Conformational changes in Proteins

Question 1

how the conformation of proteins can change in response to certain stimuli

Answer 1

1. changing membrane potential in voltage gated ion channels passes from the spinal cord down to neuromuscular fibres
2. binding of ligand in the acetylcholine receptor in the muscle
   achieved by 3. calcium binding to a muscle protein Troponin C
3. ATP hydrolysis in the myosin headpiece

Question 2

Vm membrane potential is maintained by the sodium potassium ATPase

Answer 2

the difference in electrostatic potential always exists between the inside and outside of the a cell

memenr

Question 3

action potential propagated along a neurons axon by tightly controlled voltage dependent activtion of a number of cation channels sodium potassium and calcium ions - each with a selectivity filter for its cation and in and activation gate in 3 conformational states

1. deactivated -closed
2. activated - open
3. inactivated closed - the cause of refractory period in the action potential

axon terminal is a specialised structure at the end of the axon that is used to release neurotransmitter chemicals and communicate with target neurons

Answer 3

when an excitable cell is depolarised beyond a threshold the cell will undergo an action potential
swing of polarity of the membrane potential from negative to positive and back for a few milliseconds

1 action potential - sodium in potassim out via voltage gated channels
2 secretor vesicles containing acetylcholine in presence of calciumion influx

3 neurotransmitter secreted into synaptic cleft
4 binds to receptor in postsynaptic neuron
channel that allows calcium ions to move into cell
5 acetylcholine receptor ion channels = action potential continues

ions repumped to natural level before another action potential comes along

Question 4

conformtional change voltage gated sodium channel of neurons

Answer 4

only alpha subunit essential
4 homologous domains wrapped around central transmembrane channel lined w polar amino acids containing 6 helices
helix 4 in each domain is a voltage sensor
helix 6 is activation gate
helices 5 6 loops line channel pore forming a selectivity filter

domains 3 4 inactivation gate closes soon after activation gate opens= milllisecons cycle time transmission of action potential

movement of helix 4 perpendicuar to the plane of the membrane in response to a change in the membrane potential - positivyely charge helix 4 is pulled inwards by the inside negative membrane potential

depolarisation lessens the pull and helix 4 relaxes outwards = helical movement is communicated to action gate inducing conformational changes that open the channel in response to depolarisation

The movement of.
one helix in response to a changing membrane potential which enables ions to flow and enables propagation of the membrane potential.

Question 5

neuromuscular junction

Answer 5

links axon terminal w the highy excitable region of muscle fibre plasma membrane = motor end plate responsible for initiation of action potentials across muscle surface = contraction

signalling via acetylcholine released from axon terminal pon infux of calcium ions

Question 6

conformational change in the nicotinic acetylcholine receptor ion channel

Answer 6

5 subunits each w 4 transmembrane helices (m1-4)
each alpha subunit can bind acetylcholine

the 5 subunits surround a central pore which is lines with m2 helices

top and bottom of channel are rings of -vly charged residues

there are 5 non polar leucine side chains - 1 frm each M2 helix protruding into the channel preventing small ions such as sodium, potassium and calcium passing through

hydrophobic residues with restricted
when 2 acetylcholine molecules bind to this compex tey initiate a twisiting of these domains
takes the hydrophobic residues out of channel opning up channel enabling passage of ions
hydrophobic lined channel to a polar lined channel by the rotation of the subunits

to shut the channel = acetylcholineesterase hydrolyses acehtylcholine into choline and acetic acis = neuron returns to resting state after activation
high turnover rate

sarin nerve gas inactivates enzyme by transferring a methyl phosphonate group to the seriene disabling it = leaving muscle contraction on

Question 7

skeletal muscle

Answer 7

muscle fibres consisting of single elongated multinuclear cells from fusion of many precursor cells

within fires are myofibrils surrounded by membranous sarcoplasmic reticulum

the organisation of thick and thin filaments in the myofibril gives it a restricte appearance

when muscles contract = bands narrow + Z disk come closer

thick filaments = myosin molecules in a biolar structure

2 heavy chains and 4 light chains

long carboxyl terminus tail and short head

coiled coil structure

n terminus = globular head domains containing ATP hydrolysis site

myosin + trysin = light meromyocin + heavy meromyosin + pepsin = can purify single subunits

ATP hydrolysis induces conformation changes in myosin head = muscle contraction

Question 8

actin

Answer 8

in all eukaryotes

G-actin single actin monomers associate to form F actin long polymers

think filaments = active + troponin + tropomyosin

each actin monomer binds tightly to 1 myosin head

muscle contraction occurs by sliding of the thick and thin filaments past each other so that the Z disks in neighbouring I banks approach each other

think + thick filaments are interleaves = each thick filaments is surround by 6 thins

Question 9

conformational change in troponin regulates actin myosin nteraction

Answer 9

at rest myosin binding sites on F actin are blocked by tropomysosin

conformational changes in troponin induced by calcium ion bindin displaces the tropomyosin and exposes the binding sites for myosin on actin

the tropomyosin molecules form a long alpha helical coiled coils

measuring troponin = to tell if someones had a heart attack because when muscle cells die they shed troponin

Question 10

crossbridge movements during muscle contraction

Answer 10

1. ATP binds to myosin head = dissociation from actin
2. A tightly bound ATP is hydrolysed a conformational change occurs. ADP + inorganic phosphate remain associated with the myosin head
3. myosin head attaches to actin filament = release of inorganic phosphate which triggers a power stroke = 4. conformational change in myosin = ADP release 4
   ACtin and Myosin filaments move relative to one another

Question 11

conformational changes are in response to

Answer 11

conformational changes are in response to1. membrane potential change
2. binding of acetylcholine or calcium ion ligands

3. ATP hydrolysis
   in active transport SERCA and muscle contraction