012 muscle proteins and contractile mechanisms Flashcards
1
Q
what is the functional unit of muscle called?
A
- sarcomere
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
3
Q
what is the dark A band in sarcomere?
A
- myosin and some actin overlap
4
Q
what is the light I band in sarcomere?
A
- actin only
5
Q
What is the H zone of sarcomere?
A
centre of A band, myosin only
6
Q
what is the M line of sarcomere?
A
- centre of A band of myosin
7
Q
what is the Z line of sarcomere?
A
- centre of I band of actin
8
Q
what are the diameters of actin and myosin?
A
- myosin = 15nm
- actin = 9nm
9
Q
what % of muscle is contractile protein?
A
60%
- 33% = myosin
- 14% = actin
- 3.5% = tropomyosin
- 3.5% = troponin
10
Q
what is the biggest muscle protein?
A
- Titin = 34,350 amino acids long
11
Q
what % does a muscle shorten by on contraction?
A
33%
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
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
14
Q
how many types of myosin are there and which one do we talk about as doctors?
A
10 types
- myosin type 2
15
Q
how many subunits does myosin 2 have?
A
- 6
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
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
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)
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
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
21
Q
describe the quaternary structure of myosin
A
- overall appearance is a thick filament with heads
22
Q
what are the 2 forms of actin?
A
- G actin
- F actin
23
Q
describe G-actin
A
- globular
- monomeric form of actin found at low ionic strength
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
25
describe the structure and function of F-actin
- 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
what is the function of actin in all cells?
- provide lattice that supports the plasma membrane and organises the cytosol
27
How does the Angel of Death mushroom work (hint muscle filaments)?
- contains cyclic peptide toxin called phalloidin which locks F actin fibres together (become paralysed)
28
what are the 4 stages of muscle contraction in terms of myosin?
1. ATP binding
2. ATP hydrolysis
3. release of phosphate
4. release of ADP
29
describe the 1st stage of muscle contraction (ATP binding ) in terms of ATP and myosin heads.
- 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
describe the 2nd stage of muscle contraction (ATP hydrolysis) in terms of ATP and myosin heads
- 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
describe the 3rd stage of muscle contraction (release of phosphate) in terms of ATP and myosin heads
- release of Pi from myosin causes myosin to strongly bind to actin
32
describe the 4th stage of muscle contraction (release of ADP) in terms of ATP and myosin heads
- 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
what happens to myosin when there is no ATP?
- myosin head bound to actin
34
what happens to myosin when ATP binds to S1 head?
- myosin head releases actin
35
what happens to myosin when ATP hydrolyses into ADP and Pi?
myosin head moves into new position
36
what happens to myosin during loss of Pi?
- binding of myosin head to actin
37
what happens to myosin during loss of ADP?
- power stroke
- myosin pulling actin filaments along, shortening sarcomere
38
describe the structure of tropomyosin
- 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
what is overall function of tropomyosin?
- a regulatory protein
- blocks S1 of myosin binding to actin when Ca is low
40
describe the 5th stage of muscle contraction, used to control the process
- 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
what are the 3 different troponin forms?
- troponin, C, I and T
42
what is troponin c for?
- binds to Ca at 2 sites = which causes a conformational change to troponin I and T (expose binding site for actin)
43
what is troponin I for?
- binds to actin
44
what is troponin T for?
- binds to tropomyosin = controls positioning of tropomyosin on actin filament (blocking or exposing actin binding site for myosin)
45
which muscle protein can you test for if someone is having a heart attack?
- troponin
- test blood for troponin as heart attack will damage muscle cells and release troponin into blood
46
describe 5 differences between skeletal muscle and smooth muscle
- 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
what are the 4 pathways of smooth muscle contraction?
1. caldesmon
2. phosphorylation
3. myosin light chain kinase
4. humoral factors
48
describe the caldesmon pathway of controlling smooth muscle contraction
- when Ca is low, caldesmon forms a complex with tropomyosin and actin and restricts myosin binding to actin = relax
49
describe the phosphorylation by kinases pathway of controlling smooth muscle contraction
- when caldesmon is phosphorylated, caldesmon cannot bind to actin, thus cannot inhibit myosin binding to actin = contract
50
describe the myosin light chain kinase pathway of controlling smooth muscle contraction
- 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
describe the humoral factor pathway of controlling smooth muscle contraction
- humoral factors e.g. hormones, can activate or inhibit contraction
52
describe the useful property of calcium to induce large conformational changes in muscle contraction
- 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
why must Ca conc be kept low and how does it do this, and what property does this give?
- 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
what are the 4 roles of calcium in muscle?
- 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
describe how calcium triggers contraction in striated muscles
- causes thin filaments to rearrange structurally so thick and thin filaments interact with each other
- myosin ATPase is activated, and filament interdigitation occurs
56
describe how calcium triggers contraction in smooth muscle
- by complexing with calmodulin and activating myosin light chain kinase to phosphorylate regulatory light chain (RLC) causing myosin-actin interactions
57
describe how calcium ensures the sealing of the sarcolemma of striated muscle fibres
- so they do not spontaneously leak Na or K and depolarise
- essential for neural control of skeletal muscle contraction
58
what is the clinical significance of sealing the sarcolemma?
- tetanus toxin and alkalosis both permeabilise the sarcolemma and allow unprogrammed contraction due to leaking ions
59
what is the significance of 'in less than 1 second, 5nM Ca partially activates phosphorylase kinase in striated muscle' ?
- 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
what is calmodulin?
- 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
what is caldesmon?
- an actin-myosin binding protein
62
why does the body stiffen after death (rigor mortis)
- depletion of ATP after death
- no ATP = myosin binds to actin = contract = stiffen
