Bioenergetics And Muscle Metabolism

chapter

Learning Objectives
• Learn how our bodies change the food we eat into ATP to provide our muscles with the energy they need to move • Examine the three metabolic systems that generate ATP

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Fuel for Exercise : Bioenergetics and Muscle Metabolism

Terminology
• Substrates
– Fuel sources from which we make energy (adenosine triphosphate [ATP]) – Carbohydrate, fat, protein

Measuring Energy Release
• Can be calculated from heat produced • 1 calorie (cal) = heat energy required to raise 1 g of water from 14.5°C to 15.5°C • 1,000 cal = 1 kcal = 1 Calorie (dietary)

• Bioenergetics
– Process of converting substrates into energy – Performed at cellular level

• Metabolism: chemical reactions in the body

Metabolism and
Bioenergetics
• Nutrients from foods are the substrates for metabolism and are provided and stored as:
– Carbohydrate – Fat – Protein

Substrates: Fuel for Exercise
• Carbohydrate, fat, protein
– Carbon, hydrogen, oxygen, nitrogen

• Each cell contains chemical pathways that convert these substrates to usable energy, a process called bioenergetics • The energy we derive from food is stored in cells in the form of adenosine triphosphate (ATP) • ATP serves as the immediate source of energy for most body functions including muscle contraction

• Energy from chemical bonds in food stored in high-energy compound ATP • Resting: 50% carbohydrate, 50% fat • Exercise (short): more carbohydrate • Exercise (long): carbohydrate, fat

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Carbohydrate
• All carbohydrate converted to glucose
– 4.1 kcal/g; ~2,500 kcal stored in body – Primary ATP substrate for muscles, brain – Extra glucose stored as glycogen in liver, muscles

Fat
• Efficient substrate, efficient storage
– 9.4 kcal/g – +70,000 kcal stored in body

• Glycogen converted back to glucose when needed to make more ATP • Glycogen stores limited (2,500 kcal), must rely on dietary carbohydrate to replenish

• Energy substrate for prolonged, less intense exercise
– High net ATP yield but slow ATP production – Must be broken down into free fatty acids (FFAs) and glycerol – Only FFAs are used to make ATP

Table 2.1

Protein
• Energy substrate during starvation
– 4.1 kcal/g – Must be converted into glucose (gluconeogenesis)

• Can also convert into FFAs (lipogenesis)
– For energy storage – For cellular energy substrate

• Proteins must be broken down to their basic units—amino acids—to be used for energy

Figure 2.1

The Lock-and-Key Action of Enzymes in the Catabolism of Compounds
• Enzymes control the rate of free energy release from substrates • Do not start chemical reactions or set ATP yield • Do facilitate breakdown (catabolism) of substrates • Enzyme names end with the suffix -ase

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Bioenergetics: Basic Energy Systems
• ATP storage limited • Body must constantly synthesize new ATP • Three ATP synthesis pathways
– ATP-PCr system (anaerobic metabolism) – Glycolytic system (anaerobic metabolism) – Oxidative system (aerobic metabolism)

ATP Is Generated Through 3 Energy Systems

ATP-PCr System
• Cells store small amounts of ATP, and phosphocreatine (PCr), which is broken down to regenerate ATP • Release of ATP from PCr is facilitated by the enzyme creatine kinase • This process does not require oxygen (anaerobic) • ATP and PCr sustain the muscle’s energy needs for 3-15 sec during an all-out sprint • 1 mole of ATP is produced per one mole of PCr

ATP-Phosphocreatine System

ATP-PCr

Glycogen Breakdown and Synthesis
Glycolysis is the breakdown of glucose; it may be anaerobic or aerobic. Glycogenesis is the process by which glycogen is synthesized from glucose to be stored in the liver or muscle. Glycogenolysis is the process by which glycogen is broken down into glucose-1-phosphate to be used for energy production.

Although ATP is being used at a very high rate, the energy from PCr is used to resynthesize ATP, preventing the ATP level from decreasing. At exhaustion, both ATP and PCr concentrations are low.

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Glycolytic System
• Anaerobic • ATP yield: 2 to 3 mol ATP/1 mol substrate • Duration: 15 s to 2 min • Breakdown of glucose via glycolysis

Glycolytic System
• Uses glucose or glycogen as its substrate
– Must convert to glucose-6-phosphate – Costs 1 ATP for glucose, 0 ATP for glycogen

• Pathway starts with glucose-6-phosphate, ends with pyruvic acid
– 10 to 12 enzymatic reactions total – All steps occur in cytoplasm – ATP yield: 2 ATP for glucose, 3 ATP for glycogen

Glycolytic System
• Cons
– Low ATP yield, inefficient use of substrate – Lack of O2 converts pyruvic acid to lactic acid – Lactic acid impairs glycolysis, muscle contraction

Glycolysis

• Pros
– Allows muscles to contract when O2 limited – Permits shorter-term, higher-intensity exercise than oxidative metabolism can sustain

Energy Sources for the Early Minutes of Intense Exercise
• Aerobic
The combined actions of the ATP-PCr and glycolytic systems allow muscles to generate force in the absence of oxygen; thus these two energy systems are the major energy contributors during the early minutes of high-intensity exercise.
Oxidative System

• ATP yield: depends on substrate
– 32 to 33 ATP/1 glucose – 100+ ATP/1 FFA

• Duration: steady supply for hours • Most complex of three bioenergetic systems • Occurs in the mitochondria, not cytoplasm

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Oxidation of Carbohydrate
• Stage 1: Glycolysis • Stage 2: Krebs cycle • Stage 3: Electron transport chain

Figure
2.8

Oxidation of Carbohydrate: Glycolysis Revisited
• Glycolysis can occur with or without O2
– ATP yield same as anaerobic glycolysis – Same general steps as anaerobic glycolysis but, in the presence of oxygen, – Pyruvic acid  acetyl-CoA, enters Krebs cycle

Figure 2.9

Oxidation of Carbohydrate: Electron Transport Chain
• H+, electrons carried to electron transport chain via NADH, FADH molecules • H+, electrons travel down the chain
– – – – H+ combines with O2 (neutralized, forms H2O) Electrons + O2 help form ATP 2.5 ATP per NADH 1.5 ATP per FADH

Oxidation of Carbohydrate: Energy Yield
• 1 glucose = 32 ATP • 1 glycogen = 33 ATP

• Breakdown of net totals
– – – – Glycolysis = +2 (or +3) ATP GTP from Krebs cycle = +2 ATP 10 NADH = +25 ATP 2 FADH = +3 ATP

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The Oxidative System
• The oxidative system uses oxygen to generate energy from metabolic fuels (aerobic) • Oxidative production of ATP occurs in the mitochondria • The oxidative system is slow to turn on • Can yield much more energy (ATP) than anaerobic systems • Primary method of energy production during endurance events

Oxidation of Carbohydrate
1.
In the presence of oxygen, pyruvic acid from glycolysis is converted to acetyl coenzyme A (acetyl CoA) 2. Acetyl CoA enters the Krebs cycle and forms 2 ATP, carbon dioxide, and hydrogen 3. Hydrogen ion created during glycolysis and through the Krebs cycle combines with two coenzymes (NAD and FAD) 4. NAD and FAD carry hydrogen ions to the electron transport chain (NAD and FAD → NADH and
FADH)

Oxidation of Fat
• Lipolysis is the breakdown of triglycerides into glycerol and free fatty acids (FFAs) • FFAs travel via blood to muscle fibers and are broken down by enzymes in the mitochondria into acetic acid, which is converted to acetyl CoA through β-oxidation • Acetyl CoA enters the Krebs cycle and the electron transport chain • Fat oxidation requires more oxygen compared with glucose because a FFA molecule contains more carbon

b-Oxidation of Fat
• Process of converting FFAs to acetyl-CoA before entering Krebs cycle • Requires up-front expenditure of 2 ATP • Number of steps depends on number of carbons on FFA
– 16-carbon FFA yields 8 acetyl-CoA – Compare: 1 glucose yields 2 acetyl-CoA – Fat oxidation requires more O2 now, yields far more ATP later

Oxidation of Fat: Krebs Cycle, Electron Transport Chain
• Acetyl-CoA enters Krebs cycle • From there, same path as glucose oxidation

• Different FFAs have different number of carbons
– Will yield different number of acetyl-CoA molecules – ATP yield will be different for different FFAs – Example: for palmitic acid (16 C): 129 ATP net yield

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Oxidation of Protein
• Rarely used as a substrate
– Starvation – Can be converted to glucose (gluconeogenesis) – Can be converted to acetyl-CoA

Common Pathways for the Metabolism of Fat, Carbohydrate, and Protein

• Energy yield not easy to determine
– Nitrogen presence unique – Nitrogen excretion requires ATP expenditure – Generally minimal, estimates therefore ignore protein metabolism

Interaction Among Energy Systems
• All three systems interact for all activities
– No one system contributes 100%, but – One system often dominates for a given task

Interaction of the Energy Systems

Table 2.3

Oxidative Capacity of Muscle
. • Oxidative capacity of muscle (QO2) is a measure of its maximal capacity to use oxygen • Representative enzymes to measure oxidative capacity – Succinate dehydrogenase – Citrate synthase • Factors that determine oxidative capacity – Enzyme activity – Fiber type composition, endurance training – O2 availability versus O2 need

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Figure 2.14

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 – More oxidative enzymes per mitochondrion

Oxidative Metabolism
Key Points
• The oxidative system involves the breakdown of substrates in the presence of ( ) • Oxidation of carbohydrate involves glycolysis, the Krebs cycle, and the electron transport chain, resulting in the formation of H2O, CO2, and ( ) molecules of ATP • Fat oxidation involves ( ) of free fatty acids, the Krebs cycle, and the electron transport chain to produce more ATP than carbohydrate • The maximum rate of ATP formation from fat oxidation is too low to match the rate of ATP utilization during high intensity exercise
(continued)

Oxidative Metabolism (continued)
Key Points
• Protein contributes little to energy production, and its oxidation is complex because amino acids contain ( ), which cannot be oxidized • The oxidative capacity of muscle fibers depends on their oxidative enzyme levels, ( ), and oxygen availability

Exercise
• •

From Textbook Chapter 2: Study Questions # 2, 3, and 8 (p.67)

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