Question

Which of the following statements about the molecular arrangement of actin in myofibrils is incorrect? 1\. Each actin (thin) filament is made of two ' \(\mathrm{F}\) ' (filamentous) actins helically wound to each other. 2\. Each ' \(\mathrm{F}\) ' actin is a polymer of monomeric ' \(\mathrm{G}\) ' (Globular) actins. 3\. Two filaments of another protein, tropomyosin also runs close to the 'F' actins throughout its length. 4\. A complex protein troponin is distributed at regular intervals on the tropomyosin. (a) 1 and 2 only (b) 3 only (c) Only 4 (d) None of these

Short answer

Option (d) 'None of these' is the correct answer. As all the statements about the molecular arrangement of actin in myofibrils are correct.

Step-by-step solution

Verification of Statement 1

Statement 1 suggests that each actin filament consists of two 'F' actins coiled around each other. This is a well-accepted biological concept. Indeed, each thin filament in myofibrils is composed of two 'F' actins twisted around each other. Hence, statement 1 is correct.

Verification of Statement 2

Statement 2 claims that each 'F' actin is a polymer of 'G' actin. 'F' actin indeed is a polymer made up of many individual 'G' actin molecules. So, statement 2 is also accurate.

Verification of Statement 3

Statement 3 states that two filaments of another protein, tropomyosin, run close to the 'F' actins throughout their length. This is also true, as it is well known that each actin filament is associated with a pair of tropomyosin, which is located in the grooves of the actin filaments. Thus, statement 3 is correct.

Verification of Statement 4

Statement 4 mentions that a complex protein troponin is distributed at regular intervals on the tropomyosin. This is an accurate statement as troponin complexes are inserted at regular intervals along the tropomyosin. Hence, statement 4 is correct.

Key concepts

Actin Filament

An actin filament is a fundamental component of muscle contraction. It is known as a 'thin filament' and is found within the myofibrils of muscle cells. This filament is actually a double-stranded helix structure.

Each actin filament is made up of smaller globular actin (G-actin) molecules. These G-actin molecules polymerize to form a long chain, which is referred to as filamentous actin (F-actin).
The F-actin strands then coil around each other to establish the helical structure of the actin filament widely seen in muscle cells.

Actin filaments play a critical role in muscle contraction by interacting dynamically with another filament type known as myosin. This interaction is the basis of muscle contraction, leading to the sliding filament theory, where actin and myosin filaments slide over each other to shorten the muscle fiber.

• Structure: Double-stranded helix.
• Components: G-actin polymers into F-actin.
• Function: Allows for muscle contraction by sliding motion against myosin filaments.

Tropomyosin

Tropomyosin is a protein molecule that is integral to muscle contraction. Unlike actin, tropomyosin does not form a filament by itself but instead binds along the grooves of the actin filament helix.
This binding covers the actin filaments, blocking the sites where myosin normally attaches to produce a contraction. In resting muscle, this means no contraction occurs.

Each tropomyosin molecule spans about seven actin subunits and is arranged head to tail along the actin filament. In the presence of calcium ions and associated changes induced by troponin, tropomyosin shifts its position. This exposes the myosin binding sites on actin, allowing for muscle contraction.

• Location: On the grooves of the actin filaments.
• Role: Covers myosin-binding sites on actin.
• Function: Regulates access of myosin to actin in response to calcium ion changes.

Troponin

Troponin is a complex of three regulatory proteins present on the actin filaments within myofibrils. It plays a crucial role in controlling the process of muscle contraction.

Troponin is attached to tropomyosin and interacts directly with calcium ions. It consists of three subunits: Troponin T (TnT) which connects to tropomyosin, Troponin C (TnC) which binds calcium ions, and Troponin I (TnI) which prevents the contraction of muscles in the absence of calcium.

During muscle activation, calcium ions bind to TnC, causing a conformational change within troponin. This change moves tropomyosin away from the binding sites on actin filaments, allowing myosin heads to attach and enable contraction.

• Subunits: TnT, TnC, and TnI.
• Interaction: Binds to calcium ions causing structural change.
• Function: Regulates muscle contraction through calcium signaling.

Myofibrils

Myofibrils are the basic rod-like units of a muscle cell. They are composed of long chains of repeating units called sarcomeres, which are the functional units responsible for muscle contraction.

Sarcomeres are organized arrays of actin and myosin filaments. Within a myofibril, sarcomeres are arranged end-to-end, giving the muscle fiber its striated appearance.
The overlapping of actin (thin filaments) and myosin (thick filaments) within these sarcomeres is essential for their contraction.

During muscle contraction, myofibrils shorten in length, and because they are part of a muscle fiber, this affects the entire cell, leading to overall muscle contraction.

• Units: Composed of sarcomeres.
• Role: Basic structures within muscle cells.
• Character: Contribute to striated muscle appearance and function by facilitating contraction.