The sarcomere and the contractile proteins · Biology… | StemPy Lab
Course 16.1·Lesson 3 of 3
The sarcomere and the contractile proteins
A myofibril is striped because thin actin filaments and thick myosin filaments are arranged in a regular pattern. The part between two Z lines is the sarcomere, the unit of contraction. Actin carries tropomyosin and troponin, and the head of a myosin molecule is an enzyme that splits ATP.
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Why does a muscle look striped
Look at a myofibril under a microscope. It shows dark bands and light bands one after the other. The pattern depends on two proteins, actin and myosin. Both are arranged as rods, parallel to each other and to the length of the myofibril.
The thin filaments are made of actin.
The thick filaments are made of myosin.
The bands and the lines
One sarcomere of a relaxed myofibril, drawn between two Z lines. The thin actin filaments, in green, are fixed to the Z lines and reach toward the middle. The thick myosin filaments, in red, lie in the middle and are held together at the M line. A bracket above marks the A band, which is as long as the thick filaments. Brackets mark the I band on each side of a Z line, where only thin filaments lie. A bracket in the middle marks the H zone, the part of the A band that the thin filaments do not reach. A bracket at the bottom marks the sarcomere, from one Z line to the next.
The light band contains actin. It is called the I band, or the isotropic band.
The dark band contains myosin. It is called the A band, or the anisotropic band. The thin filaments also reach into its ends.
In the centre of each I band is an elastic fibre, the Z line. It bisects the I band, and the thin filaments are firmly attached to it.
In the middle of the A band, a thin fibrous membrane called the M line holds the thick filaments together.
The H zone is the central part of the thick filaments that the thin filaments do not overlap, in a relaxed muscle.
The A bands and the I bands lie alternately along the whole myofibril. The part of a myofibril between two successive Z lines is the sarcomere. It is the functional unit of contraction.
Region
What lies in it (muscle relaxed)
I band
thin filaments only
A band
thick filaments, with the ends of the thin filaments overlapping them
H zone
thick filaments only
The thin filament
An actin filament is made of three kinds of protein.
Actin. Two filaments of F actin (filamentous actin) are wound round each other like a rope. Each F actin is a chain, a polymer, of beads of G actin (globular actin).
Tropomyosin. Two threads of this protein run close to the F actin all along its length.
Troponin. This complex protein sits on the tropomyosin at regular intervals. In a resting muscle, one subunit of troponin masks the active sites on the actin where myosin could bind.
The thick filament
A myosin filament is a polymer. It is made of many monomers called meromyosins. Each meromyosin has two parts.
A globular head with a short arm, called heavy meromyosin (HMM).
A tail, called light meromyosin (LMM).
Two filaments drawn one above the other. Above, a thin filament: two strands of F actin wound round each other as a double helix, each strand a chain of G actin beads. Two threads of tropomyosin lie along the grooves of the helix, and a knob of troponin sits on them at regular intervals. In the resting state the troponin masks the active binding sites for myosin on the actin. Below, a thick filament made of many meromyosin molecules, with the heads sticking out from its surface as cross arms. One molecule is drawn larger: a long tail, the light meromyosin, and a globular head with a short arm, the heavy meromyosin. The head has a binding site for ATP and an active site for actin.
The head and the short arm stick out from the surface of the filament at regular distances and angles. They are called the cross arms. The head is an active ATPase, an enzyme that splits ATP. It has a binding site for ATP and an active site for actin.
You should find that the A band keeps its length, while the I band and the H zone change with the length of the sarcomere.
Try it in Python
The program works out the three zones of a relaxed model sarcomere.
Practise with new numbers
Common mistakes
Saying that the I band is dark. The I band is light. The A band is dark.
Putting the Z line in the A band. The Z line lies in the middle of the I band. The M line lies in the middle of the A band.
Saying that troponin binds myosin. Troponin sits on the thin filament and masks the sites on the actin.
Calling the head of the myosin the light meromyosin. The head with its arm is the heavy meromyosin. The tail is the light meromyosin.
Putting the H zone in the I band. The H zone is the centre of the A band.
Remember these
Sarcomere: from one Z line to the next. It is the functional unit of contraction.
I band: light, actin. A band: dark, myosin. Z line: in the middle of the I band. M line: in the middle of the A band. H zone: centre of the A band.
Thin filament: F actin (a polymer of G actin), tropomyosin, troponin. Thick filament: meromyosins with HMM (head and short arm) and LMM (tail).
The myosin head is an ATPase with binding sites for ATP and for actin.
Check yourself
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
What is a sarcomere, and why is it important?
Which band contains actin, and which contains myosin?
What is the H zone?
Name the three proteins of a thin filament.
Which part of a meromyosin acts as an enzyme, and what does it split?
References
Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 10, Muscle Tissue.
Alberts B. et al. (2015). Molecular Biology of the Cell, 6th edition, Garland Science, chapter 16, The Cytoskeleton.
NCERT (2023). Biology, Textbook for Class XI, chapter 17.
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