Lipid metabolism

By Dr. Hoda Gad

OBJECTIVES BY THE END OF THESE LECTURES, STUDENT SHOULD BE ABLE TO:

Understand the structure of lipids including Fatty acids (their different numbering methods & the essential

fatty acids) ,Triacylglycerols, phospholipids & cholesterol

Understand how lipids are digested, absorbed and transported in the blood.

Know how fatty acids are synthesized and oxidizedIdentify the types of ketone bodies, how they are synthesized by the

liver, how they are utilization by the peripheral tissues to produce energy and their excessive production in diabetes mellitus

Know how triacylglycerols, the major form of stored energy, are formed & mobilized from their storage

sites Differentiate between the different lipases involved in

lipid metabolism .Know how cholesterol is synthesized in human body

cell and its regulation.know the types, composition & metabolism of plasma

lipoproteins and how they can be separated from a plasma sample

LIPID CHEMISTRYDefinition & properties

They are naturally occurring substances, water insoluble, but soluble in non-polar solvents e.g. ether

Therefore they are transported in the blood by specific proteins called lipoprpteins

Importance & functions

1 .Major source of energy for the body .

2 .Source of fat soluble vitamins .

3 .Source of essential fatty acids

4 .Essential components of cell membranes.

Classes of important lipids A) Fatty acids

Properties of fatty acids:

F.A is a hydrocarbon chain with one carboxyl group hydrophobic R-CooH hydrophilic head

hydrocarbon chain

–Example: CH3-CH2-CH2-CH2-CH2-COOH

Often un-branched except few F.As e.g., phytanic acid

Mostly contain even number of carbon atoms

They are either saturated or unsaturated

They may be essential or non essential

Saturated FA:•They do not contain carbon-carbon double bonds•They are either

–Short chain (2-10 C)–Long chain (>10C)

•Their general formula is CH3–(CH2)n–COOH

where n= umber of CH2 groups.•Example of short chain saturated FA :

Butyric acid (4C): CH3–CH2–CH2–COOH•Examples of long chain saturated FA:

Palmitic acid (16C ): CH3–(CH2)14–COOH Stearic acid (18C): CH3–(CH2)16–COOH

Unsaturated fatty acids:•They contain carbon-carbon double bonds•They are either

–Monounsaturated (one double bond) –Polyunsaturated (more than 1 double bond)

•Example of monounsaturated FA : Oleic acid 18C (18: 1; ∆9)

•Examples of polyunsaturated FA : a) Linoleic acid 18C (18: 2; ∆9,12) b) Linolenic acid 18C (18:3; ∆9,12, 15) c) Arachidonic acid 20C (20: 4; ∆5,8, 11,14)

Essential F.A•Must be supplied in the diet because the body can not

form them•They are present mainly in vegetable oils •They are the polyunsaturated fatty acids :

» linoleic acid» linolenic acid

•Arachidonic acid (20C) becomes essential only if linoleic acid is deficient

Non essential F.A •They can be formed in the body •Example of non essential FA palmitic acid

How to number the carbon atoms in FA?

Counting from the carboxylic group which will be considered as carbon no 1

Counting from the carbon next to the carboxylic group which will be numbered as α and the following carbons as β, γ …etc

Counting from the methyl group (omega numbering) which will be considered as carbon no 1 and the carbon next to it will be

carbon no 2.

6 5 4 3 2 1

CH3-CH2-CH2-CH2-CH2-COOH

ε δ γ β α

ω1 ω2 ω3 ω4 ω5 ω6

Caproic acid (6C)

The numeric designations for fatty acids:First, write the number of carbon atoms in the fatty acidThen, write the number and sites of double bonds E.g., palmitic acid, 16-carbon fatty acid with no double bonds, is

designated by (16: 0).2 systems for designating the unsaturated fatty acid

.1The Delta system) ( ∆: –Oleic acid is written as 18:1 ∆9 indicating that oleic

acid contains (18) carbon atoms & one double bond (1) between carbon number 9 and carbon number 10 starting numbering from carboxyl carbon

COOH which is (C1)

.2The omega system (ω): –Oleic acid may be written as 18:1, ω9 indicating a double

bond between carbon number 9 and carbon number 10 starting numbering from the omega carbon (CH3) which is C1.

1-The Delta system (∆)

H

methyl

ω9

2- The omega system (ω)

B) TRIACYLGLYCEROLThey are esters of 3 fatty acids with the alcohol

glycerol: - R1 is saturated F.A - R2 is unsaturated F.A -

R3 is saturated or unsaturated F.A

They are the most common type of lipids in the body

They are the storage form of lipid as it is tightly packed

If derived from animal sources, they would contain saturated FA ,

If derived from plant sources; they would contain unsaturated fatty acids

Triacylglycerol

C) CHOLESTEROLMonohydric alcohol present in

animal tissues

Precursor for many important compounds such as vitamin D

and steroid hormones

Essential component of membranes

The body can synthesize approximately half of the daily

body needs.

Some Cholesterol Derivatives

D) PHOSPHOLIPIDS•Major component of cell membranes•Types:

.1Glycerophospholipids: more commonFormed of glycerol, 2 F.A, phosphoric acid & base

Bases are choline, ethanolamine, serine & inositolMain types of glycerophospholipids :

Phosphatidylcholine (lecithin) phosphatidylethanolamine (cephaline) ,Phosphatidylserinephosphatidylinositol

.2Sphingomyelins:In myelin sheaths of nervous tissuesFormed of sphingosine, F.A, phosphoric acid & choline

Glycero

ph

osp

ho

lipid

s

Phosphatidyl serine

Phosphatidyl ethanolamine (Cephalin)

Phosphatidyl inositol

Phosphatidyl choline (lecithin)

E) GLYCOLIPIDS •Composed of Sphingosine + FA+ Carbohydrate•Sphingosine + FA is called Ceramide•So, they are composed of ceramide + carbohydrate•Main types:

I– Cerebrosides Carbohydrate is 1 glucose or 1 galactose

II– Gangliosides Carbohydrate is a chain of at least 3 sugars

Sphingosine F.ASphingosine F.A

glucose glucose or galactoseor galactose

CerebrosideCerebrosidess

Sphingosine F.ASphingosine F.A

glucose galactose sialic acidglucose galactose sialic acid

GangliosidesGangliosides

F) LIPOPROTEINSComposed of lipids and proteins

They transport lipid in blood

4 main types:

Chylomicrons (CM)

Very low density lipoproteins (VLDL)

Low density lipoproteins (LDL)

High density lipoproteins (HDL)