Chapter 12:Lipids and Cell Membranes

Copyright © 2007 by W. H. Freeman and Company

Berg • Tymoczko • Stryer

BiochemistrySixth Edition

Lipids

• Classified by solubility, not structure.

• Lipids are any molecules that can be extracted from cells using nonpolar organic solvents.

• Lipids are non-polar molecules and are not water soluble although some are amphipathic.

Lipid Classes

Triglycerides (triacylglycerols)

Glycerol based phospholipids

Sphingosine derivatives: sphingomyelins and gangliosides

Steroids/sterols: cholesterol (C27), bile acids (C24), adrenocortical hormones (C21), and sex hormones (C19 & C18)

Eicosinoids: prostaglandins, thromboxanes, leucotrienes and prostacyclins

Fat-soluble vitamins (pg 424)

Triacylglycerol (triglyceride)

Triglyceride (as shown below) is derived from glycerol plus one molecule each of palmitic acid, oleic acid, and stearic acid, the three most abundant fatty acids.

CH2OC(CH2)14CH3

CH2OC(CH2)16CH3

CH3(CH2)7CH=CH(CH2)7COCH

O

O

O

oleate (18:1)

stearate (18:0)

palmitate (16:0)

A very non-polar molecule (storage fat)

Triacylglycerol

• Physical properties of triglycerides depend on the fatty acid components.– the melting point increases as the number of

carbons in the hydrocarbon chain increases and as the number of double bonds decrease.

– triglycerides rich in unsaturated (cis double bonds) fatty acids are generally liquid at room temperature and are called oilsoils.

– triglycerides rich in saturated fatty acids are generally semisolids or solids at room temperature and are called fatsfats.

Fatty AcidsNaturally occuring components of triacylglycerols and phospholipids

Naming Fatty Acids

omega end () carboxyl end (1)

Table of Fatty Acids

Phospholipids

• Phospholipids are the second most abundant group of naturally occuring lipids.– they are found almost exclusively in plant and

animal membranes. Membranes vary in the lipid vs protein content depending upon function.

– the most abundant phospholipids are derived from phosphatidic acid, a molecule in which glycerol is esterified with two molecules of fatty acid and one of phosphoric acid.

– a second class of phospholipids are the sphingomyelins derived from N-acylsphingosine (ceramide) and a phosphate.

Phosphatidic Acid

Glycerol residue

phosphate

Glycerophospholipid cartoon

Head-polar

Tail – nonpolar

An amphipathic structure

Head Pieces

Glycerophospholipids

Sphingophospholipids

PalmitoylSCoA + Serine Sphingosine

Trans db

Trans db

Ceramide phosphate

CH

CH

CH2-O-P-O-

OH

N

O

from serine

from palmitic acid

stearic acid O

O-

• N-acylsphingosine-P (ceramide phosphate)

– the N-acyl fatty acid is usually unsaturated. – the alcohol normally attached to the phosphate is

choline.

Trans db

Glycolipids (no phosphate)

(Ceramide + glucose or galactose)

Glycolipids

A complexcarbohydrate

Gangioside

(Ceramide + a complex carbohydrate)

Isoprenoids & the Sterol Family

Isoprenoids are derived from isopentenyl-PP (related to 5-carbon isoprene). Compounds include the terpenes, sterols and fat soluble vitamins.

Sterol Family:

Cholesterol: (C27) Parent molecule and a membrane component

Bile acid salts: (C24) Emulisfy fats for digestion

Adrenocortical Hormones & Progesterone: (C21) Communication

Sex Hormones: Male (C19) and Female (C18)

Cholesterol, C27

Bile acids, C24

C21, C19 and C18

AdrenocorticalHormone

Sex Hormones

Male Female

Eicosanoids

ProstaglandinsProstacyclins, Thromboxanes,Leucotrienes,

Cis - - eicosatetraenoic acid

Eicosanoids

Eicosanoids

Lipid Aggregation

Lipids aggregate to form large non-covalent assemblies.

Micelles are spontaneously formed by fatty acids to provide stabilization in aqueous media.

Phospholipids spontaneously for lipid bilayers. The non-polar tails associate with each other and the polar heads interact with water.

Singer & Nicolson proposed the fluid-mosaic membrane model to incorporate proteins and provide functionality.

Tail (nonpolar) Head (Polar)

Similar structures

Soap Micelle (cross section)

Fatty acid salts

Non-polar inside; Polar outside

Lipid Biayer Cartoon

Phospho lipids

Non-polar inside; Polar surface

Space Filling Model

Close pack with saturated sidechains (less fluid)

Space Filling ModelPacking disrupted by unsaturated sidechains

(more fluid)

Liposome (Lipid vesicle)

Spontaneous formation of liposomes

Trapping Glycine inside of liposomes

Membrane Permeability

Low permeability for ions. Small non-polar molecules (O2, CO2, HOH) pass more easily.

Integral (a,b,c) and peripheral (d,e) Proteins

Bacteriorhodopsin ( strands)

Porin ( strands)

Permits free transport of small molecules.

Diagonal lines show hydrogen bonding between anti-parallel strands.

Non-polar residues are in yellow.

Protein Domain Anchor

Membrane Anchors

Mannose

NAcGlc

Glycophorin in red cells

Hydropathy Plot

Fluid Mosaic Model

Fluid Mosaic Model

Membrane Composition

Inner and Outer Leaflets

Membrane Diffusion

Membrane components are fluid.

The two sides are assymetric.

Phase Transition Temperature

Transition Temperatures

Fat Soluble Vitamins

Saturated

Unsaturated (cis)

Lipoproteins

Endocytosis

End of Chapter 12

Copyright © 2007 by W. H. Freeman and Company

Berg • Tymoczko • Stryer

BiochemistrySixth Edition