Structure of contractile proteins

Hyderabad: This article is in continuation to the previous articles focusing on ‘Locomotion and Movement.’ As discussed in the previous article, the thin filaments are firmly attached to the ‘Z’ line while the thick filaments in the ‘A’ band are also held together in the middle of this band by a thin fibrous membrane called […]

Hyderabad: This article is in continuation to the previous articles focusing on ‘Locomotion and Movement.’

As discussed in the previous article, the thin filaments are firmly attached to the ‘Z’ line while the thick filaments in the ‘A’ band are also held together in the middle of this band by a thin fibrous membrane called ‘M’ line.
• The ‘A’ and ‘I’ bands are arranged alternately throughout the length of the myofibrils.
• The portion of the myofibril between two successive ‘Z’ lines is considered as the functional unit of contraction and is called a sarcomere.
• In a resting state, the edges of thin filaments on either side of the thick filaments partially overlap the free ends of the thick filaments leaving the central part of the thick filaments.
• This central part of thick filament, not overlapped by thin filaments is called the ‘H’ zone.
Structure of contractile proteins
• There are two important contractile proteins – Actin and Myosin

Actin (thin) filament
• Each actin (thin) filament is made of two ‘F’ (filamentous) actins helically wound to each other.
• Each ‘F’ actin is a polymer of monomeric ‘G’ (Globular) actins.
• Two filaments of another protein, tropomyosin, also run close to the ‘F’ actins throughout its length.
• A complex protein Troponin is distributed at regular intervals on the tropomyosin.
• In the resting state a subunit of troponin masks the active binding sites for myosin on the actin filaments.
Myosin (thick) filament
• Each myosin (thick) filament is also a polymerised protein.
• Many monomeric proteins called Meromyosins constitute one thick filament.
• Each meromyosin has two important parts, a globular head with a short arm and a tail, the former being called the heavy meromyosin (HMM) and the latter, the light meromyosin (LMM).
• The HMM component, i.e., the head and short arm, projects outwards at regular distance and angle from each other from the surface of a polymerised myosin filament and is known as cross arm.

• The globular head is an active ATPase enzyme and has binding sites for ATP and active sites for actin.

To be continued…
By
Dr. Modala Mallesh
Subject Expert

Palem, Nakrekal, Nalgonda

9989535675