BIOC 212 Study Guide - Midterm Guide: Cytosol, Hydrolysis, Tyrosine
28 May 2018
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Membrane Proteins --Summary & Comparison
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Compare & Contrast
• Look at different lipids and how they compare to amino acids
o Chemical structures of lipids
Size
Polarity, charge
(Head groups)
Hydrophobicity
Phospholipids
> aa
< protein
Very polar, charged
(+1/-1 or +1/-2)
Charged head,
Hydrophobic end
Glycolipids
> PL
Polar, some acidic
charges (-)
Large polar groups
Hydrophobic end
Cholesterol
< PL
Weal polarity
(hydroxyl)
Small polar head group (OH)
Very hydrophobic
Free fatty acids
Becomes
part of PL
Charged carboxyl
Hydrophobic part of PL
Amino acid side chains
< PL
Varies greatly
Varies greatly
• Whole proteins 100-300aa is much larger than phospholipid
o But phospholipid larger than individual amino acid; i.e. serine as head group in PS
• Glycolipids have lots of carbohydrate groups as head group; sialic acid charged
• Cholesterol is smaller, more hydrophobic, rigid and flat
• Phospholipids are always charged because always have a phosphate
o Always carry charges even if net charge can be 0
!
Thought Experiments
• What happens to structure of integral membrane protein in the absence of a
membrane? (Pull it out of the membrane)
o Unstable; TM helix with mostly hydrophobic side chains is now exposed to water (polar)
• Hydrophobic residues cannot interact with water properly, want to be covered up
• Protein aggregation --interact with other hydrophobic residues
• Hopefully, if chaperones around, might bind to prevent aggregation and eventually
lead to degradation
o Same for transmembrane b-barrel
• Still have a lot of hydrophobicity; will either aggregate or be degraded
• What about lipid-anchored & peripheral proteins?
o If have only one fatty acid chain, do come in and out of the membrane fairly easily
o Fatty acid chain is hydrophobic, although smaller than typical protein
• Perhaps other things will try to cover up the exposed hydrophobic regions
• Sometimes the fatty acid forms a binding side for another protein
• Peripheral proteins are attached by non-covalent interactions to lipid or other protein
o So might not be very disruptive if take the peripheral protein off
o Especially if interacting with lipid head group --interactions are polar & charged so
surface can interact with water very well
