.

  • When oxidative capacity is exceeded, glycolysis upregulates, producing more pyruvate, which may be shunted to lactate to regenerate NAD+.

• Practical physiology during exercise:

  • Initial exercise triggers increased ventilation and cardiovascular delivery of oxygen to tissues (oxygen debt).

  • As oxygen delivery matches tissue demand, mitochondria can oxidize more substrates (fatty acids, glucose) and glycolysis can be downregulated in favor of lipid oxidation to spare glucose.

• Summary idea:

  • Glycolysis is regulated both by energy state and by hormonal signals; the decision to oxidize or store glucose vs export glucose is coordinated to meet immediate energy needs while maintaining glucose homeostasis.

Putting It All Together: Metabolic State and Pathway Flow

• Hormonal state drives covalent control:

  • High insulin (fed state, high blood glucose): activates glycolysis pathway through transcriptional upregulation (PFK-1, PK), activation of PFK-2 and production of F-2,6-BP, and activation of PDH to push carbons toward energy production.

    • Thought process: Insulin's overall message is "Glucose is abundant! Store it, use it for energy, build things." So, it turns ON pathways that consume glucose and turns OFF pathways that release glucose.

  • High glucagon (fasting state, low blood glucose): promotes covalent phosphorylation events that inhibit glycolysis (e.g., PDH phosphorylation → inactivation) and promote gluconeogenesis and glycogenolysis through pathways like PKA signaling.

    • Thought process: Glucagon's overall message is "Glucose is scarce! Release it, make new glucose, spare what we have." So, it turns OFF glucose consumption (glycolysis) and turns ON glucose production/release.

• Energetic state drives allosteric control:

  • Low energy (high AMP/ADP): allosterically activate PFK-1 and PK to accelerate glycolysis and ATP production.

    • Simple Pathway: "Low Battery" Signal: If AMP/ADP (low energy signals) are high, the cell says, "Emergency! Make ATP NOW!" and allosterically activates key enzymes of glycolysis directly and rapidly.

  • High energy (high ATP): allosterically inhibit PFK-1 and PK; citrate accumulation provides feedback to slow glycolysis and possibly push carbon flux toward storage or gluconeogenesis.

    • Simple Pathway: "Full Battery" Signal: If ATP (high energy signal) is high, the cell says, "Battery full, slow down energy production" and allosterically inhibits glycolysis.

• Substrate-level integration:

  • Glucose entry and phosphorylation via hexokinase/glucokinase determine how glucose is committed.

  • G6P fate differs by tissue: in liver, G6P can be dephosphorylated to release glucose; in other tissues, G6P commits glucose to glycolysis or glycogen synthesis.

• Practical takeaway: the glycolytic flux is a balance of covalent and allosteric controls responding to hormonal signals and cellular energy needs, ensuring glucose utilization aligns with organismal energy balance and substrate availability.

Key Equations (for quick reference)

• Glucose phosphorylation (first committed step):
  \n $\text{Glucose} + \text{ATP} \rightarrow \text{Glucose-6-phosphate} + \text{ADP}$

• Glycolysis commitment toward fructose-1,6-bisphosphate: (PFK-1 step)
  \n $\text{Fructose-6-phosphate} + \text{ATP} \rightarrow \text{Fructose-1,6-bisphosphate} + \text{ADP}$

• Pyruvate kinase step (PEP to Pyruvate):
  \n $\text{Phosphoenolpyruvate} + \text{ADP} \rightarrow \text{Pyruvate} + \text{ATP}$

• Pyruvate dehydrogenase (link to mitochondria):
  \n $\text{Pyruvate} + \text{NAD}^+ + \text{CoA} \rightarrow \text{Acetyl-CoA} + \text{NADH} + \text{CO}_2$

• Pyruvate dehydrogenase regulation (phosphorylation):

  • Active PDH: dephosphorylated

  • Inactive PDH: phosphorylated (PDH-P)

• Pyruvate dehydrogenase covalent regulation (hormone-driven):

  • \n Glucagon promotes PDH phosphorylation (inactivation); insulin promotes dephosphorylation (activation).

• Lactate production under anaerobic conditions (Cori cycle component):
  \n $\text{Pyruvate} + \text{NADH} \rightleftharpoons \text{Lactate} + \text{NAD}^+$

• Fructose-2,6-bisphosphate allosteric activation of PFK-1 (via PFK-2):

  • Insulin increases PFK-2 activity via phosphorylation to generate \n $\text{Fructose-2,6-bisphosphate} \rightarrow \text{activates} \text{PFK-1}$

• Glucokinase vs hexokinase difference (conceptual):

  • Glucokinase in liver has Km $\approx 10 \text{ mM}$ and is induced by insulin; not inhibited by its product G6P.

  • Hexokinase has high affinity (lower Km) and is inhibited by G6P.

• Glucose-6-phosphatase (liver-specific):
  \n $\text{Glucose-6-phosphate} \rightarrow \text{Glucose} + \text{P}_i$