Ch 19 Lipids

What is a lipid?

Lipids are insoluble in water, soluble in non polar organic substances

5 categories of biochemical lipid classification

1. Energy-storage lipids (triacylglycerols)

2. Membrane lipids (phospholipids, sphingoglycolipids, and cholesterol)

3. Emulsification lipids (bile acids)

4. Chemical messenger lipids (steroid hormones and eicosanoids)

5. Protective-coating lipids (biological waxes)

Saponifiable lipids (undergo hydrolysis under basic conditions)

Triacylglycerols phospholipids, sphingoglycolipids, cholesterol, and biological waxes

Nonsaponifiable lipids

Bile acids, steroid hormones and eicosanoids

Fatty acids

naturally-occurring monocarboxylic acids with a linear (unbranched) carbon chain (typically even number of carbon atoms)

Saturated (SFA)

all C-C bonds are single bonds

will always be linear (unbranched)

numbering starts from end with the -COOH group

Unsaturated (UFA)

one or more C=C bonds

Monounsaturated

Exactly one C=C bond

cis configuration

Polyunsaturated

Two or more C=C bonds

Structural notation

(# carbon atoms): (# of C=C) delta symbol ^(C=C positions)

Omega (w)-3

Three carbon atoms away from its methyl end

Omega (w)-6

Six carbon atoms away from its methyl end

Fatty acids- water solubility

The longer the carbon chain in the fatty acid, the lower its solubility in water

Fatty acids- melting point

Length of carbon chain and number of cis C=C present

Increase in # double bonds, decrease in melting point

Triacylglycerol structure

three fatty acyl groups and one glycerol

Triacylglycerol use in the body

Triacylglycerols are stored in adipocytes “long-term storage” and function as a concentrated way to store energy

Simple Triacylglycerol

three identical fatty acyl groups on a glycerol (all R groups are identical)

Mixed Triacylglycerol

2 or 3 different types of fatty acids on the same glycerol

Fats

Solid or semi-solid at room temperature

Saturated fatty acyl groups

Animal sources

Oils

Liquid at room temperature

Unsaturated fatty acyl groups

Plants and fish

Two most important essential fatty acids

Linoleic acid (18:2)

Linolenic (18:3)

Why are they essential?

Both linoleic acid and linolenic acid are needed for proper membrane structure, synthesis of longer chain omega fatty acids

Ester hydrolysis

Requires catalyst (Acidic, Basic, Enzymes)

Complete hydrolysis

Acidic: produces alcohol and carboxylic acid (complete)

Base: produces alcohol and a carboxylate ion (complete)

Partial Hydrolysis

Enzymes: partial hydrolysis where only 1-2 ester bonds react

Saponification

hydrolysis of a triacylglycerol in basic solution to produce fatty acid salt and glycerol

Hydrogenation

addition of hydrogen (H2) across a C=C double bond

Oxidation

Double bonds in triacylglycerols are subject to oxidation with oxygen in air, leading to C=C breakage

Structure of phospholipid

1. One (or more) fatty acyl groups

2. A phosphate group

3. Platform molecule (glycerol or sphingosine)

4. An alcohol

Purpose of phospholipids

All cells are surrounded by a cell membrane that confines their contents (and serves many other purposes)

Cell membrane made up of up to 80% of lipids

Glycerophospholipid

1. One glycerol

2. Two fatty acids

3. A phosphate group

4. An alcohol

Spingophospholipid

One fatty acid

One phosphate group

One sphingosine molecule (linking #1 and #2)

An alcohol attached to the phosphate group

Sphinoglycolipids

Contain a carbohydrate instead of the phosphate/alcohol polar head

Steroid

lipid whose structure is based on a fused ring system of three 6 carbon rings and one 5 carbon ring

Cholesterol

a C27 steroid molecule that is important in human cell membranes, nerve tissue and brain tissue

Cell Membrane

Provide separation between aqueous interior of cell with outside aqueous environment

Up to 80% of plasma membrane is made of lipids (lipid bilayer, 6-9 nm thick)

Role of cholesterol

Regulates membrane fluidity by incorporating between fatty acid chains in lipid bilayer

• Too little cholesterol, and the membranes are too fluid

• Too much cholesterol, and the membranes become too rigid

Role of membrane proteins

Responsible for moving substances such as nutrients and electrolytes across the membrane

Receptors for hormones and neurotransmitters

Passive transport

Moving across a cell membrane by diffusion from a region of higher concentration to a region of lower concentration

Facilitated transport

Moving across a cell membrane with the aid of a membrane protein from a region of higher concentration to a region of lower concentration

Proteins serve as “gates”

Active transport

Moving across a cell membrane with the aid of membrane proteins against a concentration gradient (requires expenditure of cellular energy)

• Proteins involved in active transport are called “pumps”

• Cellular energy is supplied by molecules like ATP

Bile

cholesterol derivative emulsifier produced in the liver

Steroid hormones

derivatives of cholesterol

Three major groups of sex hormones

Estrogens (the “female” sex hormones)

• Androgens (the “male” sex hormones)

• Progestins (the pregnancy hormones)

Adrenocorticoid hormones

are produced by the adrenal glands (small organs located on top of each kidney)

Biological wax structure

monoester of a long-chain fatty acid and a long-chain alcohol

Saponifiable Lipids

Saponification is possible in molecules that contain the following linkages (bonds): ester, amide, glycosidic

Ex) Triacylglycerols, glycerophospholipids, sphingophospholipids, sphingoglycolipids, biological waxes