Lipids

Lipids of Physiologic Significance

BIOMEDICAL IMPORTANCE

• The lipids • The common property– insoluble in water– soluble in nonpolar solvents

• They are important dietary constituents– high energy value– fat-soluble vitamins – essential fatty acids

• Fat is stored in adipose tissue

BIOMEDICAL IMPORTANCE

• Thermal insulator• electrical insulators • cellular constituents– Membranes

• Transport – Lipoproteins

• Lipid biochemistry is necessary– obesity, diabetes mellitus, atherosclerosis,

polyunsaturated fatty acids in nutrition and health

Classification

• Simple lipids – Esters of fatty acids with various alcohols

• Complex lipids – groups in addition to an alcohol and a fatty acid – Phospholipids

• Glycerophospholipids• Sphingophospholipids

– Glycolipids – Other complex lipids

• Sulfolipids and aminolipids• Lipoproteins

Classification

• Precursor and derived lipids

• Neutral lipids– acylglycerols (glycerides),cholesterol, and

cholesteryl esters

Free fatty

• Free fatty acids– Saturated • -anoic

– Unsaturated • -enoic

• 1,2,3,…• α, β, γ ,… ω or n carbons• Δ

Saturated fatty acids

Unsaturated Fatty Acids

• Monounsaturated • Polyunsaturated • Eicosanoids – eicosa- (20-carbon) polyenoic fatty acids• Prostanoids

– Prostaglandins(PGs)– Prostacyclins (PGIs)– Thromboxanes (TXs)

• Leukotrienes (LTs)• Lipoxins (LXs)

Prostaglandins

• The number of double bonds– PG1, PG2, PG3

• Different substituent groups (on ring)– A, B,C,etc

The leukotrienes andlipoxins

• Lipoxygenase pathway• three or four conjugated double bonds• Leukotrienes cause bronchoconstriction as

well as being potent proinflammatory agents and play a part in asthma

Unsaturated Fatty Acids

• Geometric isomerism– Cis• on the same side

– Trans• if on opposite sides• as a by-product of the saturation of fatty acids• ingestion of ruminant fat

• Significance – Molecular packing in membranes

Unsaturated fatty acids of physiologic and nutritional significance

Structure of some unsaturated fatty acids

Physical and Physiologic Propertiesof Fatty Acids

• Chain Length and Degree of Unsaturation– even-numbered-carbon fatty acids– The melting points

TRIACYLGLYCEROLS (TRIGLYCERIDES)

• THE MAIN STORAGE FORMS OF FATTY ACIDS• Mono- and diacylglycerols

PHOSPHOLIPIDS

• MAIN LIPID CONSTITUENTS OF MEMBRANES• derivatives of phosphatidic acid

• Phosphatidylcholines (Lecithins)• the most abundant phospholipids of the cell

membrane • represent a large proportion of the body’s

store of choline

PHOSPHOLIPIDS

• Surfactant – Dipalmitoyl lecithin • respiratory distress syndrome

• Phosphatidylethanolamine (cephalin)• Phosphatidylserine • Phosphatidylinositol• Cardiolipin

• Lysophospholipids

Phospholipids

• Plasmalogens

Sphingomyelins

GLYCOLIPIDS

• GLYCOSPHINGOLIPIDS – NERVE TISSUES– CELL MEMBRANE

• Galactosylceramide – Sulfatide • Sulfogalactosylceramide

• Glucosylceramide– Glycosphingolipid of extraneural tissues

• Simple glycosphingolipids• Complex glycosphingolipids– Gangliosides • Sialic acid • The simplest ganglioside found in tissues is GM3

• Functions – Receptor, …

STEROIDS

• Cholesterol – Atherosclerosis – As Precursor

• Sterol – Has one or more hydroxyl

groups and no carbonyl orcarboxyl groups

• Stereoisomers – “Chair” or a “boat” form– The rings can be either cis or trans– β bonds• above the plane of the rings

– α bonds • bonds attaching groups below

an all-trans configuration between adjacent rings

a cis configuration between rings A and B

Cholesterol

• Cholesteryl ester

Ergosterol

• Precursor of Vitamin D

Polyprenoids

• Synthesized from five-carbon isoprene units • Include – Ubiquinone – Dolichol – Vitamins • A, D, E, and K, and β-carotene (provitamin A)

LIPID PEROXIDATION

• A SOURCE OF FREE RADICALS – Lipids exposed to oxygen– deterioration of foods (rancidity)– damage to tissues

• Cancer, inflammatory diseases, atherosclerosis, and aging

– Peroxide formation from fatty acids containing methylene-interrupted double bonds

– Antioxidants • Vitamin E, Beta-carotene • Urate and vitamin C

AMPHIPATHIC LIPIDS

• Form– Membranes, Micelles, Liposomes, & Emulsions

• Contain polar groups– Fatty acids, phospholipids, sphingolipids, bile salts,

and, to a lesser extent, cholesterol• Part of the molecule is hydrophobic, and part

is hydrophilic

• Micelles – When a critical concentration of these lipids is

present in an aqueous medium– Facilitating absorption of lipids

• Liposomes– Formed by sonicating an amphipathic lipid in an

aqueous medium– as carriers of drugs

• Emulsions – Much larger particles

Waxes

• Serve as– Energy Stores and Water Repellents

• Esters of long-chain (C14 to C36) saturated and unsaturated fatty acids with long-chain (C16 to C30) alcohols

• Their melting points (60 to 100 °C) are generally higher than those of triacylglycerols

• Triacontanoylpalmitate, the major component of beeswax