Biochemistry of Skeletal Muscle: Integrated Musculoskeletal System

Biochemistry of Skeletal Muscle

Overview

The biochemistry of skeletal muscle encompasses the molecular mechanisms underlying muscle contraction, energy metabolism, and adaptation to exercise. Understanding these processes is essential for appreciating how muscles generate force, utilize fuels, and respond to physiological demands.

• Muscle Fiber Types: Type I, IIa, IIb characteristics and functional differences

• Excitation–Contraction Coupling: How neuronal signals trigger muscle contraction

• ATP Regeneration Pathways: Phosphocreatine, glycolysis, and oxidative phosphorylation

• Fuel Use at Rest vs Exercise: Fatty acids, glucose, glycogen, lactate shuttle

• Regulation During Exercise: Epinephrine, AMP/AMPK, glycogenolysis

• Long-Duration Exercise Fuels: Blood glucose & fatty acids, hormonal regulation

• Effects of Training: Mitochondria, oxidative capacity, capillary density, fiber adaptations

Skeletal Muscle Fiber Types

Classification and Properties

Skeletal muscle fibers are classified based on their contractile speed, metabolic properties, and fatigue resistance. The three main types are Type I, Type IIa, and Type IIb.

• Type I (Slow-Twitch, Oxidative):

  • High mitochondria density

  • High myoglobin (red color)

  • Slow contraction, fatigue resistant

  • Primary fuel: fatty acids

• Type IIa (Fast-Twitch, Oxidative-Glycolytic):

  • Intermediate mitochondria

  • Faster contraction, moderate fatigue resistance

  • Mixed fuels: glucose & fatty acids

• Type IIb (Fast-Twitch, Glycolytic):

  • Low mitochondria

  • Low myoglobin (white color)

  • Very fast contraction, fatigue quickly

  • Primary fuel: glycogen/glucose

Fiber Type	Mitochondria	Myoglobin	Contraction Speed	Fatigue Resistance	Primary Fuel
Type I	High	High	Slow	High	Fatty acids
Type IIa	Intermediate	Intermediate	Fast	Moderate	Glucose, fatty acids
Type IIb	Low	Low	Very fast	Low	Glycogen, glucose

Mechanism of Muscle Contraction

Excitation–Contraction Coupling

Muscle contraction is initiated by neuronal signals that trigger a cascade of molecular events, leading to actin-myosin interaction and force generation.

• Motor neuron releases Acetylcholine (ACh) at the neuromuscular junction.

• Depolarization travels along the sarcolemma and into T-tubules.

• Ca2+ released from the sarcoplasmic reticulum.

• Ca2+ binds troponin C, exposing actin binding sites.

• Myosin heads form cross-bridges with actin, producing the power stroke.

• ATP required for detachment and re-cocking of myosin heads.

Stepwise Mechanism

1. Action potential arrives at axon terminal, causing synaptic vesicles to fuse with the membrane.

2. Acetylcholine (ACh) is released into the synaptic cleft and diffuses across.

3. ACh binds to its receptors on the sarcolemma (ligand-gated ion channels).

4. Opening of voltage-gated Na+ channels; Na+ enters muscle cell, creating an end-plate potential.

5. Action potential spreads along the sarcolemma, starting excitation–contraction coupling.

Cross-Bridge Cycle

1. Resting State (Myosin Primed): Myosin head holds ADP + Pi; troponin/tropomyosin blocks actin binding sites.

2. Ca2+ binds: Troponin shifts, exposing actin binding sites.

3. Cross-Bridge Formation: Myosin head attaches to actin; ADP + Pi still bound.

4. Power Stroke: Myosin releases Pi then ADP, sliding actin.

5. Detachment (ATP Required): ATP binds myosin, releasing actin.

6. Myosin Reset (Reactivation): ATP → ADP + Pi; myosin re-cocked for next cycle.

Ways Muscle Regenerates ATP

ATP Regeneration Pathways

Muscle cells utilize several metabolic pathways to regenerate ATP, depending on the intensity and duration of activity.

1. Creatine Phosphate System (Phosphocreatine):

   • Provides immediate, high-power ATP for the first 1–10 seconds of intense activity.

   • Reaction: $\text{Phosphocreatine} + \text{ADP} \leftrightarrow \text{Creatine} + \text{ATP}$

2. Anaerobic Glycolysis (from Muscle Glycogen):

   • Supplies ATP without requiring oxygen.

   • Dominates during short, high-intensity efforts (sprinting, lifting).

   • Produces lactate as a byproduct.

3. Aerobic (Oxidative) Metabolism:

   • Uses oxygen to generate large amounts of ATP.

   • Includes glucose oxidation, fatty acid oxidation, ketone oxidation (minor), and amino acid oxidation (minimal).

Fuel Use in Resting Muscle

Preferred Fuels and Regulation

At rest, skeletal muscle primarily utilizes fatty acids for energy, with glucose, ketone bodies, and amino acids playing minor roles.

• Fatty acids: Preferred fuel at rest.

• Glucose: Used when needed.

• Ketone bodies: Used in fasting or ketogenic states.

• Amino acids: Minor contribution.

Regulation by Citrate:

• High ATP → ↑ citrate

• Citrate inhibits PFK-1 (slows glycolysis)

• Citrate activates ACC → ↑ malonyl-CoA (inhibits carnitine shuttle)

• Result: Fewer fatty acids enter mitochondria when energy is sufficient

Anaerobic Use of Muscle Glycogen

Glycolysis and Lactate Export

During high-intensity exercise lasting longer than 10 seconds, muscle glycogen is broken down anaerobically to produce ATP and lactate.

• Mechanism: Glycogen → glucose-1-P → glycolysis → ATP + lactate

• Fate of Lactate:

  • Liver (Cori cycle): lactate → glucose

  • Heart: lactate used as fuel

  • Neighboring muscle fibers: lactate uptake

Regulation of Glycogenolysis: Epinephrine and AMP Effects

Epinephrine and Glycogenolysis

Epinephrine stimulates glycogen breakdown in muscle via a signaling cascade.

• Epinephrine binds β-adrenergic receptor

• Activates Gs protein

• α-subunit activates adenylate cyclase

• ↑ cAMP → activates PKA

• PKA activates phosphorylase kinase → glycogen phosphorylase → glycogenolysis

• PKA inhibits glycogen synthase → ↓ glycogenesis

AMP Effects in Exercising Muscle

AMP levels rise when ATP is consumed, activating AMPK and promoting energy-generating pathways.

• AMPK actions:

  • Glycogenolysis (activates phosphorylase)

  • Glycolysis (activates PFK-1 indirectly via PFK-2)

  • Carnitine shuttle (inhibits ACC → ↓ malonyl-CoA)

  • GLUT4 translocation → ↑ glucose uptake

Fuel Use During Exercise

Substrate Utilization Over Time

Fuel selection shifts during exercise depending on duration and intensity.

• Basal (Rest): Low overall fuel use; ~25% from blood substrates, majority from glycogen and fatty acids.

• 40 Minutes of Exercise: Largest total fuel consumption; muscle glycogen becomes dominant fuel.

• 240 Minutes of Exercise (Prolonged Activity): Glycogen stores fall; greater use of circulating fuels (lactate, glycerol, amino acids, pyruvate).

• Blood-derived substrates rise to ~45% of total fuel use.

Regulation by Energy Charge

AMP/ADP/ATP Ratios and Metabolic Control

Changes in ATP, ADP, and AMP concentrations regulate oxidative phosphorylation and β-oxidation to meet energy demands.

• ↑ ATP/ADP, ↑ ADP, ↑ AMP

• Stimulates oxidative phosphorylation → regenerates NAD+, FAD

• Enhances β-oxidation to meet ATP demand

AMP activates AMPK:

• AMPK phosphorylates Acetyl-CoA Carboxylase → ↓ malonyl-CoA

• ↓ Malonyl-CoA relieves inhibition of CPT I → ↑ FA entry into mitochondria → ↑ β-oxidation

Blood Glucose and Fatty Acid Supply in Exercise

Blood Glucose Supply in Long-Duration Exercise

• Liver glycogenolysis: Primary early source

• Hepatic gluconeogenesis (later): Lactate, alanine, glycerol, small intake from diet

Blood Fatty Acids in Aerobic Exercise

• Sources: Adipose tissue lipolysis (major)

• Hormonal activation:

  • Epinephrine: Activates hormone-sensitive lipase via cAMP/PKA

  • Glucagon: Same pathway in fasting

  • Cortisol: Increases transcription of lipolytic enzymes

Metabolic Effects of Training

Adaptations to Endurance and Strength Training

Regular exercise induces metabolic adaptations in skeletal muscle, improving performance and efficiency.

• ↑ Mitochondria (biogenesis)

• ↑ Oxidative enzymes

• ↑ Fatty acid oxidation

• ↑ Capillary density

• ↑ Glycogen storage

• ↑ Lactate clearance

• Shift toward more oxidative Type IIa fibers