Structure of Contractile Proteins — Revision Notes

⚡ 30-Second Revision

Actin (Thin Filament): — F-actin (polymer of G-actin), Tropomyosin (covers binding sites), Troponin (TnI, TnT, TnC).
Myosin (Thick Filament): — Long tail, two globular heads. Heads have actin-binding site & ATP-binding site (ATPase activity).
Troponin Subunits: — TnI (inhibitory), TnT (tropomyosin-binding), TnC ($Ca^{2+}$-binding).
Contraction Trigger: — $Ca^{2+}$ binds to TnC $ightarrow$ Tropomyosin shifts $ightarrow$ Myosin binds actin.
Energy Source: — ATP hydrolysis by myosin head for power stroke and detachment.

2-Minute Revision

Contractile proteins are the workhorses of muscle contraction, primarily actin and myosin. Actin forms the thin filaments, which are double-helical strands of G-actin monomers. Associated with actin are two crucial regulatory proteins: tropomyosin, a fibrous protein that covers the myosin-binding sites on actin in a relaxed state, and troponin, a complex of three subunits (TnI, TnT, TnC).

TnC is the key calcium sensor, TnT anchors troponin to tropomyosin, and TnI inhibits actin-myosin interaction. Myosin forms the thick filaments, composed of many myosin molecules. Each myosin molecule has a long tail and two globular heads.

These heads are vital, containing both an actin-binding site and an ATP-binding site with ATPase activity. Muscle contraction is initiated when calcium ions bind to TnC, causing a conformational change that shifts tropomyosin, exposing actin's binding sites.

Myosin heads then bind to actin, hydrolyze ATP for energy, pivot to pull the actin filament (power stroke), and detach upon binding a new ATP molecule. This cyclical interaction, known as the cross-bridge cycle, drives the sliding of thin filaments over thick filaments, shortening the sarcomere and causing muscle contraction.

5-Minute Revision

The ability of muscles to contract and generate force stems from the precise structure and interaction of contractile proteins, primarily actin and myosin, along with their regulatory partners, troponin and tropomyosin. These proteins are organized into functional units called sarcomeres in striated muscle.

Thin Filament Structure: The thin filament is a complex of three proteins. Its core is F-actin, a filamentous polymer formed by the helical twisting of two strands of G-actin (globular actin) monomers.

Each G-actin monomer has a specific binding site for myosin. Wrapped around the F-actin strands is tropomyosin, a long, fibrous protein. In a relaxed muscle, tropomyosin physically blocks the myosin-binding sites on actin.

1
Troponin I (TnI): — The inhibitory subunit, which binds to actin and prevents myosin attachment.
2
Troponin T (TnT): — The tropomyosin-binding subunit, which links troponin to tropomyosin.
3
Troponin C (TnC): — The calcium-binding subunit, which has four binding sites for $Ca^{2+}$ ions.

Thick Filament Structure: The thick filament is composed mainly of myosin molecules. Each myosin molecule is a hexamer, comprising two heavy chains and four light chains. The two heavy chains intertwine to form a long, rod-like tail (Light Meromyosin, LMM). At one end, each heavy chain forms a globular head (part of Heavy Meromyosin, HMM, specifically the S1 fragment). These myosin heads are the functional 'motors' and possess two critical sites:

1
Actin-binding site: — For reversible attachment to actin.
2
ATP-binding site: — Possesses ATPase activity, hydrolyzing ATP to ADP and $P_i$ to provide energy for movement and detachment. The four myosin light chains are associated with the neck region of the heads and modulate their activity.

Mechanism of Interaction (Brief): Muscle contraction is initiated by a nerve impulse, leading to the release of $Ca^{2+}$ into the sarcoplasm. $Ca^{2+}$ binds to TnC, causing a conformational change in the troponin-tropomyosin complex.

This shift moves tropomyosin away from the myosin-binding sites on actin. Myosin heads, already energized by ATP hydrolysis, then bind to actin, perform a 'power stroke' (pulling the actin filament), release ADP and $P_i$, and detach upon binding a new ATP molecule.

This cyclical 'cross-bridge cycle' causes the thin filaments to slide past the thick filaments, shortening the sarcomere and leading to muscle contraction. Relaxation occurs when $Ca^{2+}$ is pumped back into the sarcoplasmic reticulum, allowing tropomyosin to re-cover the actin-binding sites.

Prelims Revision Notes

Structure of Contractile Proteins (NEET Revision)

I. Thin Filament Components:

F-actin: — Filamentous actin, backbone of thin filament.

* Polymer of G-actin (globular actin) monomers. * Each G-actin has a myosin-binding site. * Two F-actin strands twist helically.

Tropomyosin: — Long, fibrous protein.

* Wraps around F-actin. * Function: Covers myosin-binding sites on actin in relaxed state, preventing contraction.

Troponin Complex: — Located on tropomyosin, composed of 3 subunits:

* TnI (Troponin I): Inhibitory subunit; binds to actin, inhibits actin-myosin interaction. * TnT (Troponin T): Tropomyosin-binding subunit; anchors troponin to tropomyosin. * TnC (Troponin C): Calcium-binding subunit; binds $Ca^{2+}$ to initiate conformational change.

II. Thick Filament Components:

Myosin: — Large protein, primary component of thick filament.

* Structure: Composed of 2 heavy chains and 4 light chains. * Heavy Chains: Form a long tail (Light Meromyosin, LMM) and two globular heads (part of Heavy Meromyosin, HMM). * Myosin Heads (S1 fragment): Crucial functional domains. * Actin-binding site: For attachment to actin. * ATP-binding site: Possesses ATPase activity (hydrolyzes ATP to ADP + $P_i$). * Light Chains: Associated with myosin heads; modulate ATPase activity.

Thick Filament Assembly: — Hundreds of myosin molecules bundled together; tails point to center, heads project outwards.

III. Key Interactions & Regulation:

Relaxed State: — Tropomyosin blocks actin's myosin-binding sites.
Contraction Initiation:

1. Nerve impulse $ightarrow$ $Ca^{2+}$ release from SR. 2. $Ca^{2+}$ binds to TnC. 3. Conformational change in troponin-tropomyosin complex. 4. Tropomyosin shifts, exposing actin's myosin-binding sites.

Cross-Bridge Cycle (Myosin-Actin Interaction):

1. ATP binding: Myosin head detaches from actin. 2. ATP hydrolysis: ATP $ightarrow$ ADP + $P_i$ by myosin ATPase; cocks myosin head (high-energy state). 3. Cross-bridge formation: Myosin head binds to exposed actin site. 4. Power stroke: $P_i$ released; myosin head pivots, pulling actin filament towards M-line. 5. ADP release: Myosin head remains bound to actin (rigor state) until new ATP binds.

Energy Source: — ATP is essential for both the power stroke and detachment of myosin from actin.

IV. Sarcomere Changes during Contraction:

Sarcomere: — Shortens.
I-band: — Shortens.
H-zone: — Shortens (can disappear).
A-band: — Length remains unchanged (thick filaments don't shorten).
Actin & Myosin Filament Lengths: — Remain unchanged.

Vyyuha Quick Recall

Three Troponins: TnI (Inhibits), TnT (Tropomyosin), TnC (Calcium).