Introduction to Lipids Structure & Functions

Introduction to Lipids Introduction to Lipids Structure & FunctionsStructure & Functions

Objectives

Functions of lipid compounds

Clinical problems

Lipid compounds of physiological importance

Complex lipids:

Phospholipids, lipoproteins and glycolipids

Lipids are heterogeneous groupheterogeneous group of water-water-insoluble molecules related to fatty acids.insoluble molecules related to fatty acids.

They are soluble in non-polarsoluble in non-polar (organic solvents (benzene, ether , chloroform & acetone).

B/c they are insoluble:1.Compartmentalized

( membrane- associated lipids or as TAG in adipocytes.

2.Associated with proteins lipoproteins

Lipids’ve many many functionsLipids’ve many many functions :Major source of energy (1g=9kcal)Hydrophobic barrierFat soluble vitamins ( coenzymes, regulationPGs& steroid hrs play a major regulatory roles in body’s homeostasis

Lipids and Related Clinical Problems

Obesity

Atherosclerosis and hypertension

Coronary heart diseases

A. Simple Lipids:

B.

Complex Lipids:

Ionized at pH=7, (pK=4.8)

Long-chain FA the hydrophobicity predominates

Fatty Acids

FA has amphipathic

nature.

CH3(CH2)n COOH

Hydrophobic Hydrophilic

>90% of FA in plasma in the form of esters contained in

lipoproteins.

Low levels of free fatty acids are in tissues, but imp. amn.

maybe found during fasting & starvationfasting & starvation

Free FA’re transported by serum albumin

=hydrocarbon chain (alkyl chain )

Free Fatty Acids (FFA)

1. Chain length:Short-chain and Medium-ChainLong-Chain e.g., Palmitic acid 16:0Very long-chain e.g., Nervonic acid 24:1

2. Degree of saturation:Saturated: No double bondsUnsaturated: Mono- or poly-unsaturated

Cis- or trans-form3. Branched Vs straight-chain4. Essential fatty acids

Arachidonic ac. is an essential if linoleic acid is missing.

Unsaturated FA:•monounsat.( one =)e.g, nervonic ac

•polyunsat. (two=)e.g., linoleic ac. ,(three=) eg α- linolenic ac.,

Addition of double bonds: ↓↓ TmTm

Increase chain length: ↑↑ ↑↑ TmTm

Tm =melting temp/

Trans F.A.Trans F.A.

Are chemically classified as unsaturated f.a., but behave more like saturated f.a.

They elevate serum LDL & increase the risk of CVS

They don’t occur naturally in plants & occur in small amounts in animals

Trans f.a. are formed during hydrogenation of liquid vegetable oils (margarine)

The presence of bouble bonds in some of LCFA of membrane FA maintain the fluid fluid nature of those lipidsnature of those lipids

Free fatty acids cannot Free fatty acids cannot be used as a fuel for be used as a fuel for RBCS & BrainRBCS & Brain

Saturated FatSaturated Fat

Consumption of saturated fats is strongly associated with high levels of total plasma high levels of total plasma cholesterol & LDL-cholesterolcholesterol & LDL-cholesterol

Thus there is ↑ risk of CVS Main sources: dairy & meat products ,

vegetable oils as coconut & palm oils

*Phytanic acid’s (3,7,11,15 tetramethyl palmitic ac.) present in ↑↑ in dairy products.

Branched F.A.Branched F.A.

Triacylglycerols

Storage form in adipose tissue

~ 90% of dietary lipids

Glycerol plus 3 fatty acids

Blood transport: Chylomicrons and VLDL

Acetone Acetoacetate 3-hydroxybutyrate

1. Water-soluble2. Diabetic Ketoacidosis

Ketone BodiesKetone Bodies:

Phospholipids & Glycolipids

PhospholipidsPhospholipids

PL are ionic polar structure formed of an alcohol that is attached by bridge to a back bone either diacylglycerol or sphinogosine

A

hydrophilic head

Long hydrophobic Tail

A. GlycerophospholipidsGlycerol-containing phospholipids 1. Phosphatidylcholine (Lecithin)

e.g., Surfactant (Dipalmitoylecithin) 2. Cardiolipin (antigenic) 3. Phosphatidyl inositol (signaling molecule)

B. Sphingo-phospholipids:Sphingosine-containing phospholipids:e.g., sphingomyelin (Myelin sheath)

Phospholipids

Sphingolipids

The sphingolipids include: sphingomyelinsglycosphingolipids

They are essential components of cell membrane (greatest in nerve tissue).

Sphingolipids

Sphingosine

Ceramide

Precursor of glycolipids

Ceramide: Parent Sphingolipid Compound

A sphingomyelin

Alcohol at C-1 is esterified to Phoshorylcholine

Important constituents of myelin in

nerve fibers

The only significant sphingolipids in humans

Clinical Significances of Glycosphingolipids

present in the outer leafletouter leaflet of the plasma membrane.

carbohydrate moieties of glycolipids : antigenic determinants on the surfaces of cells

(e.g.,blood group antigens.) cell surface receptors GM1 sources of various embryonic antigens

Cholesterol Structure

Sterols: are steroids with 8-10 carbon atoms in the side chain at C-17 & OH at C-3 Cholesterol is the major sterol in animal tissuesIt is a sterol (with 8 carbons at C17,= bet 5&6)

Overview and Functions

Major Sterol of animal tissues Component of cell membranes Precursor for:

Bile acids & saltsVitamin D Steroid hormones:

Mineralocorticoids e.g., AldosteroneGlucocorticoids, e.g., CortisolSex hormones, e.g., Testosterone

Estrogen & progesterone Hypercholesterolemia: Atherosclerosis & CAD

Cholesterol: Types

Free cholesterol Esterified cholesterol

OH at C3HydrophobicMembranesPlasmaTransport

FreeLessFoundLessLDL/HDL

Fatty acidMoreAbsentMostHDL/LDL

CONT’D

Lipoprotein Structure

Protein part: Apoproteins or apolipoproteinsAbbreviations: Apo-A, B, CFunctions:

Structural and transport functionEnzymatic functionLigands for receptors

Lipid part:• According to the type of lipoproteins• Different lipid components in various combinations

Lipoprotein StructureLipoprotein Structure

Chylomicrons

Very low densityLipoprotein (VLDL)

Low densityLipoprotein (LDL)

High density Lipoprotein (HDL)

Plasma Lipoproteins

Triacylglycerol transport: Chylomicrons: TG of dietary origin VLDL: TG of endogenous synthesis

Cholesterol transport: LDL: Mainly free cholesterol HDL: Mainly esterified cholesterol

Take Home Message

Lipids are heterogeneous group of compoundsLipids are relatively water-insolubleSimple lipids:

FFA, TG, Ketone bodies, CholesterolComplex lipids:

e.g., Phospholipids, LipoproteinsLipids have important physiological functionsLipid disorders are the basis for common human diseases, namely obesity and atherosclerosis