Lecture 10: Myofibrillar proteins/muscle structures (Hayward) Flashcards Preview

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Flashcards in Lecture 10: Myofibrillar proteins/muscle structures (Hayward) Deck (28):


the principal protein comprising thick myofilaments


what are the functional properties of myosin

has a binding capacity for ATP and actin

possess an ATPase activity (myosin-ATPase).

each molecule binds 2 molecules of ATP


what are the structural features of myosin

has an asymmetrical protein with a head and a tail region

it's composed of 2 polypeptides arranged in an alpha-helix (myosin heavy chains) that form the tail region.

at one end, the chains are folded to form a globular head region, composed of 2 globular head (one per peptide chain).

the globular head also contains 4 small polypeptides (myosin light chains)


proteolytic digestion yields 2 fractions:

1. light meromyosin (LMM) fraction - most of the tail portion of the molecule

2. heavy meromyosin (HMM) fraction - consists of a short portion of the alpha-helical chain and the two globular heads.


which fraction has the capacity to bind ATP and actin as well as the ATPase activity?

HMM fraction


HMM can be further digested and fractionated to form 2 fractions:

1. the S-1 fraction - consists of the 2 globular heads.

2. S-2 fraction - composed of the short alpha-helical segment


where dot he functional properties of myosin reside? why?

in the globular head region

because the HMM S-2 fraction retains the ATP and and actin binding properties as well as the ATPase activity.


what do the LMM tail regions form?

they are arranged to form the back bone of the thick filament


what do the HMM S1 fraction and the S2 fraction form?

the HMM globular head (S1 fraction) and short heavy chain region (S2 fraction) form the cross bridges that attach myosin of thick filaments with actin of thin filaments.


how are myosin thick filaments arranged in the center of the filament?

tail-to-tail creating a smooth central zone, free of cross bridges

the molecules are arranged in repeating helical arrays such with cross-bridges projecting at 60-degree increments with 6 cross-bridges per helical repeat


what is actin

actin is the principal structural protein of thin myofilaments


functional properties of actin

actin possesses a binding capacity for myosin and activates myosin ATPase


structural properties of actin

G-actin (globular) monomers are polymerized to form F-actin (fibrous) strands. two F-actin strands are arranged in an alpha-helix form principal structure of thin filament


what are the functional properties of troponin and tropomyosin?

regulatory proteins that regulate the interaction of actin and myosin in response to calcium ion concentrations in the sarcoplasm.


what is tropomyosin

a fibrous protein composed of two polypeptide chains that sits within the groove of thin filament, F-actin strands. it has the capacity to block 7 G-actin myosin binding sites (preventing binding of myosin to actin)


what is troponin

a globular protein bound to tropomyosin at intervals along the thin filament. its composed of 3 subunits:
1. Tn-T subunit - binds troponin to tropomyosin
2. Tn-1 subunit inhibits myosin binding to actin
3. the Tn-C subunit reversibly binds free calcium in sarcoplasm


what is the action of the troponin-tropomyosin complex?

in response to changing calcium ion concentrations, it alternates between "blocking" and "freeing" the myosin binding sites on actin.


what are the steps of action of the troponin-tropomyosin complex?

1. during the state of relaxation, the sarcoplasmic calcium ion concentration is low. the tropomyosin molecules are elevated out of grooves and through the action of the Tn-1 subunit, block the myosin binding sites on actin.

2. with the depolarization of the sarcolemma, Ca ions are released from the SR into the sarcoplasm. the released Ca ions are taken up by the Tn-C subunits. this results in conformational changes in the Tn-1 subunit that moves the tropomyosin molecules deeper into the actin grooves, exposing the myosin binding sites, and the myosin heads then bind to actin.

3. when Ca ions are removed from the sarcoplasm by the SR, the Ca content of Tn-C is reduced and the effects of the troponin and tropomysin are reversed, moving tropomyosin out of the groove to block the byosin binding sites on actin.


at rest, what are each myosin head HMM, S1) bound to?

2 molecules of ATP tightly bound


what happens when they myosin binding sites on actin are freed by the action of the troponin-tropomyosin complex?

myosin binds to actin


what is the force-generating step required to generate sliding of the filaments?

the tilting of the head


what happensin the force generating step?

the myosin cross bridge heads tilt by 45 degrees and the ATP of the head is split to form ADP + P in the presence of the myosin ATPase also present in the head.

new ATP is added to the cross bridge heads and the actin and myosin dissociate. the heads then return to their original 90 angle position ready to reapeat the process at a new actin binding site on the thin filaments.


cyclical events of muscular contraction

1. during contraction of a muscle, Ca binds to troponin. this moves tropomysin out of the way and uncovers binding sites for myosin on the actin myofilaments.

2. ADP and P are attached to myosin head from the previous cycle of movement.

3. the myosin heads attach to the exposed binding sites on the actin myofilaments to form cross-bridges and the P is released

4. energy stored in the head of the myosin myofilament is used to move the head. this causes the actin myofilament to slide past the myosin myofilament. the ADP is released from the myosin head as it moves.

5. the bond between actin and myosin head is broken when an ATP binds to the myosin head. the ATP is broekn down to ADP and P, releasing energy which is stored in the myosin head for later movement. the head of the myosin molecule returns to its upright position and is ready to bind to actin again.

6. if Ca ions are still present, the entire sequence is repeated.


what controls the combination of the myosin-products complex and actin?

the regulatory proteins troponin and tropomyosin in response to an increase in Ca ion concentration


what causes rigor mortis?

actin and myosin are bound and unable to disassociate in the absence of ATP.

disassociation of actin and myosin requires the presence of ATP in the myosin heads


what determines the tension of a muscle fiber?

the sarcomere lengths of its myofibrils


the tension generated is directly proportional to what?

the number of cross-bridges overlapped by thin filaments


with reduced stretching, the tension developed is proportional to what?

the amount of overlap (number of cross bridges exposed to actin) between thick and thin filaments.

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