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Biochemistry of Muscles

The document discusses the biochemistry of muscles, focusing on the structure and function of skeletal muscles, including the role of myofibrils, myofilaments, and various proteins such as actin, myosin, tropomyosin, and troponin in muscle contraction. It explains the sliding filament model of muscle contraction, detailing the relaxed and contracted states of muscle fibers, as well as the biochemical processes involved. Additionally, it touches on muscular dystrophies, their genetic basis, and the implications for muscle function.

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boyzmorio
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33 views16 pages

Biochemistry of Muscles

The document discusses the biochemistry of muscles, focusing on the structure and function of skeletal muscles, including the role of myofibrils, myofilaments, and various proteins such as actin, myosin, tropomyosin, and troponin in muscle contraction. It explains the sliding filament model of muscle contraction, detailing the relaxed and contracted states of muscle fibers, as well as the biochemical processes involved. Additionally, it touches on muscular dystrophies, their genetic basis, and the implications for muscle function.

Uploaded by

boyzmorio
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PDF, TXT or read online on Scribd
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Biochemistry of Muscles

Basics
• Connective tissues highly specialized to shorten, to produce
movement and changes in shape and size of body parts using
chemical energy
• They are made of aggregates of elongated cells arranged in parallel
arrays
• The basic mechanism of contracting is similar in skeletal, cardiac, and
smooth muscles.
• We use example of skeletal muscles
Skeletal muscles
• They are called muscle fibres
because the cells are elongated
to provide a long axis for
contraction.
• Each cell surrounded by
electrically excitable plasma
membrane called sarcolemma
General structure
The cell (muscle fibre) contains many
myofibrils held together by connective
tissues
Myofibrils are made up of bundles of
myofilaments
• Thick myofilaments (Myosin)
• Thin myofilaments (actin, tropomyosin and
troponin)
• Alternating dark A (anisotropic) bands
and light I (isotropic) bands
• H-zone is less dense region than the rest
of the A band
• Z-disc is a narrow line that dissects the I
band and is made of α-actinin and desmin
• Sarcomere is the smallest functional unit
of a muscle and it lies between successive
z-lines
Protein composition of Muscles
1. Contractile proteins = actin and myosin
2. Regulatory proteins = tropomyosin and troponin
3. Minor or accessory proteins = stabilising the structure and function
of muscles, and they include α-actinin, Cap-Z, Titin, Nebulin,
Desmin and Dystrophin
Myosin

Six polypeptide chains


• 2 identical heavy chains – double
helix. Amino endof each has globular
structure (Myosin Head)
• ATP binding site
• Actin binding site
• ATPase activity
• 4 light chains
• each associates with the myosin heavy
chain heads
Actin

• Major constituent of thin filament of muscle fibre


• Made of a polymer of globular-shaped subunit called G-actin (G for
globular).
• G-actin polymerizes spontaneously in the presence of Mg2+ ions, to form a
fibrous or filamentous form called F-actin
• Each G-actin molecule of thin filaments have a binding site for myosin.
Tropomyosin
• Made up of two polypeptide
chains, which are wrapped spirally
around the sides of the F-actin helix
• In the resting state, tropomyosin
lies on top of the active sites of the
actin strands so that attraction
cannot occur between actin and
myosin filaments to cause
contraction, thus regulating the
attachment of actin and myosin
Troponin
• Attached to tropomyosin
• Three polypeptide chains
1. Troponin C (TnC) - binds calcium ions in the initiation of
muscle contraction
2. Troponin I (TnI) - binds to actin and inhibits actin
myosin attachment
3. Troponin T (TnT) - binds tropomyosin to anchor the
troponin complex
Mechanism and biochemistry of muscle
contraction
The “sliding filament model” suggests that the thin filaments slide past
the thick filaments during contraction so that the total length of the
fibre is shortened
Relaxed state
1. ATP is bound to the myosin head inducing conformational changes in
the myosin head.
2. This reduces the affinity of myosin for actin and causes myosin head to
detach from actin.
3. Tropomyosin and troponin occupy actin and prevent myosin binding
actin
4. ATPase activity of myosin head hydrolyses ATP to ADP and Pi and
products remain bound to the head.
5. The energy released is then stored in the myosin molecule therefore the
myosin-ADP-Pi complex is a high energy state and is most prevalent in
relaxed muscle.
6. Nervous system (action potential) activates muscle cells
7. Ca2+ ions are released from the sarcoplasmic reticulum
8. Troponin binds Ca2+ and both troponin and tropomyosin undergo a
change in both their shape and their position on the thin filaments
9. Myosin-binding sites on the actin become exposed and the myosin
heads immediately attach to binding sites forming a cross-bridge (90°
angle).
10. Inorganic phosphate (Pi) is released from myosin head, which results in
increasing the strength of the myosin-actin attachment and sliding
begins
Contraction state
1. Myosin attachment to actin causes the myosin heads to snap (90°
to 45° angle) and the thin filaments are slightly pulled toward the
centre of the sarcomere (power stroke)
2. ADP is released and this increases the myosin-actin binding a
state called rigor configuration.
3. ATP provides the energy needed to release each myosin head so
that it is ready to attach to a binding site farther along the thin
filament
4. Each cross-bridge attaches and detaches several times during a
contraction, generating tension that helps to pull the thin
filaments toward the centre of the sarcomere
5. Event occurs simultaneously in sarcomeres throughout the
muscle cell, the length of the sarcomere decreases but the
lengths of the individual thick and thin filaments do not change
(they simply slide past each other), but the H-zones and I-bands
shorten.
6. Action potential ends and calcium ions are returned to storage
areas, the regulatory proteins resume their original shape and
position, and again block myosin binding to the thin filaments
and the muscle cell relaxes and settles back to its original length
Some animations
• https://www.youtube.com/watch?v=dpxalWACO7k
• https://www.youtube.com/watch?v=nTZnBdeIb5c
Muscular
Dystrophies
• Characterized by progressive muscle
weakness and wasting without
involvement of the nervous system
• X-linked disorder caused by mutations
in the gene coding for the protein
dystrophin
• Different types of muscular dystrophy
affect different sets of muscles and
result in different degrees of muscle
weakness
1. Duchenne muscular dystrophy
(DMD) – Generally affect skeletal
and heart muscles
2. Becker muscular dystrophy (BMD) –
affects the leg and pelvis muscles
Brain (B), muscle (M), and Purkinje (P) promoters; R, B3, S, and G represent the
Dp260 (retinal), Dp140 (brain3), Dp116 (Schwann cells), and Dp71 (general) promoters.
Largest known human gene, containing 79 exons and
spanning > 2,200 kb
Rigor mortis ??
• Explain the biochemistry behind rigor mortis
Energy for Muscle contraction

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