CONTRACTILE PROTEINS CLASSIFICATION

Contractile proteins appear to be found everywhere in nature, not just in motor organs, but in every living cell. They’ve just lately been isolated from the brain, liver, thyroid, and pancreas, among other places.

 Of fact, the contractile proteins found in various animal and plant cells really aren’t identical due to variances in environmental conditions and the unique type of the activity in which they are involved, but they have many of the same enzymic & physicochemical properties. As a result, actomyosin-like proteins can be considered as a group.

CONTRACTILE PROTEINS PRESENT IN MUSCLES:

The contractile proteins actin and myosin are found in muscles and are responsible for muscular contraction and relaxation.

Actin and myosin are two proteins that can be found in any form of muscular tissue. The interplay of bulky myosin filaments plus thin actin filaments causes muscle contractions and movement. Myosin is a molecular motor that converts the chemical energy in ATP into mechanical energy. The actin filaments are subsequently pulled along by the mechanical energy, causing muscle fibres to contract and generate movement.

STRUCTURE OF MUSCLES:

Muscular tissue is made up of muscle fibre bundles. Muscle fibres are long, slender cells that can be several inches long and may include several nuclei in skeletal muscle. Myofibrils, that are made up of bundles of strong myosin filaments & thin actin filaments, are long, thread-like structures found in the cytoplasm of muscle fibres. 

The sarcoplasmic reticulum (SR) is a web of tubules that stores calcium ions and surrounds the myosin and actin filaments. The SR has a role in the propagation of electrical impulses as well. These electrical signals are sent to muscle cells by neurons.

Sarcomeres are repeated contractile units that make up each myofibril. A Z disc is located at the end of every sarcomere.

Two types of bands can be found on sarcomeres. The A bands, which are black in colour and contain thick myosin filaments, and the I band, which are lighter in colour and contain just thin actin filaments, are the two types. The actin filaments are linked to the Z disc, while the myosin filaments are attached to the M line, which runs across the middle of the sarcomere.

FUNCTIONS OF ACTIN AND MYOSIN:

• Actin and myosin are two proteins that work together to cause muscle contractions and, as a result, movement. A motor neuron first sends an electrical signal from the brain to the muscle cell. A molecule called acetylcholine is released as a result of this.
• Calcium ions are removed from sarcoplasmic reticulum when acetylcholine is present. The calcium ions then bond to a protein known as troponin. Troponin is located in muscle tissue between the actin filaments and is connected to another protein termed tropomyosin.
• The form of troponin changes as calcium ions bind to it. This ‘unblocks’ tropomyosin from the myosin-binding site on the actin filament, allowing the myosin heads to attach to the filament.
• The myosin heads can attach to the accessible binding sites on the actin filaments once tropomyosin has moved out of the way. This causes muscle contraction to start by forming actin-myosin cross-bridges.
• When ATP is hydrolysed, energy is released, and myosin acts as a motor to turn this chemical energy in mechanical energy.
• That mechanical energy is used by the myosin to drive its head groups toward this sarcomere’s midsection. The actin filaments are pulled towards the sarcomere’s centre, causing the sarcomere to shrink & contract. Muscle movement is caused by the contraction of the sarcomere, which causes the muscle fiber to contract.

Sliding Filament Model was first proposed in 1954, explains the mechanics of muscular contraction.

CONCLUSION:

Actin (thin filament) and myosin are examples of contractile proteins (thick filament). Each actin filament is made up of two helical “F” actin (filamentous actin) filaments, with each ‘F’ actin containing numerous units of ‘G’ actin. 

The two filaments comprising regulatory proteins tropomyosin & troponin are present at regular intervals together with the ‘F’ actin. Troponin covers the myosin binding sites on actin filaments during muscular relaxation.

Each myosin molecule is made up of several meromyosin units, each of which has two distinct parts: a globular head termed as heavy meromyosin with a short arm as well as a light meromyosin tail. The cross is made up of the head and arms that extend from the surface of myosin filament at a regular distance and angle from each other. The head contains ATP binding sites as well as actin active sites. Let’s look at the mechanics of muscle contraction presently.