lipid metabolism l Biochemistry question for dentistry

lipid metabolism l Biochemistry & medicinal chemistry MCQs for dental students 

 

LIPIDS

DEFINITON :

•  Are groups of heterogeneous compounds characterized by their solubility properties.
• They are insoluble in water, but are soluble in organic solvents such as ether, chloroform, and benzene…

Properities

Heterogeneous:

• Biological lipids are a chemically diverse group of compounds.

Solubility properties:

• The common feature of which is their insolubility in water and their solubility in non polar (organic) solvents.

 Biochemical Importance of Lipids

1. Are concentrated fuel reserve of the body.
2. Serve as a source of fat-soluble vitamins.
3. Are constituents of membrane structure and regulate the permeability.        
4. Thermal insulators.
5. Electrical insulators.
6. Are precursors of some hormones.
7. Protect the internal organs.
8. source of 2nd messengers.
9. Relation of lipids to health & disease:

• Knowledge of lipid biochemistry is necessary in understanding many important biomedical areas include:

1. Obesity,
2. Diabetes mellitus.
3. Atherosclerosis.

CLASSIFICATION OF LIPIDS

 Simple lipids (storage)

•  Esters of fatty acids  with various alcohols

 Complex  lipids (structural)

• Esters of fatty acids containing additional groups

Precursors & derived lipids

• Include cholesterol fatty acids, glycerol,  FSV and others

(1) Simple lipids

•  They are esters of fatty acids and alcohol. Two types:

1- Fats, oils; these are esters of fatty acids with glycerol e.g. Triacylglycerol. The difference between fat and oil is physical, oil is a liquid while fat is a solid at room temperature.  

• Fats or oils; Acylglycerols such as Triacylglycerols
• Esters of fatty acids with glycerol; mono-di- or triacylglycerol (TAG).
• The main storage form of fuel in animals is TAG.
• It is stored in adipose tissues.
• It is hydrophobic molecule, therefore it is transported in blood by the lipoprotein particles mainly
• chylomicrons (transport dietary TAG) and very low density lipoprotein –VLDL- (transport synthesized TAG).

2- Waxes are esters of fatty acids with alcohols other than glycerol. These alcohols may be aliphatic or alicyclic Cetyl alcohol is most commonly found in waxes.

 

Fatty acids

• Long hydrocarbon chain with carboxylic group.
• Structural components and precursors of different lipids.
• Most of fatty acids that occur in natural lipids are of even No. carbon.
• Transported in blood bound to albumin protein.
• Some are not synthesized in animal body, thus are dietary essential e.g. (linoleic, and linolenic acids).

Saturated and unsaturated FAs:

• Saturated FAs do not contain double bonds, while unsaturated FAs contain one or more double bonds.
• Fatty acids with one double bond are known as monounsaturated and those with 2 or more double bonds are collectively known as polyunsaturated FAs (PUFA).

(2) Complex lipids (structural):

• They are esters of FAs with alcohols containing additional group such as phosphate, nitrogenous base, carbohydrate and protein.

1. Phospholipids:

• Are classified to glycerophospholipids and sphingophospholipids according to their alcoholic constituent (glycerol or sphingosine).

2. Glycolipids:

•  lipids contain carbohydrates.

3. Lipoproteins:

• lipids with proteins. Lipoproteins are utilized for transport of lipids. They are of four  types: Chylomicrons, VLDL, LDL and HDL.

1. Phospholipids:

A. Glycerophospholipids.

•  Phospholipids are complex lipids, resembling triglycerides, but containing phosphate and a nitrogenous base in place of one of the fatty acids.
• They are important components of cell membranes and lipoproteins.
• Phosphoacylglycerols contain both a polar (the head) and a nonpolar region (the tail).

Phosphatidic acid (PA):

• Is the simplest glycerophospholipid.
• It is the parent compound for all glycero-phospholipids eg. phosphatidyl-choline or –serine or –ethanolamine or – inositol .
• It is also an intermediate in the synthesis of TAG.

Lecithins (phosphatidylcholine)

• Are important constituent of plasma membrane.
• Are particularly abundant in nerve tissues, store choline required for the formation of acetylcholine which is required for transmission of nerve impulse.
• Choline containing labile methyl groups is involved in methylation reactions.

Phosphatidylinositol

• Is an important constituent of cell membrane.
• 2nd messenger for hormonal signaling.
• The action of some hormones (e.g. oxytosine and vasopressin) is mediated through Phosphatidylinositol.
• In response to hormonal action, Phosphatidylinositol is cleaved to diacylglycerol and inositol.
• Both these compounds act as second messengers for hormonal action.

Phosphatidyl serine

• Can also be regarded as a cephalin.
• The amino acid serine is present in this type of glycerophospholipid.
• Is a plasma membrane component.

Plasmalogens

• An unsaturated FA occurs at C1.
• May contain choline, inositol, serine or ethanolamine.
• Plasmalogens represent 10% of phospholipids in brain.

 Cardiolipin

• It is so named as it was first isolated from heart muscle.
• Structurally, a cardiolipin consists of 2 phosphatidic acids held by an additional glycerol (diphosphatidyl glycerol)
• Major lipid of mitochondrial membrane.

2. Sphingophospholipids.

• Sphingophospholipids contain sphingosine (an amino alcohol).
• Amphipathic, like glycerophospholipids (polar and  nonpolar regions).
• Structural component of cellular membranes.
• Major categories:

1. Sphingomyelins.
2. Glycosphingolipids.

Sphingomyelins

• In sphingomyelins, sphingosine is attached by an amide linkage to a fatty acid to produce ceramide.
• Alcohol group of sphingosine is bound to phosphorylcholine.
• Sphingomyelins is an important constituent of myelin sheath around neurons and found in good quantity in brain and nervous tissues.

 Glycolipids

• Also known as glycosphingolipids.
• Are important constituents of cell membrane and nervous tissues (particularly the brain).
• Cerebrosides are the simplest form of glycolipids.
• They contain a ceramide (sphingosine attached to fatty acid) and one or more sugar.
• Galactocerebroside and glucocerebroside are the most important glycolipid

Lipoproteins

• Lipoproteins are classified from larger and less dense to smaller and denser.
• Lipoproteins are larger and less dense when the fat to protein ratio is increased.
• They are classified on the basis of electrophoresis and ultracentrifugation.
• The protein composition differs from one lipoprotein class to another, and the protein constituents are called Apolipoproteins.

 Functions of apolipoproteins:

1. They serve to solubulise plasma lipids.
2. Activate enzymes involved in lipid metabolism (LCAT, LPL).
3. Delivery of lipids to cells via recognition of cell surface receptors.

TYPES :

1. Chylomicrons: transport TAG from intestine (dietary lipids).
2. Very low density lipoprotein (VLDL): transport synthesized lipids TAG from liver to other tissues.
3. Low density lipoprotein (LDL): transport cholesterol from liver to different tissues (bad cholesterol).
4. High density lipoprotein (HDL): transport cholesterol from different tissues to liver (good cholesterol).

(3) Derived lipids:

• These include a number of compounds possessing the characteristics of lipids.
• e.g. fatty acids, fatty alcohols, fat soluble vitamins, steroid (cholesterol, bile acids, steroid hormones), and ketone bodies.

Steroids

• Are compounds that; contain the steroid nucleus (phenanthrine ring)
• Cholesterol is the best known member of this group of compounds.
• Other steroids include bile acids, vitamin D, sex hormones and adrenocortical hormones.
• If the steroid contains one or more hydroxyl groups it is known as sterol.