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MBB161 > Moir - Molecular Biomechanics > Flashcards

Moir - Molecular Biomechanics Flashcards

1
Q

give the width of microfilaments and microtubules

A

mf: 8nm
mt: 25nm

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2
Q

what are microfilaments and microtubules made up of? which structure can be branched?

A

mf: actin can be branched
mt: tubulin never branched

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3
Q

how many human actin types are there, name the 3 groups?

A

4 alpha actin isoforms - musccle types

beta and gamma - non muscle types

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4
Q

what is the biologically active form of actin?

A

f-actin

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5
Q

by how many a/a’s do the different forms of actin vary by?

A

4 to 5

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6
Q

what is the pitch of f-actin?

A

14 monomers (7 monomers either side)

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7
Q

describe the shape of f-actin

A

helical

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8
Q

name the standard conditions at which f-actin spontaneously polymerises

A

1mM ATP 2mM Mg 50mM K

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9
Q

how can it be proved that f-actin is polar?

A

myosin binds at same angle of 45°

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10
Q

name and describe the 3 stages in the actin polymerisation graph (g-actin polymerising to f-actin)

A

lag: energetically unfavourable
extension: trimer production reaches critical level, production of f-actin energetically favourable
equilibrium: rate of the forward and backward reaction are the same

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11
Q

at which end of f-actin is g-actin added?

A

barbed(+)

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12
Q

which molecules/ions have to be bound to f-actin in order for it be stable?

A

ATP/Mg

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13
Q

what do capping proteins do and name the 2 used to cap the barbed and the pointed end

A

stop f-actin unraveling
CAPZ caps barbed
tropomodulin caps pointed end

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14
Q

what do monomer binding proteins do and name and state the function of 2 of them

A

isolates actin to build filaments without destroying them
thymosin beta 4 - binds ATP-G-actin forming a Tβ4-ATP-G-actin complex. When free ATP-G-actin is added to the F-actin filament the free ATP-G-actin concentration will go down. As this disrupts the free ATP-G-actin/Tβ4-ATP-G-actin equilibrium more Tβ4-ATP-G-actin will dissociate leading to more free ATP-G-actin.
Overall: binds ATP-G-actin to inhibit addition G-actin to F-actin. Important in removing actin clots in blood.
profilin - activated by PIP2. Binds ADP-G-actin which increases the loss of ADP from G-actin. ADP is replaced by ATP. Stops ATP-G-actin binding (-) end so therefore grows from (+) end.
Overall: promotes nucleotide exchange and filament formation.

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15
Q

what does dystrophin do? how much of the genome does it make up?

A

anchors f-actin to the cell mem via c-terminus

is 0.1% of genome

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16
Q

what is the cause of 90% of human dystrophies?

A

deletions in the dystrophin gene

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17
Q

describe the severity, molecular basis and impact of Duchenne muscular dystrophy

A

severe (life expectancy = teens/20s)
result of deletions which causes a frameshift mutation
mutant protein cannot bind actin and cell membrane => muscle weakness

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18
Q

describe the severity, molecular basis and impact of Becker muscular dystrophy

A 
mild severity (life expectancy = 50-60yrs)
result of deletion is no frameshift mutation
mutant protein is shorter but binds membrane and actin => functional muscle weakness
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19
Q

what kind of protein is fimbrin? describe its effect on actin morphology

A

actin bundling protein
rod shaped actin filaments
N&C terminal regions both actin binding sites
Allow creation of actin cables and microvilli

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20
Q

what kind of protein is filamin? describe its effect on actin morphology

A

gelation protein
bent rod shaped actin fibres
n and c terminus both actin binding sites
creates gel: important in cellular movement as it creates networks of f-actin not sheets

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21
Q

describe the function of gelsolin and the impact it has on the structure of actin

A

binds f actin and cleaves it
binds ‘barbed’ end = dissociation of the filament
important part of serum prevents actin clots in blood

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22
Q

describe the troponin I test for cardiac arrest

A

cardiac muscle tropinin I increases after heart attack
can’t use actin antibodies to measure amount of cardiac actin as it also recognises smooth muscle actin
TN-I found in striated muscle not smooth therefore its isoform is specific to the heart and can be used to diagnose heart attack

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23
Q

at what angle does actin bind pre-existing actin filaments?

A

70° from existing filament

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24
Q

how many subclasses of myosin are there? How are these identifiable?

A

20

Identifiable by ATP/Mg2+ binding site or genome sequence

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25
Q

Is the C or the N terminus the variable region in myosin?

A

C

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26
Q

Where is myosin II found?

A

highly ordered sarcomeres

actomyosin contractile bundles in non muscle cells (cell movement, cell division)

27
Q

what do myosins I and V do?

A

movement of vesicles

28
Q

what 2 different types of actin myosin interaction are there?

A

1) contraction (transient interaction with actin)

2) transport (maintenance of contact with actin)

29
Q

what unique C terminal feature does myosin II have?

A

coiled-coil sequence

30
Q

why does myosin II form a coiled-coil?

A

myosin II is a dimer made up of 2 alpha-helices.
Residues 1&4 are always located on the same side and are always hydrophobic. this creates a hydrophobic side on both the helices which have to be in contact to minimise disruption from water.

31
Q

why do some coiled-coil myosins not form a filament?

A

some myosins (myosin V) have regions of coiled-coil that contain non-helical regions. this is usually because of proline which cannot H bond with its amino group. this results in a dimer and not a coiled coil

32
Q

which myosin is monomeric?

A

myosin I

33
Q

what are nebulin and titin?

A

giant proteins
regulate length of actin and myosin in skeletal and cardiac muscle
titin is elastic and nebulin determines actin length

34
Q

why is myosin an ‘incompetent ATPase’

A

myosin hydrolyses MgATP to MgADP but only in the presence of actin. An incompetent ATPase can hydrolyse ATP to ADP but not remove an actin monomer.

35
Q

what do tropomyosin and troponin do? how do they do this?

A

tropomyosin and troponin are regulatory proteins that regulate the contraction of skeletal and cardiac muscle. tropomyosin is wrapped around actin and troponin is bound to tropomyosin. When Ca2+ is released (ie when a muscle contracts) it binds troponin, exposing binding sites on actin. Myosin heads bind these sites and pull actin past it the myosin filament.

36
Q

describe the changes in Ca2+ concentration in contracted and relaxed muscle and explain why this happens

A

In contracting muscle Ca2+ is at 10^-5M because the presence of calcium allows myosin heads to bind to actin and hydrolyse MgATP to MgADP. In relaxed muscle the Ca2+ concentration is 10^-8M meaning the actin-activated ATPase properties of myosin are inhibited as there is v little Ca2+.

37
Q

what is the incidence and effect of myosin cardiac mutations?

A

1:500

mutations impair cardiac function left ventricle is enlarged and heart failure can occur during stress

38
Q

what is the cause of myosin related deafness?

A

stereocilia made up of myosin vibrate and mutations cause impaired function

39
Q

how are different isoforms of myosin localised in a cell?

A

myosin antibodies

40
Q

what is the function of myosin I and II in cell locomotion?

A

myosin I at leading edge sends out lamelipodia

myosin II at rear of cell pushes the cell

41
Q

why is it important that actin filaments form branches?

A

to allow the precise delivery of cargo

42
Q

whats the difference in stable and transient microtubules?

A

stable MTs: found in non-differentiating cells

transient MTs: found in dividing cells

43
Q

give an example of where a stable MT may be found and where a transient MT may be found

A

stable: neurones, cilia, flagella
transient: dividing cells - essential for organisation of chromosomes in mitosis

44
Q

what are microtubules composed of?

A

an alpha beta heterodimer of tubulin

45
Q

describe the binding of GTP/Mg to alpha and beta tubulin

A

alpha tubulin: binds GTP/Mg irreversibly

beta tubulin: binds GTP/Mg reversibly and hydrolyses it to GDP/Mg

46
Q

what is taxol? where does it bind on the tubulin heterodimer?

A

taxol is an anticancer drug

it binds to beta tubulin

47
Q

how many protofilaments are there in a microtubule?

A

13

48
Q

which end of the microtubule grows?

A

the beta-tubulin (+) end grows (the heterodimer is added with the alpha monomer on the left and the beta monomer on the right - forms a helix with alpha ring at the bottom and a beta ring at the top)

49
Q

how long is an alpha-beta heterodimer

A

8nm

50
Q

how does an MT grow?

A

addition of heterodimers to the (+) end, this end grows faster than the (-) end. The dimers added contain GTP and are therefore stable. GTP in beta-tublin hydrolyses to GDP over time and therefore when the GTP cap is lost any GDP bound beta-tubulin will dissociate.

51
Q

what is the model system for microtubules and why?

A

axonal transport in neurons is used as a model system. used because they are very stable.

52
Q

where are proteins synthesised in a neuron? why is this important?

A

protein synthesis occurs in the cells which allows products to be visualised as they are transported down the axon. this has shown that intact organelles are transported around the neuron and at the end of their life they are taken back to the cell body for re-use.

53
Q

which two motor proteins travel on microtubules? which direction do they travel?

A

kinesin (+ directed for delivery - away from the nucleus)

dynein (- directed for return - towards the nucleus)

54
Q

describe kinesin structure

A

kinesins contains 2 globular head domains (which are smaller than the myosin globular head domains)
they differ in their tail domain.
composed of a coiled coil

55
Q

what are the 2 types of kinesin

A

cytosolic kinesin (carries vesicles and organelles) and mitotic kinesin

56
Q

what is the function of dyneins

A

intracellular transport and cell movement (flagella and cilia)

57
Q

how is cargo delivered precisely?

A

MTs aren’t branched whereas actin is highly branched. MTs and kinesin and dynein are used for large-scale distribution whereas actin and myosin (V) are used to precisely deliver cargo to locations. Myosin V associates with MT and waits for kinesin carrying cargo. Kinesin picks up myosin and carries it until it reaches actin. It then hands the cargo to myosin which then picks up kinesin and delivers the cargo to the specific place.

58
Q

what are MT stabilising proteins? describe their structure and name 2

A

proteins that control the spacing of adjacent microtubules
they have an MT binding domain and a projection arm (which can also bind MTs)
Tau (axons) and MAP2 (dendrons)

59
Q

which MT stabilising protein is related to Alzheimer’s disease?

A

Tau protein (abberant polymerisation of Tau is linked to neurodegenerative disorders)

60
Q

what is the effect and side effects of taxol?

A

taxol inhibits MT shortening and therefore stops cell division. Taxol works on all cells however and is therefore toxic, it also needs to be administered using organic solvents.

61
Q

define ‘inchworm’ and ‘hand-over-hand’

A

inchworm: same globular head of a motor protein always leads and the other drags behind
hand-over-hand: the leading globular head alternates ie walks like a human

62
Q

how was walking myosin studied? what was the result of the study?

A

used an optical probe to attach to one myosin head domain. they measured how far the probe traveled (36nm if inchworm and 72nm if hand-over-hand). myosin walks hand-over-hand

63
Q

is kinesin walking hand-over-hand or inchworm

A

hand-over-hand

MBB161 (10 decks)

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