Structural Biochemistry/Organic Chemistry/Lipids

Lipids

Lipids are naturally occurring (organic) compounds that are insoluble in polar solvents such as water . Their insolubility can be attributed solely to their long hydrophobic hydrocarbon chains. These hydrophobic chains may be saturated or unsaturated. Unsaturated chains contain double or triple covalent bonds between adjacent carbons while saturated chains consist of all single bonds. Lipids are composed of a glycerol molecule bonded to long hydrocarbon chain(s) (can be single or multiple) and, depending on the lipid, to other molecules--such as a phosphate group (phospholipids).

Some examples of the types of lipids are: neutral, saturated, (poly/mono) unsaturated fats and oils (monoglycerides, diglycerides, triglycerides), phospholipids, sterols (steroid alcohols), zoosterols (cholesterol), waxes, and fat-soluble vitamins (vitamins A, D, E, and K). Lipids have many different biological functions such as fuel molecules, structural building blocks for phospholipids and glycolipids, covalent attachments to guide molecules to specific membrane locations, and intracellular messengers.

"Chemical structure of the saccharolipid lipid A as found in E. Coli."

File:Lipid.gif

An example of a lipid complex.

Triglycerides

Neutral fats (triglycerides) are composed of fatty acid hydrocarbon chains bonded to a single glycerol molecule. Fatty acids consist of long hydrocarbon chains with a carboxyl group while glycerol consists of 3 carbons and 3 hydroxyl groups. Fatty acids are the building blocks of fat molecules. The method by which the three fatty acid chains in a triglyceride attach to a single glycerol molecule is called dehydration synthesis. Dehydration synthesis is also used in various other reactions, including the joining of two monosaccharides to form a disaccharide. Triglycerides function primarily in energy storage, as a form of insulation, and to protect and cushion cells and organs.

There is an image of a triglyceride molecule with three neutral fatty acid chains and a glycerol group

Saturated fatty acids contain single bonds between the carbons of the hydrophobic chain. Saturated fatty acids originate from animals and are found as component chains in a triglyceride molecule. Saturated fatty acids exist in the solid state at room temperature. Unsaturated fatty acids however contain one (monounsaturated) or more (polyunsaturated) double bond(s) between the carbons of the hydrocarbon chain, which causes the molecule to bend. Triglycerides with too many bends cannot be packed as closely together as neutral fatty acids and therefore are less dense. Below is an example of a saturated fatty acid

A Saturated Fatty Acid

Triglycerides composed of many fatty acids that melt at lower temperatures than those triglycerides with saturated fatty acids. These unsaturated fatty acids do not bind at their maximum number of hydrogen’s because of double bonding between the carbons of the chain. Unsaturated fatty acids originate from plants and are found as component chains to triglyceride molecules. Unsaturated fatty acids exist in the liquid state at room temperature.

(E)-4-oxohexadec-2-enoic acid, An Unsaturated Fatty Acid

These images depict a saturated fatty acid chain (contain single carbon bonds) and an unsaturated fatty acid chain (contain double carbon bonds).

Phospholipids

Phospholipid structure.

Phospholipids are modified triglycerides with one of the fatty acid chains replaced with a phosphate group. They are made by four distinguished groups: fatty acid chains, a platform, a phosphate group, and an alcohol attached to the phosphate. The fatty acid chains are hydrocarbon chains that are typically 14-24 carbons in length. The platform is either glycerol or sphingosine, which is an amino alcohol with a hydrocarbon chain. Phospholipids have a very characteristic non-polar fatty acid chain portion and a polar phosphate portion.The amphipathic character of phospholipids contribute in its' crucial role in phospholipid bilayers. The polar phosphate group is capable of interacting with water molecules and spontaneously forms a bilayer in an aquatic environment. Phospholipids orientate themselves so that the polar heads are facing the water molecules and the hydrophobic fatty acids are oriented toward the inside of the bi-layer. The bi-layer environment enables the non-polar fatty acid chains to stay together, avoiding the water while the hydrophilic phosphate group is oriented toward the water. Phospholipids participate in the formation of the cell membrane by the coming together of two layers of phospholipids. The phospholipids are responsible for the membrane's semi-permeability and fluidity.

The structure describe a phospholipid:

1-Acylglycerin-3-phosphoethanolamin

This image illustrates the components and orientation of the hydrophilic phosphate group and the hydrophobic fatty acid chains that form the lipid bi-layer.

Glycolipids is the components of lipids and carbohydrates. It plays a role to provide energy for cells.

Glycolipids

Glycolipids are sugar(glyco-)containing lipids. They are derived from sphingosine instead of a form of phospholipids that derives from glycerol (phospholipids exist in both derivatives from glycerol and sphingomyelin platform). Another difference from phospholipids is that glycolipids contain a sugar unit (can be glucose or galactose) instead of a phosphate group.

Examples: Glycolipid molecules exist from the most basic molecule, cerebroside which contains 1 fatty acid unit, a sphingosine backbone, and 1 sugar unit (glucose or galactose), to the most complex molecules containing branched chains of multiple sugar residues (up to seven residues in gangliosides).

Properties: When glycolipids exist in membranes, their sugar residue terminal always face the extracellular side.

Chemical structure of Glycolipids

Structure of Glycolipids'

Cholesterol

Cholesterol

Cholesterol is a form of lipids that differs from the rest of its relatives. It is relatively medium molecule that contains 4 adjacent cyclic hydrocarbon molecules with three six-member rings and one five-member ring that has a hydroxyl and a saturated hydrocarbon chain terminals.

The molecule functions as a bufferor a temperature stabilizer for the membrane in which it can make up of 25% of the membrane. When exist in membranes, the 4 cyclic molecules in the cholesterol molecule lay parallel to the fatty acid chains of the phospholipids, meanwhile the hydroxyl terminal points in the direction with the polar phospholipid heads in which it interact with.

Cholesterol molecules exist primarily in nerve cells. The molecule binds to the myelin sheath membrane which provides an outer coating that protects the nerve cell from its surroundings.

It is an essential predecessor to sex hormones that exists in males (testosterone) and females (oestradiol). Also an essential component in vitamin D that enables the body to utilize calcium to form bones.

Animals acquire very little cholesterol from the food they eat; they make cholesterol within the body. Although cholesterol is essential for many processes and structural function, it can be detrimental to have excess cholesterol. Too much cholesterol in the blood will cause blockages in the arteries which can result in heart disease, high blood pressure, and stroke. Only 0.25% of human beings retain High Cholesterol disease from heredity, however people are gaining high cholesterol in their blood from the food they eat (especially people in America).

Cholesterol and fluidity

This shows the cholesterol molecules submerged in the lipid bilayer

Cholesterol is an important factor in membrane permeability, that is, how much can flow through the cell. Cholesterol acts as a 'buffer' to prevent against any extremes. Obviously, the membranes permeability cannot be too fluid as to allow anything inside the cell(i.e harmful agents),but at the same time, the membranes permeability must be fluid enough as to let out/let in important agents that need to enter the cell.

Cholesterol is a hydrocarbon steroid with one single alcohol group, which leads to it's amphiphatic nature.

Generally,

1.AT LOW TEMPERATURES:Cholesterol in a membrane leads to a more fluid membrane.

2.AT HIGH TEMPERATURES:Cholesterol in a membrane leads to a less fluid membrane.

Ether Lipids with Branched Chains

An ether lipid.

Two major factor that separates Archea(bacteria) from Bacteria is the Archea's cell membranes phospholipid consists of ether linkages and the fatty acid hydrocarbon chains are completely saturated and branched with a methyl group every 5 carbons. These simple structural differences provides Archeabacteria drastic difference from bacteria in terms of their habitat's harsh environments. These 2 factors contribute to the chemical properties that their membrane is more resistant to hydrolysis (ether versus ester linkages) and resistant to oxidation (branched saturated hydrocarbon chains).

Phospholipids and Glycolipids Readily Form Bimolecular Sheets in Aqueous Media

Phospholipids and glycolipids have amphipathic characteristics which enables them to form a micelle or a "lipid bilayer". Due to the hydrophobic hydrocarbon tail and the hydrophillic polar head group, the lipids arrange in a form where the polar groups face water while the tail is away from water. One formation is the micelle where the lipids arrange themselves in a circle with the head groups making the circumference while the tails are inside. A more favorable formation is the lipid bilayer or the bimolecular sheet. This arrangement has the lipids form a barrier where the polar head groups face the aqueous media and the hydrophobic tail face inside away from water. This type of formation is favorable to cell membranes for it forms a barrier from the extracellular fluid and protects the cytoplasm within the cell. Integral and peripheral proteins may be present in the lipid bilayer to allow certain functions to occur such as transportation of ions or acting as pumps.

References

Berg, Jeremy; Tymoczko, John; Stryer, Lubert. Biochemistry, 6th edition. W.H. Freeman and Company. 2007.

The Organics of Biochemistry:[1]