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CHEM 472 Biochem > Final: Chapter 9 - Membrane Structure > Flashcards

Final: Chapter 9 - Membrane Structure Flashcards

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1
Q

What are the two main functions of a membrane?

A

1) Define inside and outside of the cell or organelle
2) Control permeability of ions and small molecules

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2
Q

What are membranes primarily composed of?

A

lipids and proteins

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3
Q

What are three common lipids that are found in cell membranes?

A

Glycerophospholipids, sphingolipids, and sterols

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4
Q

What percentage of protein content in membranes?

A

15-80%

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5
Q

What is the purpose of proteins in cell membranes?

A

to add function

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6
Q

What are the three types of proteins based on their association with a membrane?

A

1) Peripheral 2) Integral 3) Lipid-anchored

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7
Q

What is a peripheral membrane protein?

A

One that does not penetrate the lipid bilayer and is mainly associated to the polar groups on the exterior of the membrane via H-bonds and ionic bonds

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8
Q

What is an integral membrane protein?

A

One that penetrates into or through the lipid bilayer, associating with the hydrophobic interior with its own hydrophobic portions

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9
Q

What is a lipid-anchored membrane protein?

A

One that covalently linked to lipids in the membrane

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10
Q

How is a peripheral protein cleaved from a membrane?

A

Change in pH or salt concentration

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11
Q

How is an integral protein cleaved from a membrane?

A

A nonpolar solvent or detergent must be used to break apart the membrane

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12
Q

How is a lipid-anchored protein cleaved from a membrane?

A

Must selectively cleave protein or destroy membrane to release it

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13
Q

What is the fluid mosaic model?

A

describes how membranes are composed of a dynamic and fluid lipid bilayer containing a diverse array of proteins and molecules that move around within the membrane

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14
Q

Describe the structure of an integral membrane protein

A

The membrane spanning portion is dominated by alpha-helices and beta-sheets

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15
Q

What are two reasons that alpha-helices and beta-sheets are more favorable in the lipid bilayer?

A

1) They are stabilized by H-bonds between AAs and their backbone which prevents interactions with the non-polar inner lipid bilayer
2) alpha-helices are primarily composed of nonpolar amino acids

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16
Q

What does a negative hydropathic index value signify?

A

The AA is hydrophilic (polar)

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17
Q

Whay does a positive hydropathic index value signify?

A

The AA is hydrophobic (nonpolar)

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18
Q

Do transmembrane protein domains exhibit a negative or positive hydropathic index overall?

A

Positive

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19
Q

Do extracellular and intercellular protein domains exhibit a negative or positive hydropathic index overall?

A

Negative

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20
Q

What is a beta-barrel?

A

A pore in the cell membrane formed by beta-sheets

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21
Q

Describe AA orientation in a beta-barrel?

A

Nonpolar AAs point outwards and face the lipid bilayer
Polar AAs point towards the center of the pore

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22
Q

Name 3 characteristics that most membranes have?

A

1) transverse asymmetry
2) lateral heterogeneity
3) lipid movements

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23
Q

What is transverse asymmetry?

A

the two leaflets of a lipid bilayer have different lipid compositions

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24
Q

What is lateral heterogeneity?

A

particular lipids or proteins cluster together within the membrane

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25
Q

What is anti-gauche isomerization?

A

Switch from gauche arrangement (where bulky groups are positioned at a 60-degree angle to each other) to an anti arrangement (where the bulky groups are positioned directly opposite each other at a 180 degree angle)

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26
Q

What is an undulation?

A

membrane movements up down in a wave like motion

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27
Q

What kind of lipid movement do proteins typically assist with?

A

Flip-flop

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28
Q

Why are flip-flop proteins necessary?

A

To maintain transverse symmetry

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29
Q

How is protein assisted lipid movement powered and why?

A

using free energy from ATP hydrolysis because energy is required to move polar heads across the nonpolar bilayers

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30
Q

What is flippase?

A

A protein that assists in lipid flipflop movement from outside the cell to in

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31
Q

What is floppase?

A

A protein that assists in lipid flipflop movement from inside the cell to outside

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32
Q

What is scramblase?

A

A protein that assists in lipid flipflop movement into the cell and out of the cell

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33
Q

Why is remodeling and curvature of membranes necessary?

A

For cell division, endocytosis, and exocytosis

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34
Q

What five ways membranes can be remodeled and curved?

A

1) changing lipid composition or head group composition
2) Addition of membrane proteins
3) Amphipathic helix insertion
4) Scaffolding
5) Cytoskeleton pushing or pulling of the membrane

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35
Q

How do membrane proteins and amphipathic helix insertion cause membrane curvature?

A

Causes one leaflet to be longer than the other

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36
Q

How do scaffolding proteins cause membrane curvature?

A

Interacts with polar head groups of membrane and enforce curvature with protein structure

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37
Q

What are two reoccurring themes of membrane transport?

A

1) concentration gradients
2) transport proteins to help facilitate

38
Q

What are the three types of membrane transport?

A

1) passive diffusion
2) facilitated diffusion
3) active transport

39
Q

What is passive diffusion?

A

When transported species are moved across the membrane in a thermodynamically favorable direction without the help of specific transport proteins

40
Q

What kinds of molecules undergo passive transport and why?

A

nonpolar small molecules; can pass through hydrophobic layer and fit between lipids

41
Q

In terms of concentrations, which direction to species move during passive diffusion?

A

high concentration to low concentration

42
Q

What is facilitated diffusion?

A

transported species moves across membrane in a thermodynamically favorable direction with the help of specific transport protein

43
Q

Describe proteins involved in facilitated diffusion?

A

commonly channel proteins that display an affinity or specificity for the transport molecule

44
Q

How can facilitated diffusion be distinguished from passive diffusion experimentally?

A

by plotting saturation behavior (velocity vs concentration)

45
Q

How can facilitated diffusion and passive diffusion be differentiated using a saturation curve?

A
  • facilitated diffusion displays a square root function, indictive of saturation kinetics
  • passive diffusion displays a linear relationship between
46
Q

Why does facilitated diffusion experience saturation kinetics?

A

because its specificity implies a form of binding interactions; thus, the protein that facilitates diffusion can become saturated

47
Q

What are four themes of channels involved in facilitated diffusion?

A

1) typically comprised of quaternary structure
2) Contain a selectivity filter
3) Can be gated
4) Width provides selectivity

48
Q

Describe selectivity filters involved in facilitated channel diffusion?

A

commonly charged and use AAs to select for particular ions/molecules

49
Q

What are the three kinds of gated channels?

A

1) voltage
2) ligand
3) pH

50
Q

How does width of a channel affect selectivity?

A

Bigger channels allow more things through

51
Q

In channel proteins, what are two typical arrangements of subunits to form quaternary structure?

A

1) arrange to form a central pore
2) each subunit has a functional pore

52
Q

What form of transport do potassium channels utilize?

A

facilitated diffusion

53
Q

What is the function of a potassium channel?

A

To move K+ ions from the interior of the cell to the exterior

54
Q

Describe the quaternary and tertiary structure of a potassium channel?

A

Made of four subunits each containing a transmembrane alpha helix that form the central pore

55
Q

Describe the nonselective portion of the pore in a potassium channel

A

about 10 angstroms wide and filled with water

56
Q

How wide is a potassium channel selectivity filter?

A

3 angstroms wide

57
Q

Why does the carbonyl backbone face the center of the pore in a potassium channel?

A

Provides four negatively charged oxygens per pentapeptide fragment that can interact with K+

57
Q

Describe the peptide fragments that facilitate selectivity in potassium channels?

A

TVGYG fragments with their carbonyl backbone and one OH group (from T) facing the center of the pore

57
Q

Why do potassium channels select for K+ over Na+?

A

Energy required to desolvate K+ as it enters the potassium channel is less than the energy released when the selectivity filter resolvates K+; is favorable

57
Q

How is potassium channel direction reversed?

A

Reversing the K+ concentration gradient

58
Q

How do gated potassium channels open and close?

A

They change between open and closed confirmations

59
Q

What is active transport?

A

transported species moves across membrane in thermodynamically unfavorable direction with help of specific transport protein and energy input

60
Q

What are three possible energy sources for active transport?

A

1) ATP
2) ion gradient
3) light

61
Q

What are inhibitors of sodium pumps?

A

cardiotonic steroids

62
Q

How do cardiotonic steroids inhibit sodium pumps?

A

They form a stable complex with E2-P confirmation, preventing phosphate hydrolysis necessary to return to E1 confirmation

63
Q

How do cardiotonic steroids increase heart rate?

A

1) sodium pump in E2P causes K+ to accumulate in the cell
2) Opens voltage-gated calcium channels
3) Ca2+ accumulates in cell and triggers muscle contraction

64
Q

Describe the structure of a cardiotonic steroid

A

Contains a common steroid core with a lactone ring attached at C17 and a Oh at C14

65
Q

Why does K+ pass through potassium channels rapidly?

A

binding of additional K+ in the channel repels K+ that are currently in the channel

66
Q

What kind of transport do sodium pumps exhibit?

A

active transport

67
Q

What is the function of sodium pumps aka Na+/K+-ATPase?

A

To pump Na+ out of the cell and K+ into the cell using energy provided by ATP hydrolysis

68
Q

Describe the main components of Na+/K+-ATPase structure

A

1) three unidentical subunits
2) Asp 369 that accepts a phosphate from ATP
3) lots of alpha helices because it is an integral membrane protein

69
Q

What is an example of Na+/K+-ATPase inhibitor?

A

cardiotonic steroids

70
Q

What are the three primary components of cardiotonic steroid structure?

A

1) common steroid core
2) lactone ring at C17
3) hydroxyl (OH) at C14

71
Q

What is an ABC transporter?

A

An ATP binding cassette transporter

72
Q

What is an example of a MDR pump?

A

ABC transporters

73
Q

What are MDR pumps?

A

Multi-drug resistance pumps

74
Q

What do MDR pumps do?

A

pump cellular waste and drugs out of the cell

75
Q

What kinds of cells typically evolve MDR pumps? How do they evolve them?

A

bacterial and cancer cells; adaptation of pumps that typically expel cellular waste in response to drug stressors

76
Q

Describe the structure of an ABC transporter?

A

Two transmembrane protein domains form a pore
Two nucleotide binding domains bind to and hydrolyze ATP
The domains can be part of the same polypeptide chains or different ones

77
Q

What part of an ABC transporter functions as the ATP binding cassette?

A

nucleotide binding domains

78
Q

What is secondary active transport?

A

Transport that uses energy from an ion gradient established by primary active transport

79
Q

What is a common example of secondary active transport?

A

Na+ and H+ gradients transporting other molecules

80
Q

What is a symporter?

A

A secondary active transporter in which ions and molecules move in the same direction

81
Q

What is a antiporter?

A

A secondary active transporter in which ions and molecules move in opposite directions

82
Q

In what organism is AcrB found?

A

E. coli

83
Q

What kind of transport does AcrB undergo?

A

secondary active transport

84
Q

What kind of secondary active transporter is AcrB?

A

A antiporter

85
Q

Describe the structure of AcrB

A

composed of three identical subunits (homotrimer) that each have a pore

86
Q

What powers transport of drug molecules through AcrB?

A

H+ moving with its gradient through the channel drives subunit confirmational changes

87
Q

Describe the confirmational changes a subunit of AcrB undergoes as a drug molecule passes through it? (2 steps)

A

1) drug binds to subunit in L and triggers change to T
2) Confirmational change to O releases drug

88
Q

What are the threes confirmations possible for a subunit of AcrB?

A

L - loosely bound to drug molecule
T - tightly bound to drug molecule
O - open, not bound to drug molecule

89
Q

How do the subunits of AcrB interact?

A

cycle independently of one another

CHEM 472 Biochem (12 decks)

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