Muscle Contraction,

Contractile Proteins,
Fuels for Skeletal Muscle

Dr. Apeksha Niraula

Contractile Proteins
Mechanism of Muscle Contraction
Fuels for Skeletal Muscle
Purine Nucleotide Cycle
 Introduction
Movement is an important property of life, especially of the members of the animal
kingdom
The organism may move as a whole (walking) or movement of cells may
occur (diapedesis or sperm movement) or it may occur at the subcellular
level (transfer and exocytosis of secretory proteins)
The important contractile proteins are Actin and Myosin in muscles
Beating of cilia or sperm is achieved by Tubulin and Dynein
Tubulin, actin, microfilaments, kinesin, and intermediate
filaments are involved in the movement of secretory granules from their site of
production to their release
 MUSCLE
Striated muscle is made up of PROTEINS
multinucleated cells bound by a plasma membrane called
Sarcolemma
The sarcomere is the functional unit of the muscle
Each muscle cell contains Myofibrils about 1 mm in diameter
The myofibrils are immersed in a cytosol that is rich in glycogen, ATP, creatine
phosphate, and glycolytic enzymes
The functional unit of a myofibril is a sarcomere
The dark A bands and light I bands alternate regularly
These bands are formed by a variable combination of thick and thin
filaments
The thick filaments have a diameter of about 150 Å whereas thin filaments
have a diameter of about 70 Å
The thick filament is primarily Myosin and thin filament contains
actin, tropomyosin, and troponin
The Z line contains 2 actin molecules and the M protein is located in
the M line
Thick and thin filaments slide past each other during the muscle contraction, so that the
muscle shortens by as much as a third of its original length
However, the lengths of the thick and thin filaments do not change during muscle
contraction
A-band: Dark band seen as part of striation (Anisotropic band)
I-band: Light band area between the ends of the Myosin (Isotropic
Band)
Z-Line: Membrane that separates Sarcomeres
H-Zone: Area by which the two ends of the thin filaments fail to meet
Sarcomere: Functional unit composed of two Myofilaments
Thick Filaments (Myosin)
Thin Filaments (Actin-Troponin- Tropomyosin)
 Myosin
Myosin has 3 different biological activities:
a. Myosin molecules assemble into filaments
b. Myosin acts as the enzyme ATPase
c. Myosin binds to actin polymer which is the major component of
the thin filaments
Myosin molecules are large (about 540 kD), each with 6 polypeptide chains; 2 identical
heavy chains and 4 light chains
The myosin molecule has a double-headed globular end
Trypsin cleaves myosin into 2 parts; light meromyosin (LMM) and
heavy meromyosin (HMM) types
LMM can form filaments but has no enzymatic activity
HMM has enzymatic activity and binds actin, but cannot form
filaments
HMM can further be split into the S1 fragments having the ATPase
site plus the actin-binding site and the S2 subfragment
 Actin
Major protein of the thin filaments
Monomeric protein often referred to as G-actin due to its globular shape
It can polymerize into a fibrous form, called F-actin, which is a helix of actin
monomer
Actin Filament: Composed of three protein components; major protein of the thin
filaments
Actin
Tropomyosin
Troponin
The muscle contraction results from interaction of actin and myosin, to form
actomyosin, with energy provided by ATP
When the two thin filaments that bind the cross bridges of a thick filament are drawn
towards each other, the distance between Z lines becomes shorter
This could result in the process of contraction of muscle fibers
This needs energy from the hydrolysis of ATP, affected by the ATPase activity of myosin
The contractile force is generated by conformational changes, leading to the
dissociation of actin and S1 heads of myosin
There is a reversible attachment and detachment of myosin S1 head to actin
Actin participates in many important cellular processes:
Muscle Contraction
Cell Motility
Cell Division
Cytokinesis
Vesicle and Organelle movement
Cell Signalling
Maintenance of cell junctions and cell shape
 Troponins
The muscle contraction is modulated by troponin and tropomyosin through Ca++ which
is the physiological regulator of muscle contraction
In the resting muscle, the Ca++ is within the sarcoplasmic reticulum
The nerve impulse releases Ca++ from the sarcoplasmic stores and increases its
cytosolic concentration about 10 times (1 mM to 10 mM)
The action of calcium is brought about by 2 proteins, troponin complex and
tropomyosin located in the thin filament
The troponin complex has 3 different polypeptide chains
Troponin-C (TnC, 18 kD) binds calcium
Troponin-I (TnI, 21 kD), otherwise called “Actomyosin-ATPase
inhibitory element”, binds to actin and inhibits the binding of actin to myosin
Troponin I is a marker for myocardial infarction
Its level in serum is increased within 4 hours of myocardial infarction and remains high
for about 7 days
It is about 75% sensitive index for myocardial infarction
TnC has calmodulin like properties
In the resting muscle, only the high affinity sites are occupied by Calcium,
but when Ca++ is released from sarcoplasmic reticulum, low affinity sites
are also occupied by Ca++
This results in a conformational change that is transmitted to tropomyosin
This shift in position of tropomyosin alters the binding of actin to S1
The events may be depicted as:
Ca++ → Troponin→ Tropomyosin→ Actin→ Myosin
Troponin-T (TnT, 37 kD) binds to tropomyosin
Two isoforms of cardiac TnT, called TnT1 and TnT2 are present in adult
human cardiac tissue
Serum levels of TnT2 increases within 4 hours of myocardial infarction,
and remains high for up to 14 days
 Transduction of Chemical Energy to
Mechanical
The amount of ATP in Energy
muscle is sufficient to sustain contractile activity for less than one
second
The reservoir of high energy phosphate in skeletal muscle is Creatine phosphate
The reaction (Lohman’s reaction) is catalysed by Creatine Kinase (CK)
CK
Creatine phosphate + ADP -------→ ATP + Creatine
The ∆Go’of creatine phosphate is -10.3 kilocalories per mol, whereas that for ATP is
only -7.3 kilocalories
Resting muscle has a high concentration of the creatine phosphate (25 mM) when
compared to ATP (4 mM)
The creatine phosphate provide a high ATP concentration during
muscle contraction (In athletes, it is the major source of
energy during the first 4 seconds of a short sprint)
During muscle contraction, the ATP level remains high as long as creatine
phosphate is present
But following contractile activity, the levels of ADP and Pi rise
The reduced energy charge of active muscle stimulates glycogen
breakdown, glycolysis, TCA cycle and oxidative
phosphorylation
The red striated muscle has an active aerobic metabolism
compared to white muscle
 Process of Muscle
Excitation
Muscle: Excitable Tissue
When Stimulates shows response:
Electrical Response: Production of
Action Potential
Mechanical Response: Contraction
 Process of Excitation- Contraction
Coupling
When End Plate Potential reaches threshold level
It produces action potential
Which propagates over muscle fibre and through it along transverse(T-
tubules)
Action Potential initiated in plasma membrane spread to surface and into
muscle fibre through T tubules
When reached the tip of T tubules activates the voltage gated
Dihydropyridine receptors
Activated Dihydropyridine receptors triggers the opening of calcium
release channels on terminal cisterns ( Ryanodine receptors)
Calcium diffuses into cytoplasm and ICF Ca increases (upto 2000 fold)
Calcium ion gets attached to Troponin C and starts chain of events
Hence, Calcium acts as a link between excitation and contraction process
 Calcium and Muscle
Contraction
Sarcoplasmic reticulum (SR) regulates intracellular levels of calcium in
skeletal muscle
In the resting state, calcium ions are pumped into the SR through Ca-
ATPase
Inside the SR, calcium ions are bound with specific calcium-binding protein, called
Calsequestrin
When nerve impulse excites the sarcolemma, the calcium channel is opened, calcium ions
are released from SR into sarcoplasm
The calcium ion concentration in the cytoplasm is increased
The calcium binding sites of TnC are now saturated with calcium
The TnC-4Ca++ complex attaches with TnI and TnT, which then interacts with
tropomyosin
 Fuels for Skeletal
ATP: Currency Muscle
Immediate source: Phosphocreatine
Ultimate sources: Carbohydrate, Fats and Proteins
ATP Regeneration
Dephosphorylation of Creatine Phosphate
Anaerobic Glycolysis
Aerobic Oxidation of glucose and fatty acids
 Purine Nucleotide
Cycle
Purine nucleotide cycle is a metabolic pathway in which ammonia and
fumarate are generated from aspartate and inosine monophosphate (IMP)
Functions:
To regulate the levels of adenine nucleotides
To facilitate the liberation of ammonia from amino acids
This cycle occurs in skeletal muscle (Cytosol)
Purine nucleotide cycle occurs during strenuous exercise, fasting or starvation when
ATP reservoirs run low
The cycle comprises three enzyme catalyzed reactions:
First stage: Deamination of the purine nucleotide
adenosine monophosphate (AMP) to form inosine monophosphate (IMP), catalyzed by
the enzyme AMP deaminase:
AMP + H2O + H+ → IMP + NH3
Second stage is the formation of adenylosuccinate from IMP and the
amino acid aspartate, which is coupled to the energetically favorable
hydrolysis of GTP, and catalyzed by the enzyme adenylosuccinate synthetase:
Aspartate + IMP + GTP → Adenylosuccinate + GDP + Pi
Finally, Adenylosuccinate is cleaved by the enzyme adenylosuccinate lyase to release
fumarate and regenerate the starting material of AMP:
Adenylosuccinate → AMP + Fumarate
A recent study showed that activation of HIF-1α allows cardiomyocytes to sustain
mitochondrial membrane potential during anoxic stress by utilizing fumarate
produced by adenylosuccinate lyase as an alternate terminal electron acceptor in
place of oxygen: Mechanism should help provide protection in the
ischemic heart
Thank
You