Muscle Movement

Question 1

What drives motion?

Answer 1

Rotary motors:
Bacteria - flagella
Rotary ATPases
Most motion in life is driven by linear motors - skeletal, cardiac, smooth muscles and in non-muscle cells

Question 2

Describe skeletal muscle?

Answer 2

Nearly half human body weight
More than 600 skeletal muscles in the body - they can be trained and atrophied
Skeletal muscle is striated - due to regular protein patterns (due to intermediate filaments which link all the Z-lines of neighbouring myofibrils)

Question 3

How do muscles work generally?

Answer 3

Muscles produce only tension - not pushing force
It uses antagonistic pairs: agonist and antagonist, and when contracted it’s job is to move the joint at the axis
The activation of myosin heads leads to contraction of the myofibril and ultimately the muscle only
Expansion has to be achieved by deactivating the myosin heads and having the sarcomeres pulled apart by outside forces
This is the antagonistic pairs at work

Question 4

Describe cardiac muscle?

Answer 4

Heart beats 60-80 bpm - 100,000 beats per day
Wrapped around the heart
Heart muscle is striated
There are lots of myofibrils (>90%) of the muscle
The repeating structure is called the muscle sarcomere

Question 5

What is the gross structure of skeletal muscles?

Answer 5

Whole muscle
Bundle of muscle fibres
Single muscle fibres
Myofibril

(this is multinucleate)

Question 6

How are the long giant cells in muscle fibres formed?

Answer 6

They form from the fusion of numerous smaller cells
The sarcoplasm (cytoplasm of muscle cells) are full of myofibril - bundles of protein filaments that cause contraction

Question 7

What is involved in the structure of myofibril?

Answer 7

Actin and Myosin
(darker where they overlap)

The repeating structure = sarcomere:
A band
H zone
I band 
M line
Z line

Question 8

What do the sections of the myofibril represent?

Answer 8

A band - length of the myosin filament
H zone - myosin only
I band - actin only
M line - middle of the sarcomere
Z line - end of the sarcomere

Question 9

Describe the structural features and roles of those features within myosin?

Answer 9

Domains: Motor (head) - Lever (neck) - Tail
Motor - binds actin, nucleotide, it also hydrolyses ATP to generate movement
Lever - Binds ‘light chains’ and will move in response to ATP hydrolysis
Tail - Binds cargo, they assemble to form thick filament in myofibril (thick filament transmits force generated by many molecules)

Contraction is driven by ATP hydrolysis

Question 10

Give an overview of myosin?

Answer 10

There are two heads per tail
The head is 16 nm long, pear shaped
Hexameric molecule

2 Heavy chains - dimerise using coiled-coil formation
S-1 - globular region, and each one binds 1 essential (ELC) and 1 regulatory light chain (RLC)
Nucleotide binding site - this is where ATP binds
Relay helix - monitors what is happening in the binding sites and relays this information to the lever in order for it to make conformational changes
Converter - this makes a ‘pocket’ for the lever to insert into and the lever rotates in that picket

Question 11

How does myosin come together within the filament?

Answer 11

Within the central region myosin uses antiparallel packing (bipolar packing) and as you move further out myosin uses parallel packing
It gives a regular ‘stagger’ between molecules

Question 12

Describe the ‘thin’ actin filament track?

Answer 12

Actin monomer is a globular protein (Mr - 42kDa) that polymerises into filaments
The filament is helical
○ G-actin = the globular protein
○ F-actin = the polymerised filament

It can be a two stranded right handed helix, pitch - 72 nm
Or alternatively a shallow left handed helix (genetic helix), pitch - 5.9 nm

F-actin is polar as the Z-disc of myofibril (‘barbed’ or ‘plus’ end)
○ Barbed end - fast growing end, found in the Z-disc (actin monomers will polymerise at this end)
○ Pointed end - slow growing end, found towards the middle of the sarcomere

Question 13

What are other thin filaments involved?

Answer 13

Tropomyosin - rod shaped, wound in the grooves of F-actin, contacting 7 consecutive actin subunits

Troponin - 3 subunits, TnC - Ca2+ binding protein, Tnl - binds to actin and TnT - binds to tropomyosin

Question 14

What are some minor proteins that help within the structural organisation of the muscle?

Answer 14

Titin - keeps the thick filament centered on the sarcomere

Nebulin - sets the length of the thin filament by acting as a template for actin polymerisation

Tropomodulin - caps the (-) end of F-actin
CapZ - caps the (+) end of F-actin

Question 15

What is the sliding filament theory/tilting crossbridge hypothesis?

Answer 15

This is how myosin interacts with actin in order to produce force
• Thin filament - actin
• Thick filament - myosin
• Crossbridge - connected to the thick filament by myosin (sub-fragment 2)

The thin filament ‘slides’ past the thick filament do to force
Movement of the crossbridge - causes the movement of the actin filament
The myosin is always trying to move towards the barbed end of the filament
It pulls the actin filaments in towards the middle of the muscle sarcomere
This shortens the sarcomere by 0.2 microns (around 10%) in each sarcomere
This leads to dramatic shortening in the overall muscle

All driven by ATP hydrolysis

Question 16

Why can muscle contraction take place?

Answer 16

Across the neuromuscular junction
Post synaptic side - the sarcolemma can transmit an action potential along it
The T-tubials in the sarcolemma, bringing the action potential directly to the sarcoplasmic reticulum

Question 17

Describe the first part of the sliding filament mechanism?

Answer 17

Tropomyosin prevents the myosin head from attaching to the binding site on actin
Ca2+ ions are released from the sarcoplasmic reticulum - binding to troponin, changing the tertiary structure causing the tropomyosin to pull away from the binding site on the actin
The myosin head attaches to the binding site on the actin forming a crossbridge

Question 18

Describe the ATP coupled part of the sliding filament mechanism?

Answer 18

ATP binds to a myosin causing myosin’s actin-binding site to open up and release its bound actin
Myosin’s active site closes around the ATP and hydrolysis into ADP+Pi, alters the myosin head into a ‘high energy conformation’
The myosin head binds weakly to another actin monomer
Myosin releases Pi causing its actin-binding site to close
A power stroke follows
ADP is released completing the cycle

Question 19

How are Ca2+ ions released from the sarcoplasmic reticulum?

Answer 19

The action potential reaches the sarcoplasmic reticulum, from direct route through the T-tubials
If the action potential exceeds threshold the Ca2+ ions are released

Question 20

What is some evidence for the sliding filament theory/tilting hypothesis?

Answer 20

Myosin is an ATPase:
Actin activates the myosin ATPase, so during ATP hydolysis the phosphate release step is greatly faster - 500 fold when actin is bound

Cycling crossbridge mechanism

The solving of the structure of the motor/lever domains

Question 21

What is the cycling crossbridge mechanism?

Answer 21

The power stroke - alters the direction of the myosin head, causing it to tilt as a result of losing a phosphate
However, the crossbridge needs to be re-set = synthesis of ATP

There are alterations within the head and the lever of the myosin
• Head - there are very small changes with the motor
• Lever - there is a large change in the orientation
We can see as myosin interacts with actin to generate force, the position of the lever changes dramatically, but the motor stays in the same relative orientation

Question 22

Describe the findings in solving the structure of the motor domain and lever?

Answer 22

The motor domain itself is composed of an N-terminal domain, a 50kDa domain and a converter domain which is linked to an alpha helix that forms the lever arm
○ The essential light chain is essential - required for motor activity
○ The regulatory light chain can regulate motor activity in some myosin’s, where it needs to become phosphorylated to activate the myosin
The two light chains bind to the alpha helix (from the heavy chain) in the lever arm to stabilise the lever

Question 23

What do the light chains bind to?

Answer 23

They bind to IQ motifs in the lever of the heavy chain:
I - Isoleucine
Q - Glutamine
R - Arginine
G - Glycine
However, some myosin my bind light chain similar proteins e.g. Calmodulin, or low molecular weight calmodulin-like proteins in other myosin’s

Question 24

What can also affect the shape of myosin other than ATP?

Answer 24

Actin activates the ATPase, so when it binds it must alter myosin

With a closed cleft on the myosin - this was the best fit and strongest binding to the actin and internal tension therefore when actin binds to myosin - the myosin cleft closes
This results in the opening of the nucleotide binding cleft - therefore ADP and Pi are released
This results in a rotation of the converter domain and the power stroke

Question 25

Give an example of myosin?

Answer 25

Myosin 5:
Identified in a mouse with a patchy coloured coat
Single molecules thought to carry cargoes
Each head has 6 light chains and the lever is 3 times as long = very large steps
The helical repeat of the strides will give a linear walk

This myosin holds on to its phosphate and ADP a little longer = bound to the tract for a longer time
It only detached briefly spending the majority of its time attached to the actin filament
= ADP release is the rate limiting step in the ATPase cycle

Question 26

How do the 2 heads within myosin 5 influence each other?

Answer 26

The 2 heads influence each other and there is strain between the two
When one head releases it’s phosphate and is ready to move on, the strain means it can’t complete it’s power stroke = gating of ADP release
The lead head therefore has slower ADP release than the rear head (trail)
ATP will preferentially binds to the rear head, as the rear head more likely to detach first

Question 27

What does myosin 6 do?

Answer 27

An additional connector in between the motor and the lever in order to point the lever the other way = motor goes backwards

Question 28

Where is actin also found?

Answer 28

Non-muscle actin known as microfilaments play roles in: cell shape, cell division, endocytosis, and organelle transport
Treadmilling - where the microfilament maintains a constant length by the subunits hat have added to the (+) end move toward the (−) end where they dissociate