Lecture 10: Myofibrillar proteins/muscle structures (Hayward)

Question 1

myosin

Answer 1

the principal protein comprising thick myofilaments

Question 2

what are the functional properties of myosin

Answer 2

has a binding capacity for ATP and actin

possess an ATPase activity (myosin-ATPase).

each molecule binds 2 molecules of ATP

Question 3

what are the structural features of myosin

Answer 3

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)

Question 4

proteolytic digestion yields 2 fractions:

Answer 4

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.

Question 5

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

Answer 5

HMM fraction

Question 6

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

Answer 6

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

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

Question 7

where dot he functional properties of myosin reside? why?

Answer 7

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.

Question 8

what do the LMM tail regions form?

Answer 8

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

Question 9

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

Answer 9

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.

Question 10

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

Answer 10

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

Question 11

what is actin

Answer 11

actin is the principal structural protein of thin myofilaments

Question 12

functional properties of actin

Answer 12

actin possesses a binding capacity for myosin and activates myosin ATPase

Question 13

structural properties of actin

Answer 13

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

Question 14

what are the functional properties of troponin and tropomyosin?

Answer 14

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

Question 15

what is tropomyosin

Answer 15

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)

Question 16

what is troponin

Answer 16

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

Question 17

what is the action of the troponin-tropomyosin complex?

Answer 17

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

Question 18

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

Answer 18

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.

Question 19

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

Answer 19

2 molecules of ATP tightly bound

Question 20

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

Answer 20

myosin binds to actin

Question 21

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

Answer 21

the tilting of the head

Question 22

what happensin the force generating step?

Answer 22

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.

Question 23

cyclical events of muscular contraction

Answer 23

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.

Question 24

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

Answer 24

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

Question 25

what causes rigor mortis?

Answer 25

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

Question 26

what determines the tension of a muscle fiber?

Answer 26

the sarcomere lengths of its myofibrils

Question 27

the tension generated is directly proportional to what?

Answer 27

the number of cross-bridges overlapped by thin filaments

Question 28

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

Answer 28

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