Lipids

Question 1

What are complex lipids as opposed to simple lipids?

Answer 1

Complex: bonded to additional sugars, nucleotides, other molecules etc.
Simple: just three fatty acid chains bonded to one glycerol molecule

Question 2

Give two examples of essential fatty acids, and explain why they are essential

Answer 2

Omega-6 linoleic acid and omega-3 linolenic acid.
They are essential as they cannot be synthesised by the body from other acetyl-CoA sources such as sugars. They both contain a carbon=carbon double bonds past the C9 position (moving in the direction starting from the carboxylic acid group), and C9 marks the furthest point that the ER is able to make the required double bond modifications to form these fatty acids.

Question 3

What conformation are most double bonds found in in unsaturated fatty acids?

Answer 3

Cis

Question 4

In IUPAC or delta nomenclature, which is the first carbon (C1) of a fatty acid?

Answer 4

The carbon of the carboxylic acid

Question 5

What does the delta^superscript mean after the cis/trans sign?

Answer 5

Indicates the position of the double bond, with the superscript being the lower numbered carbon

Question 6

Delta nomenclature - If you saw [20:4 delta^5,8,11,14] what would that be indicating?

Answer 6

20 = # of carbons is 20

4 = # of double bonds is 4

delta^5,8,11,14 = position of double bonds at C5=C6, C8=C9 etc.

Question 7

Omega nomenclature - If you saw [18:1 omega-9] what would that be indicating

Answer 7

18 = # of carbons is 18

1 = # of double bonds is 1

omega-9 = position of double bond is between C9=C10 (however since it is omega nomenclature this is counting with the methyl end as C1 and the carboxylic acid end being C18)

Question 8

What is the only difference between delta and omega nomenclature?

Answer 8

In delta, C1 is the carboxylic acid carbon, and in omega, C1 is the terminal methyl carbon.

Question 9

How are phospholipids formed? What property does it have?

Answer 9

A non-specific fatty acid tail will be replaced with a phosphate group, forming an amphipathic molecule which has hydrophilic (negatively charged phosphate group) and hydrophobic (nonpolar fatty acid tail) parts

Question 10

What is phosphatidic acid? What is its function?

Answer 10

Glycerol 3-phosphate bonded at both C1 and C2 to a fatty acid chain.

It is a precursor to many other phospholipids
It can induce a high curvature to a membrane
It acts as a signalling molecule to recruit cytosolic proteins

Question 11

What is phosphatidylethanolamine? What is its function?

Answer 11

It is phosphatidic acid with a primary amine bonded to the phosphate group

It is an important structural feature of neural membranes

Question 12

What is phosphatidylcholine? What is its function?

Answer 12

It is phosphatidic acid with a choline group bonded to the phosphate group

Due to it being a zwitterion (phosphate is negatively charged, choline is positively charged), it is very useful in membrane packing, and is thus the major constituent of cell membranes present mostly in the extracellular leaflet
Also important constituent of pulmonary surfactant

Question 13

What is phosphatidylserine? What is its function?

Answer 13

It is phosphatidic acid with a serine group bonded to the phosphate group[

Present mainly in the cytoplasmic leaflet, especially in brain tissue, and important in intracellular signalling, especially the apoptotic pathway and perhaps cognition

Question 14

What is phosphatidylinositol? What is its function?

Answer 14

It is phosphatidic acid with an inositol (carbocyclic sugar) bonded to the phosphate group

Plays a minor structural role
Very important in signalling, and are often phosphorylated to phosphoinositide (important phosphoinositide is PIP2 which can be broken down to produce IP3, the IP3 calcium induced calcium release channel receptor ligand

Question 15

What is phosphatidylglycerol? What is its function?

Answer 15

It is phosphatidic acid with a glycerol bonded to the phosphate group

Important lung surfactant, and in spreading the surfactant over the whole lung surface

Question 16

What is pulmonary surfactant, how does it relate to phospholipids and how is it important in disease?

Answer 16

A surface active lipoprotein complex that allows lungs to stretch/recoil and prevents airway collapse

In infant respiratory distress syndrome, premature infants are developmentally deficient in this surfactant, important constituents of which are phosphatidylglycerol and phosphatidylcholine

Question 17

What is cardiolipin? What is its function?

Answer 17

Two phosphatidic acids bridged by a glycerol bonded to each phosphate group

Important components of the mitochondrial inner membrane in stabilising the proteins of the electron transport chain

Question 18

How are phospholipids broken down?

Answer 18

Phospholipases A1 and A2 hydrolyse the ester linkage between the Fatty Acyl chains, and Phospholipase C hydrolyses the phosphate ester bond formed between the glycerol and the phosphate head (so between the O-P)

Question 19

What are sphingolipids? Name and outline the basic constituent of all sphingolipids

Answer 19

Another type of cell membrane lipid. Its basic constituent contains a structure called sphingosine.

Sphingosine doesn’t contain glycerol, but instead a three carbon backbone, with C1 and C3 bonded to hydroxyl groups and C2 bonded to an amine. There is also a long unsaturated hydrocarbon chain bonded to C1

Question 20

What is sphingomyelin? How is it implicated in Niemann-Pick’s disease?

Answer 20

A constituent of myelin sheaths.

Basic structure is a sphingosine with:

• a fatty acid attached to the amine via an amide bond
• a phosphate attached to the hydroxyl group of C3 via a phosphate ester bond; bond to this phosphate group is a choline molecule, also attached via a phosphate ester bond

In Niemann-Pick’s disease, there is a defect in the gene coding for an enzyme that degrades excessive sphingomyelin. The result is that it builds up in the liver, spleen, brain and bone marrow, and the child dies within several years

Question 21

What are cerebrosides and, by extension, gangliosides? How are they implicated in Tay-Sachs disease?

Answer 21

Cerebrosides are sphingosines bonded to a fatty acid via an amide bond and a carb chain (most often glucose and galactose) via the hydroxyl group of C3.

Glucocerebrosides are found in the cell membranes of macrophages and galactocerebrosides in the cell membranes of brain cells

Gangliosides are cerebrosides with the sugar bonded to multiple sialic acids

Tay-Sachs occurs in patients who have defects in genes coding for specific enzymes that degrade excessive gangliosides. They thus build up in the brain and the spleen, and can cause severe mental retardation.

Question 22

What are sterols? What forms can they adopt? What structural features make them suitable for membranes?

Answer 22

A subgroup of the steroid class of organic molecules

They are amphipathic lipids synthesised from acetyl-CoA via the HMG-CoA reductase pathway.

They are mostly planar, and are composed of three 6 membered rings attached in a row, at one end bonded to a 5 membered ring and at another bonded to a single OH group.

They can be found in free form, or conjugated to a fatty acid, in which the hydroxyl group is esterified to form a non-polar molecule

There is limited movement allowed due to the bonds that form sterols, therefore the molecule is rigid and can be compacted, making it useful in membrane packing

Question 23

Outline the molecular structure of cholesterol

Answer 23

Cholesterol has a structure derived from sterol, with some of the hydrogen atoms substituted by other chemical groups:

• OH group on the first 6 membered ring remains, so it retains its amphiphilic characteristics
• there is an aliphatic chain bonded to the 5 membered ring
• between the first and second 6 membered rings, and between the third 6 membered ring and the 5 membered ring, there is a methyl group bonded
• it is unsaturated due to a double bond forming within the second 6 membered ring

Question 24

Label the two groups of steroid hormones, and then name them on notability

Answer 24

Notability–>Biochem and med gen–> steroid hormones

Question 25

Describe the typical type of fatty acid at position 1 and 2 of glycerol

Answer 25

Position 1: saturated

Position 2: polyunsaturated

Question 26

What structure is cholesterol?

Answer 26

Planar

Question 27

What is cholesterol an important precursor of?

Answer 27

Bile acids

Steroid hormones

Question 28

Describe cholesterol synthesis

Answer 28

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