Biochemistry Sixth Edition Berg • Tymoczko • Stryer Chapter 12: Lipids and Cell Membranes Copyright © 2007 by W. H. Freeman and Company

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

Fatty Acids Naturally occuring components of triacylglycerols and phospholipids

Naming Fatty Acids omega end (w) 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 phosphate Glycerol residue

Glycerophospholipid cartoon Tail – nonpolar An amphipathic structure Head-polar

Head Pieces

Glycerophospholipids

Sphingophospholipids PalmitoylSCoA + Serine    Sphingosine Trans db Trans db

Ceramide phosphate N-acylsphingosine-P (ceramide phosphate) the N-acyl fatty acid is usually unsaturated. the alcohol normally attached to the phosphate is choline. C H 2 - O P OH N from serine from palmitic acid stearic acid Trans db

Glycolipids (no phosphate) (Ceramide + glucose or galactose)

Glycolipids A complex carbohydrate 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 Adrenocortical Hormone Sex Hormones Male Female

Eicosanoids Prostaglandins Prostacyclins, Thromboxanes, Leucotrienes, Cis - D5, D8, D11, D14 - eicosatetraenoic acid Prostaglandins Prostacyclins, Thromboxanes, Leucotrienes,

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 Model Packing 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 (a strands)

Porin (b 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

Biochemistry Sixth Edition Berg • Tymoczko • Stryer End of Chapter 12 Copyright © 2007 by W. H. Freeman and Company