Gluconeogenesis and Glucose Homeostasis Notes

Gluconeogenesis and Glucose Homeostasis

Gluconeogenesis

• Synthesis of glucose from noncarbohydrate precursors.
• Examples of precursors:
  • Lactate
  • Amino acids (e.g., alanine)
  • Glycerol (from triacylglycerols)

Learning Objectives

• Define gluconeogenesis and identify organs and physiological states where the pathway is active.
• Explain the reciprocal regulation between glycolysis and gluconeogenesis.
• Explain metabolic homeostasis.
• Evaluate the roles of hormones like insulin and glucagon in controlling metabolism.

Glucose Requirements

• Many body tissues (e.g., RBCs, brain, kidney medulla, exercising skeletal muscle) require or strongly prefer glucose.
• Daily glucose requirement: $160g$
• Glycogen stores: $190g$
• Glycogen stores last approximately one day.

When is Gluconeogenesis Active?

• Exercise (from lactate)
• Short-term fasting (from alanine)
• Diabetes (insulin insensitivity)
• Trauma (peripheral insulin resistance)
• Location:
  • (Mostly) in the cytosol of cells in the liver, kidney, and small intestine

Glucose Metabolism Reminder

• Glycolysis occurs in the cytosol.
• Oxidation of pyruvate to acetyl CoA occurs in the mitochondrion.
• TCA cycle also occurs in the mitochondrion.
• Key irreversible steps in glycolysis:
  • Hexokinase
  • Phosphofructokinase (PFK)
  • Pyruvate kinase
• Gluconeogenesis bypasses these irreversible steps with different enzymes.

Gluconeogenesis Pathway

• Reversal of glycolysis, except for 3 steps, which:
  • Require different enzymes.
  • Require direct energy input.
  • Pyruvate to PEP
    • Fructose-1,6-bisphosphate to fructose-6-phosphate.
    • Glucose-6-phosphate to glucose.

Gluconeogenesis: First Half

• Reversing the single-step reaction from PEP to pyruvate in glycolysis requires two steps in gluconeogenesis.
• This requires energy input in the form of ATP and GTP.
• Pyruvate to PEP:
  • The intermediate is oxaloacetate, which is also the final step in the TCA cycle.

Pyruvate to Oxaloacetate

• Enzyme: Pyruvate carboxylase (mitochondrial)
• Reaction: $pyruvate + CO_2 + ATP \rightarrow oxaloacetate + ADP + Pi$

Oxaloacetate to PEP

• Enzyme: PEP carboxykinase (cytosol)
• Reaction: $oxaloacetate + GTP \rightarrow PEP + GDP + CO_2$

Interconnected Metabolic Pathways

• TCA cycle is amphibolic, involved in both catabolic and anabolic processes.
• Energy production and biosynthesis.
• TCA cycle components used for synthesis must be replaced.
• Pyruvate carboxylase contributes by converting pyruvate into oxaloacetate (OAA).

Glyceraldehyde 3-phosphate to Glucose

• Two further reactions are catalyzed by gluconeogenesis-specific enzymes.
• Glycerol from lipid breakdown enters via DHAP.
  • Fructose 1,6-bisphosphatase
  • Glucose 6-phosphatase

Stoichiometry of Gluconeogenesis

• $2 pyruvate + 4 ATP + 2 GTP + 2 NADH + 6 H_2O \rightarrow Glucose + 4 ADP + 2 GDP + 6 Pi + 2 NAD^+ + 2 H^+$
• Occurs when glucose levels are low and ATP levels are high.

Substrates for Gluconeogenesis

• Lactate and some amino acids (e.g., alanine) enter via pyruvate.
• Other amino acids can enter via oxaloacetate (glucogenic amino acids).
• Glycerol from triacylglyceride breakdown enters gluconeogenesis through DHAP.

Fatty Acids & Gluconeogenesis

• Free fatty acids (FFAs) CANNOT form glucose.
• FFAs are broken down to form acetyl CoA.
• Humans cannot convert acetyl CoA into pyruvate.
• Entry into the TCA cycle leads to the production of $CO_2$, not more oxaloacetate.

Ethanol Inhibits Gluconeogenesis

• Metabolism (oxidation) of ethanol generates very high levels of NADH.
• Excess NADH favors alternative reactions to regenerate $NAD^+$ (e.g., lactate & malate).

Regulation of Gluconeogenesis

• Gluconeogenesis and glycolysis must not be active simultaneously to avoid futile cycles.
• Pathways are reciprocally regulated.
• Allosteric regulation plays a key role.

Hormonal Regulation: Enzyme Levels

• Insulin: Promotes synthesis of key glycolysis enzymes (PFK, PK & PFK2) and inhibits synthesis of PEP carboxykinase.
• Glucagon: Increases synthesis of PEP carboxykinase & fructose 1,6 bis-phosphatase.

Gluconeogenesis: Summary

• Gluconeogenesis is the formation of glucose from non-carbohydrate precursors.
• Differs at three points from glycolysis.
• Main role is maintaining blood glucose levels.
• Occurs in the liver, kidneys, and small intestine.
• Substrates include alanine, glycerol, and lactate.
• Reciprocally regulated with glycolysis via allosteric and hormonal mechanisms.

Metabolic Homeostasis

• Humans must meet several metabolic requirements:
  • Molecules not available in the diet
  • Adaptation to changing external conditions
  • Protection from toxins
• Four basic types of metabolic pathways:
  • Oxidative pathways (energy generation)
  • Fuel storage & mobilization
  • Biosynthetic pathways
  • Detoxification/waste-disposal pathways

Glucose Metabolism Overview

• Glycolysis: Glucose to pyruvate.
• TCA cycle: Pyruvate to $CO_2$.
• Oxidative phosphorylation: ATP and $e^-$.
• Glycogen synthesis & breakdown.
• Gluconeogenesis: Pyruvate to glucose.

Metabolic Homeostasis

• Balancing of:
  • Intake of fats, protein, carbohydrates.
  • Oxidation (catabolic) rates.
  • De novo synthesis.
  • Mobilization to/from storage.
• In eukaryotes, involves interactions between different tissues and organs.
  • Liver, adipose tissue, muscle, brain, etc. have different roles reflected in different enzyme pathways.

Glucose Requirements & Levels

• Hypoglycemia (too little; < $3.3 mmol/L$):
  • Limits brain metabolism (low affinity blood-brain barrier transporters).
• Hyperglycemia (hyperosmolar hyperglycemic state; > $11 mmol/L$):
  • Neurologic deficits & coma.
  • [Glucose] rises above the renal tubular threshold.
  • Non-enzymatic glycosylation of proteins.
• Normal range for resting blood glucose levels: $4-6 mmol/L$.

Major Metabolic Hormones

• Fuel storage & mobilization regulated by two main hormones:
  • Insulin: Promotes storage of fuels (& use for growth).
  • Glucagon: Promotes mobilization of fuels.

Inter-tissue Integration

• Regulated by:
  • Blood nutrient levels (e.g., fatty acid concentrations determine whether skeletal muscle uses fatty acids or glucose).
  • CNS control (e.g., adrenaline released by the sympathetic NS signals immediate need for energy).

Insulin

• 51 amino acid polypeptide synthesized in the pancreas as pre-prohormone (proinsulin).
• Released in proportion to blood glucose levels.
• Acts on three main tissues.
• Effects include:
  • Promotion of glycogen synthesis in the liver & muscle.
  • Synthesis of TAGs in the liver.
  • TAG storage in adipose cells.
  • Glucose uptake by cells (muscle & adipose).
  • Protein synthesis in muscle & liver.

Counter-Regulatory Hormones

• Glucagon is the most important counter-regulatory (contra-insular) hormone.
  • Produced in pancreatic α cells in response to high insulin or low glucose in plasma.
• Release also promoted by:
  • Catecholamines (e.g., adrenaline).
  • Amino acids (meal composition important).

Glucagon

• 29 amino acid polypeptide.
• Acts at the liver and adipocytes to increase blood glucose levels.
• Very short half-life (5 minutes).
• Effects include increased:
  • Glycogen breakdown.
  • Gluconeogenesis.
  • Fatty acid mobilization (and therefore ketogenesis in the liver).

Hormone Actions

• All hormones act on cells through receptors.
• Affect enzyme activity levels through phosphorylation or by regulating gene expression.
• Insulin: Tyrosine kinase receptor, signaling cascade.
• Glucagon: αs-linked GPCR, cAMP activates PKA.

Glucose Homeostasis: Summary

• Catabolic and anabolic processes are mediated by interconnected metabolic pathways.
• Regulated by substrate concentrations, hormones, and the CNS.
• Pancreas is the site of generation of many metabolic hormones.
  • Insulin is the major anabolic hormone.
  • Glucagon is the major catabolic hormone.
• A balance between these hormones controls the metabolic processes of the body's major organs.