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9: Self assembly – Lipids, Membranes

Last updated

Jun 3, 2019
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- 8: Protein Structure, Self assembly – Multimeric Proteins, Cytoskeletal fibers, Protein Evolution
- 10: Protein purification

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Lecture 10. Self assembly – Lipids, Membranes

Reading & Problems: LNC p. 357-361, 362-364, 366-368, 368-370, 385-395 Figs. 11-17 and 11-18; p. 381 prob. 1, 2, 7, 9, 11, 12

I. Lipids - hydrophobic cellular "small" molecules - non-protein components of cells that partition into organic solvents like xylene or hexane.

A. Fatty acids- long chain carboxylic acids; aphipathic (having both hydrophobic and hydrophilic regions on one molecule); salts of FA are soaps; form micelles in aqueous solutions(the structure of a micelle can be viewed at this site). Fatty acid properties are determined by chain length and number of unsaturations.

B. Triglycerides (triacylglycerols) - three FA chains attached to a glycerol backbone via ester linkages. Called fats (if mostly saturated and solid at room temperature) or oils (if unsaturation leads them to be liquid at room temperature). Primarily used for energy storage.

C. Glycerophospholipids - glycerol backbone with two FA chains and a phosphate (which may have additional functional groups attached). A major class of membrane lipids. Will spontaneously form bilayers or liposomes in aqueous environment.

D. Sphingolipids - structures based on a sphingosine backbone. A major class of membrane lipids consist of a sphingospine backbone with a hydrophilic group attached to the backbone hydroxyl group and a FA attached to the backbone amino group via an amide linkage (peptide bond). Sphingomyelin structure shows that it is similar to a glycerophospholipid in overall shape. (Image Source: https://commons.wikimedia.org/wiki/F...structures.png)

E. Cholesterol. In membranes of animals. Broadens melting curve of membrane enabling maintenance of appropriate fluidity over a range of temperatures.

II. Membranes - biological lipid bilayers with associated/included proteins.( here's a simulation of motion in a lipid bilayer).

A. Fluid mosaic model of membranes

B. Membrane proteins

1. Integral membrane proteins - can only be removed from membrane by methods that lead to dissolution of the membrane. Surfaces of proteins in contact with interior of the membrane are hydrophobic.

2. Peripheral membrane proteins - can be removed by salt or other treatments that leave membrane largely intact. Can be bound to surface of integral membrane proteins, partially enter membrane, or be anchored by attached lipid molecules (e. g. prenylation).

C. Inner and outer leaflets of membranes can have different composition.

D. Lateral movement in membranes is rapid, movement between leaflets (flip) is slow.

Some take home information

Glycerolipids - have a glycerol backbone
Sphingolipids - have a sphingosine backbone
Phospholipids - include a phosphate group
Glycolipids - have attached sugar residues
Some Other Vocabulary: amphipathic, micelle, liposome, ester, vesicle, sonication, vortexing, ethanolamine, choline

Check out the structures that can be viewed under the "2. Lipids" heading on this page, click on the "JSmol" button and you won't need Java to view the structures.

Contributors

Charles S. Gasser (Department of Molecular & Cellular Biology; UC Davis)