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• Membrane lipids form a __ and thereby establish __, whereas specific proteins __; they __
o Membrane lipids create the appropriate environment for the action of such proteins
- permeability barrier
- compartments
- mediate all other membrane functions
- transport chemicals and info across a membrane
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• Membranes differ in __; percentages range
o The protein components of the membrane can be visualized by __ because membranes performing different functions contain __
• The ease with which a protein can be dissociated from a membrane indicates what?
Some can be solubilized by mild means. Others are bound more tenaciously.
There are __ and __ due to this.
- protein content
- SDS-PAGE
- different repertoires of proteins
- how intimately it is associated with the membrane
peripheral and integral
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Integral proteins
interact extensively with the hydrocarbon chains of membrane lipids and can be released only by agents that compete for these nonpolar interactions; span the lipid bilayer
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Peripheral
membrane proteins are bound to membranes by electrostatic and hydrogen-bond interactions with the head groups of lipids; can be disrupted by adding salts or change in the pH; bound to the surfaces of integral proteins, on either the cytoplasmic or extracellular side; others are anchored to the lipid bilayer by a covalently attached hydrophobic chain
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• Proteins can span the membrane with __
o __uses light energy to transport protons from insde to outside the cell, generating a proton gradient used to form bacteria
- alpha helices
- Bacteriorhodopsin
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Structure of bacteriorhodopsin
built almost entirely of alpha helices; seven closely packed alpha helices, arranged almost perpendicularly to the place of the cell membrane; mostly nonpolar amino acids allowing contact with the hydrocarbon core of the membrane or with one another
• Membrane spanning alpha helices are the most common structural motif in membrane proteins
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• A channel protein can be formed from __, like __, located in the outer membranes of bacteria; have essentially no __
Arrangement of beta strands is simple: __
- beta strands
- porin
- alpha heliceso
each strand is hydrogen bonded to its neighbor in an antiparallel arrangement, forming a single beta sheet, which curls up to form a hollow cylinder that forms a pore in the membrane
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The outside surface of porin is __ and the inside is __ and __
• This arrangement of nonpolar and polar surfaces is accomplished by the __
- nonpolar
- hydrophilic and filled with water
- alternation of hydrophobic and hydrophilic amino acids along each beta strand
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• Embedding part of a protein in a membrane can do what, such as is the case with __, which catalyzes the conversion of arachidonic acid into prostaglandin H2
o Prostaglandin H2 promotes __and modulates __; and the enzyme that produces it is a __with mostly __
- link the protein to the membrane surface
- prostaglandin H2 synthase-1
- inflammation
- gastric acid secretion
- homodimer
- alpha helices
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Prostaglandin H2 is not __but instead lies along the __, firmly bound by a set of __ with __ that extend from the bottom of the protein into the membrane
• This linkage is so strong that what; it is classified as an __, although it does not span the membrane
- largely embedded in the membrane
- outer surface of the membrane
- alpha helices with hydrophobic surfaces
only the action of detergents can release the protein from the membrane
integral membrane protein
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o The localization of __is crucial to its function
The substrate, __, is a hydrophobic molecule generated by __; __ reaches the active site of the enzyme from the membrane without entering an aqueous environment by doing what?
prostaglandin H2 synthase-1
arachidonic acid
hydrolysis of membrane lipids
arachidonic acid
traveling through a hydrophobic channel in a protein
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• The parts of the protein that interact with the hydrophobic parts of the membrane are coated with __, whereas those parts that interact with the aqueous environment are much more __•
The structures positioned within the membrane are quite __and, in particular, all backbone __ participate in __
o Breaking a hydrogen bond within a membrane is quite unfavorable because __
- nonpolar amino acid side chains
- hydrophilic
- regular
- hydrogen-bond donors and acceptors
- hydrogen bonds
- little or no water is present to compete for the polar groups
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• Some proteins associate with membranes through __
Soluble proteins can associate with membranes if __.
Some examples are:
---> A __ attached to a specific __residue by a __
--> A __ attached to a __residue at the __
--> A __ attached to the __.
covalently attached hydrophobic groups
- hydrophobic groups are attached to the proteins
- palmitoly group
- cysteine
- thioester bond
- farnesyl group
- cysteine
- carboxyl terminus
- glycolipid structure termed a glycosylphosphatidylinositol (GPI anchor)
- carboxyl terminus
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o These modifications are attached by __ that recognize __ near the __
- enzyme systems
- specific signal sequences
- site of attachment
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• __ can be accurately predicted from amino acid sequences
o Many membrane proteins employ __ to do what?
We can identify transmembrane helices by asking what?
- Transmembrane helices
- alpha helices
- span the hydrophobic part of a membrane
- whether it is likely to be more stable in a hydrocarbon environment or in water, estimating the free-energy change when a helical segment is transferred from the interior of a membrane to water
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• The hydrocarbon core of a membrane is about __ wide, a length that can be traversed by an __
We can estimate the __that takes place when a __ is transferred from the __ to __
- 30 angstroms
- alpha helix consisting of 20 residues
- free-energy change
- hypothetical alpha helix formed of residues 1 through 20
- membrane interior to water
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o The span of 20 residues chosen for this calculation is called a __; and the free-energy change for each window is plotted against the first amino acid at the window to create a __
A peak of __ or more in a __ based on a window of 20 residues indicates that a polypeptide segment could be a __
Yet, some membrane proteins contain __ that escape detection by these plots
- window
- hydropathy plot
- +84 kJ mol-1
- hydropathy plot
- membrane-spanning helix
- membrane-spanning features
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