BCH 4053 Biochemistry I
Fall 2001
Dr. Michael Blaber


Lecture 14

Membranes, Structure of Membrane Proteins


All cells have a cytoplasmic membrane ("plasma membrane"). One major function of the plasma membrane is to separate the cytoplasm from the outside environment. Other functions related to the plasma membrane include:

Membrane environments may be necessary for certain chemical processes essential for life. Thus, the membrane is not only a physical envelope, but a specialized environment for biochemical processes.

Spontaneously formed lipid structures

Monolayers and micelles

Fatty acids spontaneously form various structures in aqueous solution. These are formed because of the hydrophobic effect (i.e. entropy considerations of solvent) that promote the formation of structures that remove the hydrophobic tail of the fatty acid from solvent accessibility.

Several unilamellar vesicles may be located within each other, like the layers of an onion (or those Matrioshka Russian stacking dolls). This organization is called a Multilamellar Vesicle. Note that each compartment can contain a different aqueous environment. (In the following picture the lipid bilayer is represented as a double line)

The bilayer of vesicles has polar surfaces and a hydrophobic middle, and is a barrier to the passage of polar molecules and charged ions into and out of the inner compartment. However, the hydrophobic middle is an excellent environment for non-polar molecules.

The Fluid Mosaic Model

Proposed by Singer and Nicolson in 1972. The model describes the lipid bilayer of vesicles as a dynamic, liquid-like environment that allows the free motion of non-polar molecules throughout its structure. The model also characterizes the lipid bilayer as a complex mixture of both phospholipids and proteins. Both the phospholipids and the proteins may be further conjugated with other groups, such as carbohydrates.

Membrane Proteins

Proteins associated with the lipid bilayer are of two general types:

 

Membranes are asymmetric structures

Lipid bilayers are typically heterogenous in their constituent lipids. Some lipids may have charged head groups, others may have polar head groups. The different lipid groups may be randomly distributed, or like-molecules may cluster together to produce patches within the bilayer.

In addition to lateral asymmetry within the plane of the bilayer, membranes also exhibit transverse asymmetry (i.e. dissimilar properties on the inside versus the outside facing side of the bilayer.

 

Membrane Phase Transitions

Lipid bilayers can undergo a temperature-dependent phase transition, somewhat like a solid to liquid phase transition.

Features of this phase transition:

 

Structure of Membrane Proteins

Lipid bilayers comprise the structural barrier that contains cytoplasm. However, it is through the action of membrane proteins that the functionalities such as transport and receptor signalling occur. We will focus here on the integral membrane proteins and a new class of proteins termed the lipid-anchored proteins.

Integral Membrane Proteins

Generally-speaking, these are of two types.

Proteins with a hydrophobic anchor

The anchor is often a single a-helix.

Proteins that are largely embedded

Embedded integral membrane proteins are often globular, as opposed to simply having a linear hydrophobic portion for an anchor. Their globular structure confers their functionality, its just that they require a hydrophobic environment for "solubility".


Click on the above image to view a VRML file of bacteriorhodopsin


Click on the above image to view a VRML file of a porin molecule

Lipid-Anchored Membrane Proteins

These are soluble globular proteins that have a lipid prosthetic group covalently attached that inserts into the lipid bilayer and anchors the protein to the bilayer

Four types of lipid-anchoring motifs have been identified

 


© 2001 Dr. Michael Blaber