Locomotion & Movement

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Actin contains 2 other proteins (tropomyosin & troponin).

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Two filaments of tropomyosin run along the grooves of the F-actin

double helix.

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Troponin (having 3 subunits) is seen at regular intervals on

tropomyosin. In the resting state, a subunit of troponin masks the

binding sites for myosin on the actin filaments.

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Each myosin filament is a polymer of many monomeric proteins

called meromyosins.

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A meromyosin has 2 parts:

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Heavy meromyosin or HMM or cross arm (globular head + short

arm): It projects outwards.

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Light meromyosin or LMM (tail).

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The globular head is an active ATPase enzyme and has binding

sites for ATP and active sites for actin.

[AIPMT 2006]

+

the center of the sarcomere results in return

of myosin head back to low energy state.

Myosin head (low-

energy configuration)

energy configuration)

energy configuration)

Myosin head (low-

Thick

filament

Myosin head (high-

ATP

ATP

5

Thick filament

Thin filaments

1

The myosin head hydrolyzes

ATP to ADP and Pi

(High energy configuration)

The thin filament and the

myosin head create a cross

bridge when the myosin head

bind to actin.

2

3

4

Actin

Myosin-

binding sites

Cross-bridge

ADP

Pi

Pi

ADP

ADP

Pi

The myosin head is linked to

ATP (at low energy configura-

tion)

Thin filament

ADP and Pi are released

and the myosin head

pushes acting filament

inverts

toward

the

centre of the sarco-

mere. This is called

power stroke.

Myosin head is released from

actin by the binding of a new

molecule at ATP and a new cycle

begin.

The thin filament moves toward

Mechanism of muscle contraction