topic 13

This topic reviews the critical aspects of energy metabolism, including the regulatory roles of hormones (insulin, glucagon), the body's response to varying energy states, and the implications of metabolic disorders like diabetes mellitus.

Case Study: Patrick (Pyruvate Dehydrogenase Deficiency)

Patrick, at age 21, exhibited severe physical weakness, particularly an inability to move his head. Initially diagnosed with a demyelinating disease, his condition did not improve with treatment. Further investigation revealed a metabolic rather than a neurological issue.

Biochemical Clue and Diagnosis

• Symptoms: Weakness, difficulty with basic movements (e.g., picking up a glass, climbing stairs), beginning at age 16.

• Biochemical Finding: High levels of lactate and pyruvate in his blood, indicative of lactic acidosis.

• Diagnosis: Inherited mutations in the pyruvate dehydrogenase (PDH) gene, resulting in low enzyme activity. This caused symptoms to manifest later in life than typically observed in complete deficiencies.

Metabolic Consequences of PDH Deficiency

• Reduced production of acetyl-CoA from carbohydrates, as PDH converts pyruvate to acetyl-CoA.

• Decreased flux into the citric acid cycle, limiting oxidative phosphorylation.

• Impaired ATP synthesis, critically affecting high-energy-demand organs like the brain, which primarily relies on glucose metabolism.

• This metabolic disruption leads to severe neurological problems.

Recommended Diet

A low-carbohydrate, high-fat diet is recommended for individuals with PDH deficiency:

• Fatty acids can be broken down directly into acetyl-CoA (via beta-oxidation), bypassing the deficient PDH enzyme and feeding directly into the citric acid cycle for ATP production. This allows for sufficient energy generation even when carbohydrate metabolism is impaired.

• While protein can also provide energy, some amino acids are glucogenic and can be converted to glycolysis intermediates, which would still require the problematic PDH step for complete oxidation.

Potential Research Goals for Treatment

Ideal research strategies focus on enhancing the existing, albeit low, PDH activity:

• Increase pyruvate dehydrogenase activity: By activating the gene responsible for PDH synthesis, aiming to increase the amount of active enzyme in the system.

• Inhibit pyruvate dehydrogenase kinase: This kinase inhibits PDH by phosphorylating it. Inhibiting this inhibitor ($PDH\text{ kinase}$) would promote PDH activity and keep the enzyme in its active, dephosphorylated state.

• Incorrect approach: Increasing fatty acid synthase activity would be counterproductive, as it promotes fat storage (conversion of acetyl-CoA to fatty acids) rather than energy production (ATP synthesis from acetyl-CoA oxidation).

Outcome

Patrick unfortunately passed away at age 21 in 2006 due to organ shutdown and inability to fight infections, underscoring the vital importance of these core metabolic pathways for survival.

Energy Metabolism in the Fed State (After a Meal)

Upon consuming a meal, particularly one rich in carbohydrates, the body prioritizes insulin-mediated energy storage and biosynthesis to handle the influx of nutrients.

• Glucose Uptake: Insulin promotes increased transport of glucose into cells, followed by its phosphorylation to glucose-6-phosphate.

• Immediate Energy Production: The central pathways of energy metabolism (glycolysis, citric acid cycle, oxidative phosphorylation) are activated to meet immediate ATP requirements.

• Energy Storage: Once immediate ATP needs are met, excess glucose is stored:

  • Glycogen Synthesis: Glucose is converted into glycogen and stored in the liver and muscle cells.

  • Triacylglycerol Synthesis: Excess glucose, after conversion to acetyl-CoA, can be synthesized into fatty acids, which are then stored as triacylglycerols in adipocytes (fat cells).

• Biosynthesis: The fed state supports the synthesis of essential molecules such as lipids (for membranes), cholesterol, and amino acids (for protein synthesis).

• Pentose Phosphate Pathway: Activated to produce NADPH (a reductant crucial for biosynthesis) and ribose-5-phosphate (a precursor for nucleotide synthesis, vital for RNA and DNA).

Energy Metabolism in the Unfed/Starvation State (Prolonged Fasting)

As time passes without food, the body shifts its primary energy sources and metabolic priorities to maintain essential functions, especially brain glucose supply.

Phase 1: Glycogen Utilization (Short-term, ~16-24 hours)

• Initial Response: Blood glucose levels begin to fall.

• Primary Source: Stored glycogen is broken down (glycogenolysis).

  • Liver Glycogen: Released as glucose into the bloodstream to maintain blood glucose levels for the brain and other tissues.

  • Muscle Glycogen: Used primarily by the muscle cells themselves for their own energy needs.

• Limitation: Glycogen stores are limited and typically depleted within 16-24 hours of normal activity (or much faster with high activity like marathon running).

Phase 2: Gluconeogenesis from Protein (Intermediate-term, ~16-72 hours)

• After Glycogen Depletion: The brain still requires glucose.

• Primary Source: Gluconeogenesis, primarily from amino acids obtained through the breakdown of muscle protein.

  • Muscle protein (e.g., in skeletal muscle) is catabolized to release amino acids, which are then transported to the liver and converted into glucose.

• Fatty Acids: Are released from triacylglycerol stores (lipolysis) and utilized by other tissues (e.g., liver, muscle) to produce acetyl-CoA for ATP. However, fatty acids are not directly used by the brain for energy.

• Ketone Body Transition: Fatty acid metabolism gradually increases, and ketone body production by the liver begins, but it is not yet the primary brain fuel.

• Important Note: Humans cannot produce glucose from fatty acids.

Phase 3: Ketone Body Utilization (Prolonged Starvation, >72 hours)

• After ~72 hours (3 days): Enzymes for fatty acid oxidation and ketone body synthesis/utilization are maximally expressed.

• Brain's Main Energy Source: Ketone bodies, produced by the liver from acetyl-CoA derived from fatty acid breakdown, become the predominant fuel for the brain.

• Liver's Energy Source: The liver primarily uses fatty acids for its own ATP needs.

• Other Tissues: Muscles and other peripheral tissues also adapt to use ketone bodies and fatty acids for energy.

• Fat Stores Duration: A person with an average healthy starting weight has enough stored fat to last for 2-3 months. Extremely obese individuals under medical supervision have been observed to fast for up to 12 months.

• Terminal Stage: If fat stores are depleted, the body reverts to breaking down essential proteins for energy, leading to severe weakness, organ shutdown, and increased susceptibility to infections, often resulting in death.

Hormonal Regulation: Insulin and Glucagon

These two hormones, produced by the pancreas, are central to regulating blood glucose levels and overall energy metabolism.

Insulin (Released in Fed State)

• Stimulus: High blood glucose levels.

• Source: Pancreatic $\beta$-cells.

• Effects: Acts as a signal to the body that glucose is abundant.

  • Increases glucose uptake by muscle and adipose tissue.

  • Promotes glycogen synthesis in the liver and muscles.

  • Favors triacylglycerol synthesis and storage in adipocytes, while inhibiting triacylglycerol breakdown.

  • Stimulates glycolysis and the pentose phosphate pathway.

• Overall: Decreases blood glucose concentration by promoting its uptake and storage.

Glucagon (Released in Unfed State)

• Stimulus: Low blood glucose levels.

• Source: Pancreatic $\alpha$-cells.

• Effects: Counteracts insulin's actions, signaling to the body to release stored energy.

  • Promotes glycogenolysis (glycogen breakdown) in the liver, releasing glucose into the blood.

  • Promotes gluconeogenesis in the liver from non-carbohydrate precursors (e.g., amino acids, glycerol).

  • Stimulates lipolysis (triacylglycerol breakdown) in adipose tissue, releasing fatty acids for energy.

  • Favors ketone body production in the liver from fatty acid breakdown.

• Overall: Increases blood glucose concentration and shifts energy utilization towards fats and ketone bodies.

Diabetes Mellitus

A metabolic disorder characterized by chronically high blood glucose levels (hyperglycemia).

Type 1 Diabetes

• Cause: Autoimmune destruction of pancreatic $\beta$-cells, leading to an inability to produce insulin. Typically manifests in childhood.

• Symptoms/Consequences: The body acts as if it is starving despite high blood glucose because cells cannot effectively take up and utilize glucose without insulin.

  • Increased breakdown of stored energy (triacylglycerols, glycogen).

  • Favored ketone body production, leading to characteristic breath odor (ketone breath).

  • Glycosylation of proteins: High glucose levels can non-enzymatically attach to proteins (e.g., in eyes, nerves), leading to complications like blindness and nerve damage.

  • Increased urination and thirst: The body tries to excrete excess glucose through urine, leading to fluid loss.

Type 2 Diabetes

• Cause: A combination of reduced insulin production from pancreatic $\beta$-cells (often due to wear-out over time) and insulin resistance (tissues not responding effectively to insulin). Develops over time, usually in adulthood.

• Risk Factors: Age, higher body weight, sedentary lifestyle. Genetic predisposition also plays a role.

• Consequences: Similar to type 1 if untreated, leading to hyperglycemia and associated complications due to impaired glucose uptake and utilization.

Historical Treatment

• Insulin Discovery: Frederick Banting and Charles Best, under John Macleod's supervision, discovered insulin in Toronto. James Collip, a biochemist, purified insulin preparations, making them safe for human use.

• This discovery revolutionized the treatment of type 1 diabetes, significantly improving patient prognosis.

Traditional Treatments: Cree (Nehiyaw) Plant Remedies for Diabetes

Indigenous knowledge has provided insights into plant-based treatments for diabetes:

• Balsam Fir Extract: Activates glycogen synthase, which pulls glucose from the bloodstream for storage and prevents glucose release from the liver (inhibits glycogen breakdown and gluconeogenesis).

• Lingonberry: Promotes glucose uptake by tissues like muscle.

• Both actions effectively reduce blood glucose levels, demonstrating traditional ecological knowledge aligned with modern biochemical understanding.