The proteins actin
interact inside the sarcomeres and cause the sarcomeres and whole myofibril to shorten, which results in the contraction of the muscle.
Their relative movement is explained by the sliding filament theory
This simple diagram shows the arrangement of actin and myosin in a sarcomere.
Myosin heads projecting from the sides of thick filaments go through a cycle of forming and then breaking bonds ('actomyosin
' cross-linkages or bridges) with the thin filaments of actin.
This repeated action - which resembles the movement of feet when walking, or oars when rowing - draws the actin on each end of the sarcomere inwards towards the centre of the sarcomeres.
After contraction: central gaps between ends of actin filaments reduced
This bridge breaking is assisted by ATP, which also causes the head to become 'cocked' and gives energy for the movement ('the power stroke') after the myosin head binds with specific sites on the actin filament. After this another crossbridge can be formed, further along the actin filament.
(Myosin shown in blue)
1 ATP hydrolysis
causes 'cocking' of myosin head
ADP and Pi still attached
2 Myosin forms cross-bridge
with Actin making actomyosin:
Pi released, ADP still attached
3 'Power stroke
': myosin neck changes angle,
thick filament moves towards Z-line, pulling thin filament of actin towards centre of sarcomere and shortening myofibril:
4 ATP binds
breaking previous cross-bridge
The cycle may then repeat, causing more contraction.
Proteins on the pull
There are several proteins which interact in muscles:
The two main ones actin and myosin make up 90% of protein in muscles.
Although actin is a globular protein, it forms a fibrous structure (F-actin) which associates with others to form a double helical structure
- rather like strands in rope.
It is attached to the Z-discs at the ends of sarcomeres and extends outwards into the cylindrical spaces within them forming 'thin filaments
Actin has attachment sites for myosin but these are initially obscured by the protein tropomyosin.
Double myosin structure and thick filament (many 'tails' wound together)
Myosin has a distinctive structure with a head, neck and tail. The tails bundle together to form double structures and 'thick filaments
', with heads splaying outwards like a bunch of flowers all along the bundled filaments. The head is at an angle so it is sometimes drawn looking like a golf club. At the tip of the head is a binding site can which attach to actin, and it interacts with ATP at a second site to change the angle in the neck region.
It can thus be said to have ATPase activity. It moves along the actin fibre by a 'rowing' action.
It is attached (via Titin, see below
) to the Z-discs at the ends of sarcomeres.
Tropomyosin blocks the binding sits on actin, but calcium ions cause it to move aside
As mentioned above, tropomyosin can associate with actin and prevent access by myosin. This happens when there are no calcium ions present (before/after muscle activity).
However in the presence of Ca2+
released from the sarcoplasmic reticulum, tropomyosin changes shape (assisted by another protein troponin
) and tropomyosin moves out of the way of the binding sites on actin, thus allowing actomyosin
crossbridges to form and causing filaments to move relative to one another.
- also known as connectin - is the third most abundant protein in muscle (after myosin and actin).
This forms a molecular spring giving 'passive elasticity' to muscle, as a result of the folding of about 33,000 amino acid residues , arranged in 244 protein domains. It runs between the Z-line and M-line, inside the myosin thick filament, so it spans half the length of the sarcomere.
Titin is said to be a giant protein, and it holds a number of records:
It is thought to be the largest known protein: over 1 µm in length. Its gene has the largest number of exons in a single gene, as well as the longest single exon.
And its full name - or the concatenation of the amino acid residue names which make its primary structure - has been claimed to be the longest word in English (189,819 letters)!
The M-line is composed of other proteins including Myomesin
which anchors the thick myosin filaments to titin filaments.
The Z-line is composed of a protein Alpha-actinin
which cross-links with actin and titin.