Chapter 21: Lipids

K. DunlapChem 104

Lipids• like CHO, lipids are compose of carbon, hydrogen and oxygen

• unlike, CHO they do not contain multiple hydroxyl groups for hydrogen bonding, and therefore are insoluble in water

• Lipids include a large variety of molecules:- fatty acids- triglycerides- waxes- eicosanoids- steroids- fat soluble vitamins

• insoluble in water

• wide range of functions: insulate and protect internal organs, signaling molecules, energy

• most efficient form in which energy is stored in the body

• fats in animals absorb and sequester nonpolar contaminants, such as DDT, PCBs, organomercury

•excess glucose, lipids and proteins are stored in adipose cells

General info on Lipids…

Lipids: diverse class of molecules

1) Energy and storage – fatty acids, triacylglycerides, waxes

2) Membrane structure – phospholipids, glycolypids, sterols

3) Signaling, cofactors and pigments – Eicosanoids, sterols, fat soluble vitamins

Energy & storage Lipids• Fatty Acids

• Triacylgerols

• Waxes

Fatty Acids• long chain, even number carboxylic acids, typically between 12 and 26 carbons

• Saturated fatty acids have no double bonds• Unsaturated fatty acids have at least one double bond

Unsaturated Fatty Acids

• Most naturally occurring fatty acids are in the cis form• causes a bend in the molecule• chains cannot pack tightly, and therefore are less dense, with lower melting points

Cis, Trans, & Saturated Fatty Acids

Common Fatty Acids

Formation of Triglycerides• glycerol backbone, with three fatty acids, joined by ester linkages• The fatty acids that compose the triglyceride affect their properties, such as melting point • Fats have FAs with fewer double bonds• Oils have FAs with more double bonds

Triglycerides• The lower melting points of triglycerides rich in

unsaturated fatty acids are related to differences in their three-dimensional shape.– Hydrocarbon chains of saturated fatty acids can lie

parallel with strong London dispersion forces between their chains; they pack into well-ordered forms and melt above room temperature

– Because of the cis configuration of the double bonds in unsaturated fatty acids, their hydrocarbon chains have a less ordered structure and London dispersion forces between them are weaker; these triglycerides have melting points below room temperature.

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Triacylglycerols: stored fat

• Insulation - seals, walruses, penguins• Energy – Hibernation

Hydrogenation• Hardening: reduction of some or all of the

carbon-carbon double bonds of an unsaturated triglyceride using H2/metal catalyst.

– In practice, the degree of hardening is carefully controlled to produce fats of a desired consistency.

– The resulting fats are sold for kitchen use (Crisco, Spry, Dexo, and others).

– Margarine and other butter substitutes are produced by partial hydrogenation of polyunsaturated oils derived from corn, cottonseed, peanut, and soybean oils.

Hydrogenation of Unsaturated Fats

• Oils can be converted to semi-solids through hydrogenation that converts the double bonds to single bonds• In the process, some double bonds are converted to trans form

Complex Lipids

• Phospholipids– contain an alcohol, two fatty acids, and a

phosphate ester.– In glycerophospholipids, the alcohol is glycerol.– In sphingolipids, the alcohol is sphingosine.

• Glycolipids– Complex lipids that contain a carbohydrate.

Complex Lipids• Figure 21.1 Schematic diagram of simple and

complex lipids.

Membranes• Complex lipids form the membranes around

cells and small structures within cells.

• In aqueous solution, complex lipids spontaneously form into a lipid bilayer, with a back-to-back arrangement of lipid monolayers.– Polar (hydrophilic) head groups are in contact with

the aqueous environment.– Nonpolar (hydrophobic) tails are buried within the

bilayer– The arrangement of hydrocarbon tails in the

interior can be rigid (if rich in saturated fatty acids) or fluid (if rich in unsaturated fatty acids).

Lipids in Membranes: Fluid Mosaic Model

• Polar heads and nonpolar tails

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Storage vs. Structural lipids in membranes

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Glycerophospholipids

• Polar or charged group is attached to the third carbon of glycerol– Basis for

nomenclature

• Most abundant lipid in biological membranes

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Sphingolipids

• One fatty acid joined to sphingosine

• Polar head group

• Sphingomyelin, Glycosphingolipids,Gangliosides

• Sphingomyelin - animal cells, especially myelin

• Some involved in signal transduction & cell surface recognition

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Glycosphingolipids: determinants of the blood groups

Cholesterol• Cholesterol is the major sterol in animal

tissues– It is a component in plasma membranes in all

animal cells.– It is the precursor of all steroid hormones, some

vitamins and bile acids.

Steroids

• SteroidsSteroids:: a group of plant and animal lipids that have this tetracyclic ring structure.

Steroids

• cholesterol, bile acids, vitamin D, and many hormones

• Oxidized sterols

• Lipid soluble and enter cells

• Bind nuclear receptors and alter gene expression and metabolism

Cholesterol• Cholesterol is the most abundant steroid in the

human body, and also the most important.– It is a component in plasma membranes in all

animal cells.– It is the precursor of all steroid hormones and bile

acids.

Lipoproteins• Cholesterol, along with fats, are transported by

lipoproteins

Lipoproteins• Figure 21.5 Schematic of a low-density

lipoprotein.

•is the most abundant steroid and is the starting material for all other steroids

• present in animal fats• major factor in the development of atherosclerosis

• transported by lipoproteins:

VLDL (very low density lipoproteins)- transport triglyceridesLDL (low density lipoproteins)- transport cholesterol from liver to tissuesHDL (high density lipoproteins)- transport cholesterol from tissues to the liver for elimination

Cholesterol

Bile Salts• Bile saltsBile salts, the oxidation products of cholesterol.

– synthesized in the liver, stored in the gallbladder, and secreted into the intestine where they emulsify dietary fats and aid in their absorption and digestion

Fats • recommended 30% of diet-10% monounsaturated-10% saturated-10% unsaturated

Essential Fatty acids:Omega-3

EPA, DHA, linoleic

Omega-6Arachidonic acid, -linolenic

• EPA, DHA, AA are 20 carbon hormone-like fatty acids that play an important part in the immune function

Eicosapentaenoic acid (20:5ω-3)

Omega-3 Fatty Acids

Prostaglandins & Leukotriene• are not stored in tissues as such, but are

synthesized from membrane-bound 20-carbon polyunsaturated fatty acids in response to specific physiological triggers.– One such polyunsaturated fatty acid is arachidonic

acid.

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Eicosanoids: paracrine hormones

• Derived from the 20 carbon polyunsaturated fatty acid (arachidonic acid)

• Involved in reproduction, inflammation, fever and pain• 3 classes: Prostaglandins, thromboxanes, leukotrienes

Regulate synthesis of cAMP production fever, pain, blood flow, and uterine contraction

Produced by platelets, important in clotting and blood flow

Inflammation, asthma, allergy

Phospholipid membrane

Phospholipase A2

Cyclooxygenases (COX)

Prostaglandins (PG)

&

Thromboxanes (TX)

Lipoxygenases (LOX)

Leukotrienes

O

HO

O

HOArachidonic acid Eicosapentaenoic acid

-poor substrate for cyclooxygenases

-gives rise to series 5 leukotrienes

More on Lipids……

• fats insulate and protect internal organs• most efficient form in which energy is stored in the body

-9 kcal/ gram as opposed to 4

• fats in animals absorb and sequester nonpolar contaminants, such as DDT, PCBs, organomercury

• excess glucose, lipids and proteins are stored in adipose cells

Endocrine Disruptors•Hormones are substances produced in specialized glands such as the pituitary, thymus, hypothalamus, thyroid, adrenals, pancreas, gonads which make up the endocrine system• direct and control reproduction, prenatal development, growth, and many regulatory processes• endocrine disruptors refer to contaminants that mimic or interfere with the function of hormones: PCB, dioxin, DDT

1. What is a polyunsaturated fatty acid?

2. What happens when a polyunsaturated FA is hydrogenated? What happens to the physical properties of the oils?

3. What molecules react to form a triglyceride?

4. What is an endocrine disruptor?

This membrane lipid is classified as a ____________?

5. In contrast to water-soluble vitamins, which must be part of our daily diet, fat-soluble vitamins can be stored in the body in amounts sufficient for many months. Suggest an explanation for this difference.

6. The melting points of a series of 18-carbon fatty acids are: stearic acid, 69.6 C°; oleic acid, 13.4 C°; linoleic acid, -5 C°; and linolenic acid, -11 °C. What structural aspect of these 18-carbon fatty acids can be correlated with the melting point?

What are some important biochemical roles of lipids in Alaska?