Energy Transfer in the Body

Chapter 6: Energy Transfer in the Body

High-energy Phosphates

  • Adenosine triphosphate (ATP)

    • Definition: ATP is referred to as the energy currency of the cell.

    • Functions:

    • Powers all of the cell's energy-requiring processes.

    • Provides potential energy extracted from food.

    • Energy is stored in the chemical bonds of ATP, although there is a limited store.

    • Energy is readily transferred to do biological work.

    • Formation of ADP:

    • ADP (adenosine diphosphate) forms when ATP reacts with water.

    • This process involves the release of the outermost phosphate group.

    • The reaction is catalyzed by the enzyme ATPase.

    • Energy released in this reaction is approximately 7.3 kcal/mol.

    • ATP Resynthesis:

    • Cells maintain a small quantity of ATP, necessitating its continuous resynthesis.

    • The body typically stores around 80 to 100 grams of ATP under normal resting conditions, enough to power approximately 2 to 3 seconds of maximal exercise.

Phosphocreatine (PCr): The Energy Reservoir

  • Anaerobic ATP Resynthesis:

    • Process: Phosphocreatine (PCr) is hydrolyzed (i.e., “split”) into creatine by the enzyme creatine kinase (CK).

    • ADP Phosphorylation: The reaction phosphorylates ADP back into ATP.

    • Reaction Dynamics: High levels of ADP favor this reaction, reaching maximum energy yield in about 10 seconds.

    • Storage Capacity: Cells store approximately 4–6 times more phosphocreatine than ATP.

    • Adenylate Kinase Reaction:

    • Assists in ATP regeneration:
      2 ext{ADP}
      ightarrow ext{ATP} + ext{AMP}

Important By-products of ATP Production

  • Generated By-products:

    • From PCr hydrolysis and the adenylate kinase reaction:

    • Inorganic phosphate (Pi)

    • AMP

    • ADP

  • Stimulation of Other Processes:

    • These by-products stimulate metabolic processes such as:

    • Glycogenolysis/Glycolysis: The breakdown of glucose.

    • Oxidative Phosphorylation: The process of cellular respiration.

Different Ways to Produce ATP

  • Methods of ATP Production:

    • Cellular Oxidation: The burning of dietary macronutrients: carbohydrates, lipids, and proteins.

Quick Chemistry Review

  • REDOX Reactions in Energy Metabolism:

    • Reduction: A reduced molecule gains electrons.

    • Oxidation: An oxidized molecule loses electrons.

    • NAD+ and FAD:

    • Major electron acceptors in cellular respiration.

    • NAD+ is reduced to NADH.

    • FAD is reduced to FADH2.

The Electron Transport Chain (ETC)

  • Overview:

    • The ETC is the final common pathway forelectrons extracted from hydrogen atoms that pass to oxygen.

    • Mitochondrial Oxygen:

    • Mitochondrial oxygen levels drive the respiratory chain by serving as the final electron acceptor, resulting in the formation of water.

    • This process is aerobic.

Oxidative Phosphorylation

  • Definition:

    • Consists of the Krebs Cycle (also known as the TCA cycle) and the electron transport chain.

    • Involves aerobic metabolism.

  • Function:

    • This process transfers electrons from the oxidation of fuel sources (carbohydrates, fats, proteins) to NADH or FADH2, and finally to oxygen to produce ATP.

    • Produces ATP slowly but yields a high amount of ATP.

  • P/O Ratio Efficiency:

    • The ratio of phosphagen bonds formed to oxygen atoms consumed.

  • Location:

    • Occurs in the mitochondria, termed the cell's "energy factory."

    • Utilizes a proton pump to generate a gradient and enables chemiosmotic coupling that joins ADP and inorganic phosphate (Pi).

  • Estimated Efficiency:

    • The efficiency of electron transport is approximately 50%.

The Electron Transport Chain (ETC) Continued

  • Process of ATP Synthesis:

    • ATP is synthesized by transferring electrons from NADH and FADH2 to oxygen, effectively using the electron energy to phosphate ADP into ATP.

    • About 90% of ATP synthesis is derived from oxidative phosphorylation through oxidation reactions connected with phosphorylation.

    • Energy Conservation: Each mole of ATP formed from the reaction of ADP and inorganic phosphate conserves about 7 kcal of energy.

    • The relative efficiency of energy harnessing via oxidative phosphorylation is 34%.

Glycolysis

  • Definition:

    • Glycolysis is an anaerobic energy system that serves as an intermediate energy source.

    • It is crucial for physical activities that require maximal effort for up to 90 seconds and relies solely on carbohydrates (CHO).

  • Location:

    • Occurs in the cytosol.

  • Reactions:

    • NAD is reduced to NADH during glycolysis.

    • Generates limited ATP through substrate-level phosphorylation (no oxygen utilized).

    • Glucose is cleaved into 2 pyruvate molecules, which may convert to lactate depending on conditions.

Regulation of Glycolysis

  • Key Rate-limiting Enzymes:

    • Hexokinase

    • Phosphofructokinase (PFK)

    • Pyruvate Kinase

  • Transport into Sarcoplasm:

    • Dependent on GLUT-4 transporters that respond to elevated insulin (post high CHO meal) and physical activity regardless of insulin.

  • Refueling Post-Exercise:

    • Regulates “refeeding” effect following exercise, dependent on available glycogen.

The Role of Lactate

  • Process:

    • If pyruvate is not utilized for aerobic glycolysis, it converts into lactate through the enzyme lactate dehydrogenase.

    • This reaction is reversible and allows for the recycling of NAD+ from NADH for continued glycolytic cycle participation.

  • Acid-base Balance:

    • As proton concentration rises, pH decreases, causing the “muscle burn” feeling attributed not merely to lactic acid, but by the accumulation of hydrogen ions.

  • Accumulation of Lactate:

    • When lactate accumulates without being utilized, fatigue sets in, but lactate itself is not harmful; rather, it is the accumulation that signals anaerobic metabolism.

Lactate Is Not a Waste Product

  • Potential Uses of Lactate:

    • Lactate Shuttle: Lactate produced in fast-twitch muscle fibers can be transported to other fast-twitch or slow-twitch fibers for conversion back to pyruvate when oxygen becomes available.

    • Gluconeogenesis: Lactate can be converted back to pyruvate and then glucose in the liver via the Cori Cycle, presenting new glucose available for muscle use.

The Krebs Cycle

  • Alternate Names:

    • Citric Acid Cycle

    • Tricarboxylic Acid (TCA) Cycle.

  • Function:

    • Continues the oxidation process of carbohydrates post-glycolysis, fatty acids post-beta oxidation, and some amino acids post-deamination.

  • Entry Point:

    • Begins with acetyl CoA formation (derived from pyruvate) joining oxaloacetate to form citrate.

Krebs Cycle Outputs

  • Cycle Outputs:

    • Each turn of the cycle produces the following (per 2 turns total due to 2 pyruvate from glucose):

    • 3 NADH

    • 1 FADH2

    • 1 ATP (somewhat)

    • Additional ATP significance from NADH (3 ATP per molecule) and FADH2 (2 ATP per molecule).

  • Substrate Flexibility:

    • Triglycerides and proteins can enter the Krebs cycle once converted to Acetyl CoA.

Total Energy Transfer From Glucose Catabolism

  • Discrepancy in Estimates:

    • Total estimated ATP yield ranges from 32 to 38 ATP based on the shuttle system used to transport NADH + H+ (either malate-aspartate or glycerol-phosphate shuttles).

Regulation of Energy Metabolism

  • Overall state and direction:

    • The energy state of the cell significantly influences metabolic pathways.

  • Rate-limiting modulators include:

    • ATP

    • ADP (most influential on rate-limiting enzymes)

    • Cyclic AMP (cAMP)

    • NAD

    • Calcium

    • pH

    • Relative concentrations such as NADH/NAD+ and ATP/ADP are crucial for determining energy production efficiency.

Energy Release From Fat

  • Storage and Mobilization:

    • Adipocytes serve as the site of fat storage and mobilization.

    • Energy Potential:

    • 1 lb of fat yields approximately 3,500 kcal.

    • Fats are primarily stored as triglycerides (glycerol + 3 fatty acids).

  • Utilization Process:

    • Initiated by lipolysis, the splitting of triglycerides into glycerol and free fatty acids (FFAs).

    • Driven by the enzyme hormone-sensitive lipase.

    • FFAs are transported in plasma bound to albumin to be reesterified to triglycerides or enter mitochondria for metabolism.

Hormonal Effects on Lipolysis

  • Stimulators of Lipolysis:

    • Epinephrine

    • Norepinephrine

    • Glucagon

    • Growth Hormone

    • Testosterone

    • Thyroid-stimulating Hormone (TSH)

  • Storage Influencers:

    • Lipid storage is stimulated by hormones like Estrogen and Insulin.

Catabolism of Glycerol and Fatty Acids (FAs)

  • Fate of Glycerol:

    • Glycerol can be converted to pyruvate through glycolysis and can also serve as a carbon skeleton in gluconeogenesis.

  • Beta Oxidation:

    • Fatty acids are converted to Acetyl CoA in mitochondria through beta oxidation, cleaving two-carbon compounds from fatty acyl CoA molecules.

    • Acetyl groups enter the Krebs Cycle that generates Acetyl CoA as from glucose catabolism.

    • Requires oxygen, thus associating with aerobic processes.

Total Energy Transfer From Fat Catabolism

  • Dietary Influences:

    • High-fat diets can enhance reliance on triglycerides as fuel, but the suitability varies based on exercise intensity; reliance on carbohydrates is crucial during high-intensity exercise.

Protein as a Fuel Source

  • Processes Involved:

    • Deamination: Removal of nitrogen from amino acids occurs in the liver and muscles, allowing entry to the Krebs Cycle for energy production.

    • Transamination: Transfers the amine group to form glutamate, with remaining carbon skeletons being utilized for ATP generation.

    • Proteins contribute to about 10% of ATP production during energy metabolism.

Glucogenic and Ketogenic Amino Acids

  • Glucogenic Amino Acids:

    • These can form pyruvate, oxaloacetate, and malate, serving as intermediates in gluconeogenesis.

  • Ketogenic Amino Acids:

    • These can be converted into Acetyl CoA and acetoacetate but cannot directly synthesize glucose.

Summary of Energy Transition from Macronutrients

  • The body can utilize carbohydrates, fats, and proteins through complex metabolic pathways to maintain energy supplies. Efficient regulation ensures energy availability based on cellular needs and demands during various physical activities.