LECTURE 2 NEW

Question 1

Myosin function

Answer 1

Myosin is an actin-based motor protein that converts ATP hydrolysis energy into mechanical work, facilitating movement along actin filaments.

functions: muscle contraction and the transport of cellular components

Question 2

Myosin structure

Answer 2

Myosins are made up of one or two heavy chains (the motor subunits) and several light chains.
→ The heavy chains do the actual movement.
→ The light chains help stabilise and regulate the motor.

The head domains of all myosins are similar:
→ They bind to actin, bind and hydrolyze ATP, and generate force to move.

The tail domains of different myosins are different:
→ These determine what cellular structure or cargo the myosin interacts with, and therefore what its specific function is.

Question 3

Myosin Classes and Functions

Answer 3

There are approximately 40 different human myosins, with about 20 distinct classes identified based on sequence relationships.
Common myosin classes include:
Myosin I: Single head domain, associates with membranes.
Myosin II: Two head domains; forms bipolar filaments for muscle contraction.
Myosin V: Two head domains; longer neck; transports organelles via hand-over-hand movement.

Question 4

Myosin Neck domain

Answer 4

The length of the myosin neck domain influences the step size and movement rate along actin filaments.

Question 5

Myosin V – Cargo Transport

Answer 5

Myosin V has a longer neck than Myosin II

It has globular cargo-binding domains at its tail

Myosin V walks in a hand-over-hand fashion — one head moves forward, then the other.

At least one head is always attached to the actin filament, ensuring stability during transport.

Question 6

Role of Calcium in muscle contraction

Answer 6

A nerve impulse generates an action potential in the muscle fibers.

The action potential stimulates the opening of voltage-gated Ca²⁺ channels in the membrane of the sarcoplasmic reticulum (SR).

Ca²⁺ ions are released from the SR into the cytosol, leading to an increase in cytosolic Ca²⁺ concentration.

Ca²⁺ binds to TN-C (troponin) - weakens bond between troponin and tropomyosin - tropomyosin moves away

Exposes myosin-binding sites on actin filaments

Question 7

Role of ATP in muscle contraction

Answer 7

Myosin heads, energized by the hydrolysis of ATP, attach to the exposed binding sites on actin, forming cross-bridges.

The myosin head hydrolyzes ATP to ADP and inorganic phosphate (Pi), causing a conformational change that pulls the actin filament inward.

ADP and phosphate bound to the myosin head are released.

After the power stroke, another molecule of ATP binds to the myosin head, breaking cross-bridge and re-exposing active site

Question 8

Role of Troponin and Tropomyosin

Answer 8

Accessory proteins
Tropomyosin (TM) is a rope-like molecule that runs along the length of actin filaments, blocking myosin-binding sites in the relaxed state.
Troponin (TN) is a regulatory complex associated with tropomyosin and consists of three subunits (TN-T, TN-I, and TN-C).
TN-C binds Ca²

Question 9

Smooth Muscle Contraction

Answer 9

Myosin II Cycling: Contraction is controlled by phosphorylation of myosin light chains in response to Ca²⁺ via calmodulin, allowing for slower, sustained contractions compared to skeletal muscle.

Regulatory Mechanisms: Involves external signals like hormones affecting contraction through signal transduction pathways

Question 10

Troponin test

Answer 10

Myocardial infarction leads to the release of cardiac-specific troponins into the bloodstream, used as a diagnostic marker for heart attacks.

Question 11

Structure of muscle tissue

Answer 11

Muscle tissue < Bundle of fascicles < fascicles < myofibers < myofibrils < sarcomeres