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Lipids & Membranes

Biochemistry

William Kimaru Dept of Biological Sciences MUST

Lipids are non-polar (hydrophobic) compounds, soluble in organic solvents. Most membrane lipids are amphipathic , having a non-polar end and a polar end. Fatty acids consist of a hydrocarbon chain with a carboxylic acid at one end. A 16-C fatty acid: CH 3

(CH

2

14

- COO - Non-polar polar A 16-C fatty acid with one cis double bond between C atoms 9-10 may be represented as 16:1 cis  9 .

There is free rotation about C-C bonds in the fatty acid hydrocarbon, except where there is a double bond. Each cis double bond causes a kink in the chain. Rotation about other C-C bonds would permit a more linear structure than shown, but there would be a kink. C O O  1 2 3 4    fatty acid with a cis- ^9 double bond

Glycerophospholipids H C OH CH 2 OH CH 2 OH glycerol Glycerophospholipids (phosphoglycerides), are common constituents of cellular membranes. They have a glycerol backbone. Hydroxyls at C1 & C2 are esterified to fatty acids. An ester forms when a hydroxyl reacts with a carboxylic acid, with loss of H 2

O.

Formation of an ester: O O R'OH +^ HO-C-R" R'-O-C-R'' +^ H 2 O

O P O O  O H 2 C CH H 2 C R 1 C O O O C O R 2 X glycerophospholipid In most glycerophospholipids (phosphoglycerides), Pi is in turn esterified to OH of a polar head group ( X ): e.g., serine, choline, ethanolamine, glycerol, or inositol. The 2 fatty acids tend to be non-linonenic acid identical. They may differ in length and/or the presence/absence of double bonds.

Phosphatidylinositol , with inositol as polar head group, is one glycerophospholipid. In addition to being a membrane lipid, phosphatidylinositol has roles in cell signaling. O P O  O H 2 C CH H 2 C R 1 C O O O C O R 2 OH H OH H H H OH OH H O H OH phosphatidyl- inositol

Each glycerophospholipid includes  (^) a polar region: glycerol , carbonyl O of fatty acids, Pi , & the polar head group ( X )  (^) non-polar hydrocarbon tails of fatty acids ( R 1 , R 2 ). O P O O  O H 2 C CH H 2 C R 1 C O O O C O R 2 X glycerophospholipid

H 2 C H C OH CH N+ CH C CH 2 CH 3 H H 3 OH ( ) 12 sphingosine Sphingosine may be reversibly phosphorylated to produce the signal molecule sphingosine-1-phosphate. Other derivatives of sphingosine are commonly found as constituents of biological membranes.

Sphingolipids are derivatives of

the lipid sphingosine , which has a long hydrocarbon tail, and a polar domain that includes an amino group. sphingosine- 1 - P H 2 C H C O CH N

CH C CH 2 CH 3 H H 3 OH ( ) 12 P O  O  O

Sphingomyelin, with a phosphocholine head group, is similar in size and shape to the glycerophospholipid phosphatidyl choline. Sphingomyelin has a phosphocholine or phosphethanolamine head group. Sphingomyelins are common constituent of plasma membranes H 2 C H C O CH NH CH C CH 2 CH 3 H OH ( ) 12 C R O O P O O H 2 C H 2 N C

CH 3 H 3 C CH 3 Sphingomyelin phosphocholine sphingosine fatty acid  

head group that is a complex oligosaccharide , including the acidic sugar derivative sialic acid. Cerebrosides and gangliosides, collectively called glycosphingolipids , are commonly found in the outer leaflet of the plasma membrane bilayer, with their sugar chains extending out from the cell surface. cerebroside with -galactose head group H 2 C H C CH NH CH C CH 2 CH 3 OH C R O OH O H H H H OH OH CH 2 OH H O H ( ) 12 A cerebroside is a sphingolipid (ceramide) with a monosaccharide such as glucose or galactose as polar head group. A ganglioside is a ceramide with a polar

liquid crystal crystal In the liquid crystal state , hydrocarbon chains of phospholipids are disordered and in constant motion. At lower temperature , a membrane containing a single phospholipid type undergoes transition to a crystalline state in which fatty acid tails are fully extended, packing is highly ordered, & van der Waals interactions between adjacent chains are maximal. Kinks in fatty acid chains, due to cis double bonds , interfere with packing in the crystalline state, and lower the phase transition temperature.

Membrane fluidity:

The interior of a lipid bilayer is normally highly fluid.

Cholesterol is largely hydrophobic. But it has one polar group, a hydroxyl , making it amphipathic. C h o lestero l H O

Cholesterol , an

important constituent of cell membranes, has a rigid ring system and a short branched hydrocarbon tail. PDB 1N83^ cholesterol

But the presence of cholesterol in a phospholipid membrane interferes with close packing of fatty acid tails in the crystalline state, and thus inhibits transition to the crystal state. Phospholipid membranes with a high concentration of cholesterol have a fluidity intermediate between the liquid crystal and crystal states. Cholesterol in membrane Interaction with the relatively rigid cholesterol decreases the mobility of hydrocarbon tails of phospholipids.

Two strategies by which phase changes of membrane lipids are avoided:  (^) Cholesterol is abundant in membranes, such as plasma membranes, that include many lipids with long-linonenic acid chain saturated fatty acids. In the absence of cholesterol, such membranes would crystallize at physiological temperatures.  (^) The inner mitochondrial membrane lacks cholesterol, but includes many phospholipids whose fatty acids have one or more double bonds , which lower the melting point to below physiological temperature.