Biochemistry/Lipids

Lipids are a structuraly diverse class of biological molecules whose defining properties are hydrophobicity and solubility in organic solvents. Lipids have several roles including energy storage and cell signalling and are the major constituent of cell membranes.

Fatty Acids

A fatty acid is simply a linear carbon chain with a carboxylic acid group at one end.

Fatty acids usually have between 10 and 24 carbon atoms and may have one or more double bonds. These double bonds are almost always in the cis configuration.

Fatty acids have both common names and systematic names. The systematic name is based on the alkane or alkene with the same number of carbon atoms, with the final e of the hydrocarbon replaced with oic acid. For example Laurate (Figure x) is a fatty acid with 12 carbon atoms and no double bonds, so the systematic name is dodecanoic acid, and the ionized form is dodecanoate. (saturated fatty acid table) If there are double bonds, the location is marked with the symbol Δ with a superscript number indicating the location of the double bond and preceded by cis or trans to indicate configuration (but almost invariably cis). Carbon atoms are counted from the carboxyl end, as in Figure x, so a dodecenoate acid with a cis double bond between carbon atoms 9 and 10 would be: cis-Δ⁹-dodecenoate.

The number 2, 3 and last carbon atom are called the α, β and ω atoms, respectively.

Phosphoglycerides And Sphingolipids

Fatty acids can be linked to glycerol to form monoacylglycerols, diacylglycerides and triacylglycerides.

One class of phospholipids are the phosphoglycerides, which are made from diacylglycerides linked to a phosphate group. Phosphoglycerides usually have the following arrangement:

• Hydroxyl #1 of glycerol is usually esterified to a saturated fatty acid
• Hydroxyl #2 of glycerol is usually esterified to an unsaturated fatty acid
• Hydroxyl #3 of glycerol is esterified to a phosphate group

There are four common phosphate groups. Serine, ethanolamine and choline are structurally similar, while inositol is different:

Sphingolipids, have the same overall shape as phosphoglycerides but have different chemistry, using sphingosine in place of glycerol. Sphingosine has a long hydrocarbon tail similar to fatty acids attached to a structure that is similar to the amino acid serine. A fatty acid can attach to the amine group, and a "head" group can attach to a hydroxyl (see Figure x). Sphingolipids are named according to this head group:

• If there is no head group it is called a ceramide
• If the head group is phosphate and choline, it is called sphingomyelin
• If the head group is a sugar, it is called a glycosphingolipid (or a glycolipid)

The majority of sphingolipids are of the third type, glycosphingolipids. It is thought they have functions in cell recognition and protection in addition to there structural role in the membrane.

When mixed in water, phosphoglycerides and sphingolipids will form two chararistics structures. Micelles are spheres constructed from these lipids, with the hydrophobic tails interacting with one another inside the sphere and the polar head groups interacting with water at the surface of the sphere. These lipids will also for whats known as a lipid bilayers. These are sheets composed of two layers of lipids where the hydrophobic tails stick into the center and the polar head groups are on the surface interacting with water. Cell membranes are composed of lipid bilayers. (picture of micelle, milcell cross section, lipid bilayer)