Lipid Bilayers are Highly Impermeable

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Lipid Bilayers are Highly Impermeable
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Membrane Proteins Carry Out Most of Membrane
Functions

• Energy conversion
• Respiration
• Photosynthesis
• Transportation
• Channels and pumps
• Signal transduction
• Cell surface recognition

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Membrane Proteins
Peripheral
45A
30A
Integral
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Membrane Proteins

• Peripheral proteins
• need high concentration of salt or change pH to
  separate from membrane
• Integral proteins
• need mild detergents to separate from membrane
• Nonionic octyl-b-glucoside
• Ionic sodium cholate

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Membrane Proteins are Difficult to Crystallize

• Bacteriorhodopsin (1990)
• Photobacterial reaction center (1991)
• Cytochrome bc1 complex (1997)
• Cytochrome c oxidase (2001)
• Structure of gt20 membrane proteins have been
  determined

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Integral Proteins
Bacteriorhodopsin
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Integral Proteins

• The transmembrane helix is about 20 residues each
  in length, which is about 30A long.
• The transmembrane helixes may be predicted from
  hydropathy plot.

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Polarity Scale for Identifying Transmembrane
Helices
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Amino Acid Sequence of Bacteriorhodopsin
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Integral Membrane Protein May not Contain
Trans-membrane Helix
Prostaglandin H2 synthase-1
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Hydrophobic Channel in Prostaglandin H2 Synthase
heme
bioactive prostaglandins and thromboxanes
Ibuprofen (Motrin IB)
Arthritis, cardiovascular disease, cancer and
various neurological disorders
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Integral proteins can also use b-sheetsStructure
of Bacterial Porin
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Amino Acid Sequence of Porin
Hydrophobic residues are highlighted in yellow.
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Structure of a Bacterial Porin
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Some Antibiotics are Membrane Channels

• A) Carriers
• Valinomycin? K
• B) Channel formers
• Gramicidin A

K
K
K
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Some Antibiotic Peptides
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Channel Formation in Membrane by Antibiotic
Peptides
Channel
a-helix
hydrophilic
hydrophobic
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Membrane Proteins and Lipids Define Membrane
Function

• Prokaryotic Cell Membranes

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Membrane Proteins and Lipids Define the Membrane
Function

• Eukaryotic Cells

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Membrane Proteins and Lipids Define the Membrane
Function

• Eukaryotic Cells

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Membrane Proteins and Lipids Define the Membrane
Function
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Movement of Nucleus-Targeted Protein