Chapter 10: Membranes

Know the terminology:Phospholipid, phosphoglyceride, sphingolipid, cholesterol, steroids, phosphotide, polar head group, fatty acid, glycerol, glycoprotein, proteoglycan

bilayer, fluidity, homeoviscous adaptation, integral membrane protein, transmembrane domain, peripheral protein, lipid raft, hydropathy plot

Membranes allow compartmentation

Biological membranes are composed of a: (i) Lipid bilayer (ii) Proteins

Lipid components of the bilayer

Phospholipids• Phosphoglycerides: glycerol, 2 fatty acids and

polar head group• Sphingolipids: sphingosine, 1 fatty acid and

polar head group

Other lipids:• Steroids (cholesterol mainly)• Fatty acids: aliphatic chains with carboxylic

acid group

Protein components of the bilayer

Simple proteins

Glycoproteins: Proteins with carbohydrate chains

Proteoglycans:Proteins with glycosaminoglycan chains

Phosphoglycerides

Composed of:(i) A glycerol backbone (with 3 positions)(ii) 2 long chain fatty acids(iii) A polar head group

Lipid bilayer

Sphingolipids

(e.g. sphongomyelin) Composed of:(i) A sphingosine backbone (ii) 1 long chain fatty acid (iii) A polar head group

Sphingolipids and phosphoglycerides

Cholesterol

Cholesterol

Cholesterol increases “tightness” of membranes but increases fluidity

Membranes are heterogeneous

(1) Inner and outer leaflets are distinct in composition

Membranes are heterogeneous

(2) Regions of membranes can be enriched in specific lipids such as cholesterol (lipid rafts)

Membrane fluidity

Membranes composed of phospholipids are highly mobile.

Membrane fluidity depends upon lipid composition

Phospholipid movement depends upon:(i) Fatty acid chain length(ii) Saturation(iii) Polar head group(iv) Physical conditions

Membrane fluidity also depends upon presence of other macromolecules:

(i) Cholesterol(ii) Glycolipids

Homeoviscous adaptation

Environmental conditions (temperature, salt concentration) can alter membrane fluidity

Cells adjust lipid profiles to maintain constant fluidity

Reduced temperature “solidifies” membranes

Cell increase fluidity by:• using shorter fatty acids• introducing double bonds into fatty acids• altering polar head groups

Membrane proteinsMany membranes are primarily protein

(e.g. mitochondrial inner membrane is about

~80% protein, 20% lipid)

Proteins can be associated with the membrane many different ways:

(1) Integral proteins are embedded within the membrane

(2) Peripheral proteins are only associated with the membrane (via various connections)

Topography and membrane proteins

Topography and membrane proteins

Transmembrane proteins

Many proteins cross completely through the membrane one or more times

Typically an alpha-helix with hydrophobic amino acids (Figs. 10-19, 10-20)

Surface hydrophocity of -helices

Hydrophobic amino acids-greenPolar-blue, Charged-red

Predicting membrane-spanning domains

Membrane spanning domains can be predicted from primary structures using hydropathy plots.

Some integral proteins are -barrels

Many large pores are composed of beta-sheets arranged into a barrel.

Controlling protein location

In a naked cell, proteins are free to move within membranes but often cells restrict movement of proteins.

(1) Some proteins interact with each other (self-assembly) (Fig 10-43)

Controlling protein location

(2) Others can interact directly with the cytoskeleton (or via linkers)

Controlling protein location

(3) Some interact via external domains (e.g. carbohydrate)

Controlling protein location

(4) Inter-cellular interactions may prevent movements.