Metabolism

Metabolism


Nutrients from foods are substrates for metabolism

3 substrates: Carbohydrate (CHO), Fat, Protein(PRO)

Kilocalorie (kcal)

  • Measure of energy in biological systems
  • Measure of heat
  • kcal - amount of heat needed to raise 1 kg of water from 1 °C, from 14.5 °C to 15.5 °C
  • Kcal is what we call a ‘calorie’ when looking at food labels

Energy Source Used

  • Rest – fairly equivalent use of CHO and fat

  • PRO - limited energy for cellular activity; building blocks
    for the body’s tissues

  • Intense, short-duration activity – mostly CHO

  • Longer, less intense activity – fats and CHO

Carbohydrate

Glucose - Main fuel for brain, muscles

Glucose is Converted to & stored as glycogen in liver, muscles (~2500 kcal)

Glycogen

  • Stored in cytoplasm of muscle cells; can be quickly used to form ATP

  • Stored in liver - converted back to glucose, transported to muscles to form ATP

Fat

  • Energy source during prolonged, low-intensity activity

  • Body stores larger than CHO (70,000+ kcal)

  • More energy from fat (9.4 kcal/g) than CHO (4.1 kcal/g)

  • Stored as triglycerides

  • Broken down to free fatty acids (FFAs) for metabolism

  • Less readily available for metabolism than CHO

Can provide plenty of ATP, but production slower than with CHO

Protein

  • Minor energy source (4.1 kcal/g)

  • Must be converted to glucose via gluconeogenesis

  • For energy use, must be broken down to amino acids

  • Can generate FFAs during starvation through lipogenesis

  • Can supply up to 5-10% of energy during prolonged exercise

Key Points

  • Energy from food converted to ATP – CHO(4.1kcal/g)
    – Fat(9.4kcal/g)
    – PRO(4.1kcal/g)

  • CHO stored as glycogen (muscles, liver), more accessible than fat or PRO

  • Glucose - usable form of CHO

  • Fat stored as triglycerides in adipose tissue, broken down to FFA

Storing Energy: High-EnergyPhosphates

• ATP stored in small amounts until needed

• Breakdown of ATP to release energy

  • ATP + ATPaseADP + Pi + energy

  • ADP is a lower-energy compound that is less useful

• Synthesis of ATP from by-products

  • ADP + Pi + energyATP (via phosphorylation) 

  • Can occur in either absence or presence of O2

ATP: Generated Through 3 Energy Systems

  1. ATP-PCr system – occurs in the absence of O2, therefore anaerobic

  2. Glycolytic system - anaerobic

  3. Oxidative system – requires O2, therefore aerobic

ATP Phosphocreatine (PCr) System

  • Phosphocreatine (PCr) stored in cells broken down to regenerate ATP

  • Reaction facilitated by creatine kinase

  • O2 not required → anaerobic

  • Provides energy for 3-15 sec during an all-out sprint

  • 1 mole of ATP produced per mole of PCr

Glycogen Breakdown and synthesis

  • Glycolysis - breakdown of glucose; can be anaerobic or aerobic

  • Glycogenesis - glycogen synthesis from glucose; for storage

  • Glycogenolysis – breakdown of glycogen to glucose-1- phosphate for energy production.

Glycolytic System

  • 10-12 reactions - glycogen or glucose to lactic acid, produces ATP

  • Occurs in cytoplasm

  • Without O2, pyruvic acid converted to lactic acid

  • Start w/ glycogen - get 3 moles ATP

  • Start w/ glucose - get 2 moles ATP

  • 1 ATP used to convert glucose to glucose-6- phosphate

  • ATP-PCr + glycolysis provide energy for ~2 min of all-out activity

• Cons

  • Low ATP yield, inefficient use of substrate

  • Lack of O2 converts pyruvic acid to lactic acid

  • Lactic acid impairs glycolysis, muscle contraction – Provides energy for ~2 min of all-out exercise

• Pros

  • Allows muscles to contract when O2 limited

  • Permits shorter-term, higher-intensity exercise than oxidative metabolism can sustain

Oxidative System

  • Uses O2 to produce energy → aerobic

  • Occurs in mitochondria

  • Yields more ATP than anaerobic systems 

  • – 32 to 33 ATP/1 glucose
    – 100+ ATP/1 FFA

  • Slow to turn on, steady energy supply for hours

  • Endurance events
  • Most complex energy system

 3 processes/stages make up oxidative system: 

  1. Stage1-Glycolysis

  2. Stage2-Krebs Cycle

  3. Stage3-Electron Transport Chain

Oxidation of CHO

  1. W/ O2 present, pyruvic acid (from glycolysis) converted to acetyl coenzyme A (acetyl CoA)

  2. One glucose yields 2 acetyl CoA

  3. Acetyl CoA enters Krebs cycle, forms 2 ATP, CO2, and H+

  4. H+ combines with two coenzymes (NAD and FAD) – to limit acidity in cell

  5. NAD and FAD carry H+ to electron transport chain (NAD and FAD → NADH and FADH)

Electron Transport Chain

  1. Splits NADH and FADH, releasing H+, recombined with O2 to produce water

  2. Electrons produced from the split of NADH and FADH provide energy for the phosphorylation of ADP to ATP

  3. One glycogen can generate 37-39 ATP

Oxidation of Fat

• Lipolysis - breakdown of triglycerides into glycerol & 3 free fatty acids (FFAs)

Enzymes – lipases

• FFAs travel to muscle fibers

• Enter muscle across concentration gradient

  • β-oxidation –conversion of FFA to acetyl CoA in
    mitochondria

  • Acetyl CoA enters Krebs cycle (ETC follows)

  • Fat oxidation requires more O2 than glucose because FFA contains more carbon

• Yields 3-4 more times ATP than CHO

Oxidation of Protein

 Not usually used for fuel • Utilized during starvation

  • Some AA converted into glucose (gluconeogenesis) or other oxidative intermediates (acetyl-CoA)

  • Energy yield difficult to determine

    • Because of presence of N in protein molecules

    • Excretion of N (e.g., conversion to urea) requires ATP expenditure

    • Most estimates ignore protein metabolism

Lactate Utilization

• Lactate an important fuel during exercise

• Muscles can use lactate in threeways: 

  1. Lactate produced in cytoplasm can be taken up by mitochondria of same muscle fiber and oxidized

  2. Lactate can be transported to another cell and oxidized there (lactate shuttle)

  3. Lactate can be taken up by liver and be reconverted to pyruvate and then to glucose through gluconeogenesis

Crossover Concept

  • Atrestand exercise below 60%VO2 max, lipids serve as the primary substrate

  • During high intensity (above 75% VO2max), CHO serve as the primary substrate

  • The crossover point is the intersection
    – Affected by exercise intensity and endurance training

Oxidative Capacity of Muscle

  • Oxidative capacity of muscle (QO2) - measure of its maximal capacity to use O2 .

  • Dependent on

    • Oxidative enzyme levels

    • Fiber-type composition • O2 availability

  • Representative enzymes to measure oxidative capacity – Succinate dehydrogenase (SDH)
    – Citrate synthase (CS)

Fiber Type Composition and Endurance Training

Type I fibers: greater oxidative capacity 

  • More mitochondria
  • High oxidative enzyme concentrations
  • Type II better for glycolytic energy production

• Endurance training

  • Enhances oxidative capacity of type II fibers
  • Develops more (and larger) mitochondria
  • Develops more oxidative enzymes per mitochondria