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Movement and Musculoskeletal Biology Emily > 012 muscle proteins and contractile mechanisms > Flashcards

012 muscle proteins and contractile mechanisms Flashcards

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1
Q

what is the functional unit of muscle called?

A
  • sarcomere
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2
Q

describe the structure of a sarcomere

A
  • myosin and actin protein filaments
  • thick filament = myosin
  • thin filament = actin
  • dark A band = myosin (and actin overlap)
  • light I band = actin (only actin)
  • Centre of A band = H zone (myosin only) and M line in centre of H zone
  • Centre of I band = Z line
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3
Q

what is the dark A band in sarcomere?

A
  • myosin and some actin overlap
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4
Q

what is the light I band in sarcomere?

A
  • actin only
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5
Q

What is the H zone of sarcomere?

A

centre of A band, myosin only

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6
Q

what is the M line of sarcomere?

A
  • centre of A band of myosin
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7
Q

what is the Z line of sarcomere?

A
  • centre of I band of actin
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8
Q

what are the diameters of actin and myosin?

A
  • myosin = 15nm
  • actin = 9nm
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9
Q

what % of muscle is contractile protein?

A

60%
- 33% = myosin
- 14% = actin
- 3.5% = tropomyosin
- 3.5% = troponin

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10
Q

what is the biggest muscle protein?

A
  • Titin = 34,350 amino acids long
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11
Q

what % does a muscle shorten by on contraction?

A

33%

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12
Q

what is the sliding filament model?

A
  • thick and thin filaments slide past each other in the sarcomeres
  • this shortens the sarcomeres, shortening the muscle and causing contraction
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13
Q

what is the mechanical process between myosin and actin in contraction?

A
  • ATP- dependent interactions between myosin and actin cause myosin heads to move
  • the myosin heads walk down’ the fixed actin filaments, bind to actin and move the filaments past them and repeat = shortening sarcomere
  • myosin is a motor protein = a mechanochemical enzyme
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14
Q

how many types of myosin are there and which one do we talk about as doctors?

A

10 types
- myosin type 2

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15
Q

how many subunits does myosin 2 have?

A
  • 6
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16
Q

what is the overall primary structure of myosin 2?

A
  • 2 copies of 3 different protein chains
  • heavy chain
  • light chain 1
  • light chain 2
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17
Q

describe the primary structure of the heavy chain of myosin 2

A
  • 2 a-helical tails
  • Light meromyosin (LMM) - fibrous - forms rods by self-association
  • head S1 = globular = contains ATP binding site, an actin binding site and light chain binding site
  • head S2 = fibrous
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18
Q

describe the primary structure of the light chains of myosin 2

A
  • light chain 1 = regulatory light chain (RLC)
  • light chain 2 = essential light chain (ELC)
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19
Q

describe the secondary structure of myosin 2

A
  • LMM of heavy chain contains 2 alpha helices coiled around each other at 3.5 residues per turn
  • hydrophobic amino acids at contact points, holding it together
  • charged residues on outside
  • 170nm in length
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20
Q

describe the tertiary structure of myosin 2

A
  • S1 is globular and contains binding sites for ATP and actin
  • 17nm long
  • S1 has 2 special features:
    - ATPase site and actin site are on opposite sides of N-terminal end
    - Long neck region which RLC and ELC light chain bind
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21
Q

describe the quaternary structure of myosin

A
  • overall appearance is a thick filament with heads
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22
Q

what are the 2 forms of actin?

A
  • G actin
  • F actin
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23
Q

describe G-actin

A
  • globular
  • monomeric form of actin found at low ionic strength
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24
Q

describe F-actin

A
  • fibrous
  • exists in thin filaments in muscle as a coiled coil
  • when ionic stress increases to physiological, G-actin polymerises to form F actin
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25
Q

describe the structure and function of F-actin

A
  • helical structure that repeats itself after 13 subunits
  • looks like 2 strings of beads wound around each other
  • each subunit touches 4 others
  • ATP binds to actin to accelerate F-actin formation
  • in all cells, actin filaments provide a lattice that supports the plasma membrane and organises the cytosol
  • in muscle = actin provides track for myosin to move along
26
Q

what is the function of actin in all cells?

A
  • provide lattice that supports the plasma membrane and organises the cytosol
27
Q

How does the Angel of Death mushroom work (hint muscle filaments)?

A
  • contains cyclic peptide toxin called phalloidin which locks F actin fibres together (become paralysed)
28
Q

what are the 4 stages of muscle contraction in terms of myosin?

A
  1. ATP binding
  2. ATP hydrolysis
  3. release of phosphate
  4. release of ADP
29
Q

describe the 1st stage of muscle contraction (ATP binding ) in terms of ATP and myosin heads.

A
  • when ATP binds, it closes the ATP cleft in S1 head and opens actin binding cleft in S1 head, so weakening binding to actin
  • myosin then dissociates from actin
30
Q

describe the 2nd stage of muscle contraction (ATP hydrolysis) in terms of ATP and myosin heads

A
  • in the free S1 head, ATP is hydrolysed to ADP and Pi
  • this causes ATP-cleft to close and S1 bends its shape ready to bind to actin again in a new position
31
Q

describe the 3rd stage of muscle contraction (release of phosphate) in terms of ATP and myosin heads

A
  • release of Pi from myosin causes myosin to strongly bind to actin
32
Q

describe the 4th stage of muscle contraction (release of ADP) in terms of ATP and myosin heads

A
  • loss of ADP from ATP-cleft causes S1 head to swivel and striaghten back up to starting position when it can bind to actin
  • power stroke
  • in course of this conformational change, neck and tail of myosin shifts along actin filament by 10nm (or 2 actin monomers)
33
Q

what happens to myosin when there is no ATP?

A
  • myosin head bound to actin
34
Q

what happens to myosin when ATP binds to S1 head?

A
  • myosin head releases actin
35
Q

what happens to myosin when ATP hydrolyses into ADP and Pi?

A

myosin head moves into new position

36
Q

what happens to myosin during loss of Pi?

A
  • binding of myosin head to actin
37
Q

what happens to myosin during loss of ADP?

A
  • power stroke
  • myosin pulling actin filaments along, shortening sarcomere
38
Q

describe the structure of tropomyosin

A
  • 40nm long 2 stranded a-helix rod with 7 actin-binding sites that is aligns along thin filaments of F-actin
  • makes up 33% of weight of thin filament
39
Q

what is overall function of tropomyosin?

A
  • a regulatory protein
  • blocks S1 of myosin binding to actin when Ca is low
40
Q

describe the 5th stage of muscle contraction, used to control the process

A
  • in absence of ATP, myosin does not interact with actin unless the inhibition from tropomyosin and troponin is switched off by a rise in Ca conc
  • in relaxed muscle, Ca is stored in the sarcoplasmic reticulum, Ca/ATPase pump (SERCA) lowers Ca conc, but nerve impulse releases Ca into muscle cytosol = contraction
41
Q

what are the 3 different troponin forms?

A
  • troponin, C, I and T
42
Q

what is troponin c for?

A
  • binds to Ca at 2 sites = which causes a conformational change to troponin I and T (expose binding site for actin)
43
Q

what is troponin I for?

A
  • binds to actin
44
Q

what is troponin T for?

A
  • binds to tropomyosin = controls positioning of tropomyosin on actin filament (blocking or exposing actin binding site for myosin)
45
Q

which muscle protein can you test for if someone is having a heart attack?

A
  • troponin
  • test blood for troponin as heart attack will damage muscle cells and release troponin into blood
46
Q

describe 5 differences between skeletal muscle and smooth muscle

A
  • smooth muscle is not striated
  • smooth muscle is autonomic, skeletal is somatic
  • smooth muscle is not organised into well-ordered sarcomeres, but instead into loose bundles of thick and thin filaments at dense bodies in cytosol
  • less ordered contraction not simple nerve innervation
  • no troponin
47
Q

what are the 4 pathways of smooth muscle contraction?

A
  1. caldesmon
  2. phosphorylation
  3. myosin light chain kinase
  4. humoral factors
48
Q

describe the caldesmon pathway of controlling smooth muscle contraction

A
  • when Ca is low, caldesmon forms a complex with tropomyosin and actin and restricts myosin binding to actin = relax
49
Q

describe the phosphorylation by kinases pathway of controlling smooth muscle contraction

A
  • when caldesmon is phosphorylated, caldesmon cannot bind to actin, thus cannot inhibit myosin binding to actin = contract
50
Q

describe the myosin light chain kinase pathway of controlling smooth muscle contraction

A
  • phosphorylates regulatory light chain (RLC) of myosin
  • RLC inhibits actin stimulation of myosin ATPase activity
  • Ca is needed to activate myosin light chain kinase, which is does by first binding to calmodulin
  • then Ca-calmodulin complex binds to RLC to activate it
  • when RLC is phosphorylated, inhibition is removed
51
Q

describe the humoral factor pathway of controlling smooth muscle contraction

A
  • humoral factors e.g. hormones, can activate or inhibit contraction
52
Q

describe the useful property of calcium to induce large conformational changes in muscle contraction

A
  • Ca can co-ordinate 6-8 oxygen atoms in asymmetric complexes and therefore cross-link different segments of a protein and induce large conformational changes
  • so, an intracellular messenger in many eukaryotic signal transducing pathways
  • e.g. vision, phosphoinositide cascade, regulation of muscle contraction
53
Q

why must Ca conc be kept low and how does it do this, and what property does this give?

A
  • phosphate esters are abundant white calcium phosphate is very insoluble = cause deposits
  • so rapid increase in cytosolic calcium through calcium channels can be used for signalling
54
Q

what are the 4 roles of calcium in muscle?

A
  • triggers contraction in striated muscles
  • triggers contraction in smooth muscles
  • ensures sealing of the sarcolemma of striated muscle fibres
  • in less than 1 second, 5nm Ca activates phosphorylase kinase in striated muscle
55
Q

describe how calcium triggers contraction in striated muscles

A
  • causes thin filaments to rearrange structurally so thick and thin filaments interact with each other
  • myosin ATPase is activated, and filament interdigitation occurs
56
Q

describe how calcium triggers contraction in smooth muscle

A
  • by complexing with calmodulin and activating myosin light chain kinase to phosphorylate regulatory light chain (RLC) causing myosin-actin interactions
57
Q

describe how calcium ensures the sealing of the sarcolemma of striated muscle fibres

A
  • so they do not spontaneously leak Na or K and depolarise
  • essential for neural control of skeletal muscle contraction
58
Q

what is the clinical significance of sealing the sarcolemma?

A
  • tetanus toxin and alkalosis both permeabilise the sarcolemma and allow unprogrammed contraction due to leaking ions
59
Q

what is the significance of ‘in less than 1 second, 5nM Ca partially activates phosphorylase kinase in striated muscle’ ?

A
  • this initiates glycogenolysis exactly as work begins, and gives muscle partial access to this large furl store
  • glycogen is the main initial fuel for all unrehearsed work
60
Q

what is calmodulin?

A
  • a multifunctional intermediate calcium binding messenger protein in all eukaryotic cells
  • it is an intracellular target of the secondary messenger Ca and the binding of Ca is required for activation of calmodulin
61
Q

what is caldesmon?

A
  • an actin-myosin binding protein
62
Q

why does the body stiffen after death (rigor mortis)

A
  • depletion of ATP after death
  • no ATP = myosin binds to actin = contract = stiffen

Movement and Musculoskeletal Biology Emily (23 decks)

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