Glycolysis and the TCA Cycle: Regulation, Pathways, and Connections

Glycolysis – Cytosolic Pathway

Overview of Glycolysis

Glycolysis is a central metabolic pathway that occurs in the cytosol of cells, converting glucose into pyruvate while generating ATP and NADH. The pathway is divided into two main phases: the energy-requiring phase and the energy-releasing phase.

• Energy-requiring phase (uses 2 ATP): In this phase, glucose is rearranged and phosphorylated, consuming ATP. Key steps include the conversion of glucose to glucose-6-phosphate and then to fructose-1,6-bisphosphate.

• Energy-releasing phase (produces 4 ATP and 2 NADH): This phase generates ATP and NADH by converting triose phosphates into pyruvate.

• Aerobic pathway: Pyruvate is oxidized in the mitochondria to generate acetyl-CoA, entering the TCA cycle.

• Anaerobic pathway: Pyruvate is reduced to lactate, regenerating NAD+ for glycolysis under low oxygen conditions.

Key equation:

$\text{Glucose} + 2\text{NAD}^+ + 2\text{ADP} + 2\text{P}_i \rightarrow 2\text{Pyruvate} + 2\text{NADH} + 2\text{ATP} + 2\text{H}_2\text{O}$

Review of Glucose Uptake

Glucose uptake and phosphorylation are regulated by key enzymes, which determine the rate and direction of glycolysis.

• Enzymes: Glucokinase (liver) and Hexokinase (most tissues)

• Glucokinase vs. Hexokinase:

  • Glucokinase has a high Km for glucose and is found in the liver and pancreas; it is not saturated at physiological glucose concentrations.

  • Hexokinase has a low Km and is found in most tissues; it is rapidly saturated and allows efficient glucose phosphorylation at low concentrations.

  • Both are isozymes—enzymes with similar functions but different amino acid sequences.

Three Regulatory Steps of Glycolysis

1. Conversion of Glucose to Glucose-6-phosphate

   • Uses ATP

   • Enzymes: Glucokinase or Hexokinase

2. Conversion of Fructose-6-phosphate to Fructose-1,6-bisphosphate

   • Uses ATP

   • Enzyme: Phosphofructokinase-1 (PFK1)

   • PFK1 is regulated by phosphorylation and allosteric effectors (see table below).

   • Fructose 2,6-bisphosphate is a feedforward activator of PFK1.

3. Conversion of Phosphoenolpyruvate (PEP) to Pyruvate

   • Enzyme: Pyruvate kinase (PK)

   • Catalyzes the irreversible conversion of PEP to pyruvate, generating ATP.

   • Regulated by allosteric effectors and phosphorylation.

Fate of Pyruvate

• Pyruvate can enter the TCA cycle via the pyruvate dehydrogenase complex.

• It can be reduced to lactate under anaerobic conditions.

• It can be transaminated to alanine.

Summary of Glycolysis Regulation

Metabolic pathway	Major regulatory enzyme(s)	Allosteric effectors	Hormonal effects
Glycolysis (pyruvate oxidation)	Glucokinase (liver), Hexokinase	GKRP, Glucose 6P (+)	Insulin/Glucagon ratio >1 → dephosphorylation
	PFK-1	Fructose 2,6-BP (+), AMP (+), Citrate (–)	Insulin/Glucagon ratio >1 → dephosphorylation of PFK2 and increased production of F 2,6-BP
	Pyruvate kinase	ATP, alanine (–), Fructose 1,6-BP (+)	Insulin/Glucagon ratio >1 → dephosphorylation
Pyruvate dehydrogenase complex	PDC	Acetyl-CoA, NADH, ATP (–)	Insulin/Glucagon ratio >1 → dephosphorylation

Summary of TCA Regulation

Enzyme (Mitochondrial)	Co-factors	Regulated	Other information
Pyruvate dehydrogenase complex	Thiamine, Lipoate, FAD	Acetyl CoA, NADH (–), phosphorylation by pyruvate dehydrogenase kinase	Phosphorylation inactivates the complex; deficiencies can result in lactic acidosis
Citrate synthase		NADH (–)	Inhibited by fluorocitrate
Aconitase			Inhibited by fluoroacetate
Isocitrate dehydrogenase		ADP (+), Ca2+ (+)	Physiologically unidirectional step in the TCA
α-ketoglutarate dehydrogenase	Thiamine, Lipoate	NADH (–), Ca2+ (+)	Physiologically unidirectional step in the TCA; inhibited by arsenic
Succinate thiokinase			Substrate level phosphorylation
Succinate dehydrogenase			Embedded in the mitochondrial membrane; part of Complex II in the ETC
Malate dehydrogenase			Oxaloacetate production depends on NADH/NAD+ ratio

Connections to Other Pathways

Fatty Acid Synthesis

• Citrate is shuttled out of the TCA cycle and cleaved to generate acetyl-CoA and oxaloacetate in the cytosol.

• Acetyl-CoA is used for fatty acid synthesis.

• Oxaloacetate is reduced to malate, then decarboxylated to pyruvate, which can re-enter the TCA cycle.

• This conversion by malic enzyme produces NADPH, essential for biosynthetic reactions.

Anaplerotic Reactions

• Reactions that replenish TCA cycle intermediates.

• Pyruvate carboxylase converts pyruvate to oxaloacetate.

• Transamination reactions transfer amino groups to TCA intermediates (e.g., glutamate to α-ketoglutarate).

• Products can be metabolized to propionyl-CoA.

Shuttle Systems

Glycerophosphate Shuttle

• Transfers reducing equivalents from cytosolic NADH to mitochondrial FAD via glycerol-3-phosphate dehydrogenase.

• FAD is tightly bound to the enzyme in the mitochondria.

Malate-Aspartate Shuttle

• Oxaloacetate does not cross the mitochondrial membrane.

• In the cytosol, oxaloacetate is reduced to malate, which enters the mitochondria.

• Malate is oxidized back to oxaloacetate in the mitochondria.

• Oxaloacetate can undergo transamination with glutamate to form aspartate and α-ketoglutarate.

• Aspartate can move in/out of the mitochondria and be converted back to oxaloacetate.

• Each NADH equivalent yields approximately 2.5 ATP molecules.

Key Terms and Definitions

• Glycolysis: The metabolic pathway that converts glucose into pyruvate, generating ATP and NADH.

• TCA Cycle (Krebs Cycle): A series of reactions in the mitochondria that oxidize acetyl-CoA to CO2, producing NADH, FADH2, and ATP.

• Isozyme: Enzymes with different amino acid sequences but catalyze the same reaction.

• Allosteric Regulation: Regulation of enzyme activity by binding of effectors at sites other than the active site.

• Feedforward Activation: A regulatory mechanism where a metabolite produced earlier in a pathway activates an enzyme downstream.

• Anaplerotic Reaction: A reaction that replenishes intermediates of a metabolic cycle.

Example Applications

• Clinical relevance: Deficiencies in pyruvate dehydrogenase can lead to lactic acidosis.

• Metabolic integration: Glycolysis, TCA cycle, and fatty acid synthesis are interconnected through shared intermediates and regulatory mechanisms.

Additional info: Some details on transamination reactions and shuttle systems were expanded for clarity and completeness.