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MBB266 > Membrane Proteins (Baker) > Flashcards

Membrane Proteins (Baker) Flashcards

1
Q

What are the major classes of membrane proteins?

A
  • Peripheral
  • Integral
  • Lipid-Anchored
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2
Q

What is a requirement of membrane spanning helices?

A

Main chain C=O and N=H must be hydrogen bonded to neutralise their charges.

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3
Q

On the Kyte and Doolittle Scale (from Arg = -4.5 - Ile =+4.5), what is the minimum number that predicts a transmembrane helix?

A

+1.6

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4
Q

How many residues are used in the window on a hydropathy plot, and why?

A

19, as this is the number of residues in a typical transmembrane helix.

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5
Q

On a hydropathy plot, what value must peaks be above to be a transmembrane helix?

A

+1.6

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6
Q

Can Kyte Doolittle analysis be performed on transmembrane ß-barrels?

A

No - as the residues alternate between hydrophobic-hydrophilic

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7
Q

What are three kinds of hydrophobic protein anchors?

A
  1. Acylated
  2. Prenylated
  3. GPI Anchor
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8
Q

What is myristoylation?

A

An amide bond to an N-terminal glycine

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9
Q

Where is myristoylation typically found?

A

The inner leaflet of eukaryotes

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10
Q

What is prenylation?

A

A thioester link to C-terminal cysteine

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11
Q

When does prenylation occur?

A

Post-translationally

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12
Q

What is a thioesterase?

A

An enzyme that reversibly cleaves thioester lipid anchors.

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13
Q

What is GPI?

A

Glycosyl phophatidylinositol

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14
Q

Where do GPI anchors face?

A

Exoplasm

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15
Q

What is the general structure Aquaporin?

A
  • 6 transmembrane helices + 2 short helices per subunit
  • Aquaporin is a tetramer
  • Each subunit has a pore
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16
Q

How wide is the aquaporin pore?

A

2.8Å

17
Q

Which two highly conserved residues form the gate?

A

Arg195 and His180

18
Q

Is the pore lined with hydrophobic or hydrophilic residues?

A

Hydrophobic

19
Q

What happens in the aquaporin channel if an uninterrupted chain of water molecules forms?

A

The chain becomes a ‘proton conducting wire’, co-translating protons from one side of the membrane to another, forming hydronium ions

20
Q

What mechanism stops proton co-translation in aquaporin?

A

Water is stabilised by two highly conserved Asn residues.

They hydrogen bond the lone pairs on the O on water.

21
Q

What is the general structure of the Potassium Channel?

A
  • Four identical subunits
  • Each subunit has two membrane spanning helices
  • One channel per tetramer
  • P segment (selectivity filter)
22
Q

What the are key features of the potassium channel p segment?

A
  • Selectivity filter to allow only potassium
  • Homologous in all potassium channels
  • Mutations stop selectivity
  • Functional even if bacterial segment replaced with mammalian segment
23
Q

What is the rate of transport of the potassium channel?

A

108 ions/second

24
Q

What is the diameter of the potassium channel selectivity filter?

A

3Å

25
Q

What is the diameter of the potassium channel vestibule?

A

10Å

26
Q

What form of potassium travels through the selectivity filter?

A

Desolvated potassium

27
Q

Since potassium is larger than sodium, how is it selected?

A

It is transported as the desolvated ion - the desolvation energy is matched by specific coordination by the filter.

28
Q

What part of the selectivity filter coordinates the desolvated potassium ion?

A

The main chain carbonyl groups that line the channel.

29
Q

What is the difference in activation energy that favours potassium transport over sodium transport in the potassium channel?

A

1000X

30
Q

How many potassium binding sites are there in the selectivity filter?

A

4

31
Q

Which potassium binding sites can be occupied similtaneously, and why?

A

1 & 3 or 2 & 4, this prevents electrostatic repulsion.

32
Q

What is the purpose of the potassium channel vestibule?

A

It limits the hydrophobic distance that the ions have to travel through.

33
Q
A

MBB266 (4 decks)

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