Question

For each lipid derivative below, draw a representative molecule. State what lipid type they belong to and describe what their functions is
a) Cerebroside
b) Phosphatidylethanolamine

Answer 1

a) Cerebroside

Cerebrosides is the common name for a group of glycosphingolipids called monoglycosylceramides which are important components in animal muscle and nerve cell membranes. They consist of a ceramide with a single sugar residue at the 1-hydroxyl moiety. The sugar residue can be either glucose or galactose; the two major types are therefore called glucocerebrosides (also known as glucosylceramides) and galactocerebrosides (also known as galactosylceramides). Galactocerebrosides are typically found in neural tissue, while glucocerebrosides are found in other tissues.

b) Phosphatidylethanolamines are a class of phospholipids found in biological membranes.They are synthesized by the addition of cytidine diphosphate-ethanolamine to diglycerides, releasing cytidine monophosphate. S-Adenosyl methioninecan subsequently methylate the amine of phosphatidylethanolamines to yield phosphatidylcholines. It can mainly be found in the inner (cytoplasmic) leaflet of the lipid bilayer.

Function of PE

1. In cell: Phosphatidylethanolamines are found in all living cells, composing 25% of all phospholipids. In human physiology, they are found particularly in nervous tissue such as the white matter of brain, nerves, neural tissue, and in spinal cord, where they make up 45% of all phospholipids. Phosphatidylethanolamines play a role in membrane fusion and in disassembly of the contractile ring during cytokinesis in cell division.Additionally, it is thought that phosphatidylethanolamine regulates membrane curvature. Phosphatidylethanolamine is an important precursor, substrate, or donor in several biological pathways. As a polar head group, phosphatidylethanolamine creates a more viscous lipid membrane compared to phosphatidylcholine. For example, the melting temperature of di-oleoyl-phosphatidylethanolamine is -16 °C while the melting temperature of di-oleoyl-phosphatidylcholine is -20 °C. If the lipids had two palmitoyl chains, phosphatidylethanolamine would melt at 63°C while phosphatidylcholine would melt already at 41 °C. Lower melting temperatures correspond, in a simplistic view, to more fluid membranes.

2. In humans, metabolism of phosphatidylethanolamine is thought to be important in the heart. When blood flow to the heart is restricted, the asymmetrical distribution of phosphatidylethanolamine between membrane leaflets is disrupted, and as a result the membrane is disrupted. Additionally, phosphatidylethanolamine plays a role in the secretion of lipoproteins in the liver. This is because vesicles for secretion of very low-density lipoproteins coming off of the Golgi have a significantly higher phosphatidylethanolamine concentration when compared to other vesicles containing very low-density lipoproteins. Phosphatidylethanolamine has also shown to be able to propagate infectious prions without the assistance of any proteins or nucleic acids, which is a unique characteristic of it. Phosphatidylethanolamine is also thought to play a role in blood clotting, as it works with phosphatidylserine to increase the rate of thrombin formation by promoting binding to Factor V and Factor X, two proteins which catalyze the formation of thrombin from prothrombin.

3. In bacteria: Where phosphatidylcholine is the principal phospholipid in animals, phosphatidylethanolamine is the principal one in bacteria. One of the primary roles for phosphatidylethanolamine in bacterial membranes is to spread out the negative charge caused by anionic membrane phospholipids. In the bacterium E. coli, phosphatidylethanolamine play a role in supporting lactose permeases active transport of lactose into the cell, and may play a role in other transport systems as well. Phosphatidylethanolamine plays a role in the assembly of lactose permease and other membrane proteins. It acts as a 'chaperone' to help the membrane proteins correctly fold their tertiary structures so that they can function properly. When phosphatidylethanolamine is not present, the transport proteins have incorrect tertiary structures and do not function correctly.