LIPIDS

 

A. Fatty Acids  (see Table 9-1)

·        Rarely free in nature.

·        Esterified as the major components in lipids.

·        In higher animals and plants:  Predominantly as C16 and C18 species, e.g. palmitic acid (16:0), linoleic acid (18:2), and stearic acid (18:0).  Over half are unsaturated.

·        Most have an even number of C.

·        For most unsaturated FAs, the 1st double bond between C9 and C10.

·        For polyunsaturated FAs, the double bonds tend to occur every 3 C atoms.

·        Saturated FAs as individual molecules are highly flexible.  A wide range of conformation allowed by single bonds between two neighboring carbons.  The fully extended conformation is the most stable one.

·        Double bonds in FAs are almost always cis, with a rigid 30° bend.  The reduced van der Waals interactions cause melting point to drop.

 

 

B. Triacylglycerols

·        Triacylglycerols (= Triglycerides) or neutral fats

·        A major component of the oils and fats in plants and animals.

·        Triacylglycerols are energy reservoirs in animals.  Less oxidized than carbohydrates, proteins.  Less hydrated, lighter in weight.

·        Simple triacylglycerols: only one type of FA

·        Mixed triacylglycerols: 2 or 3 types of FA.

·        Plant oils are usually richer in unsat. FA residues than are animal fats.

 

 

C. Glycerophospholipids (Phosphoglycerides)

 

 

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D. Sphingolipids

·        Also major lipid components of biological membranes.

·        Derivatives of the C18 amino alcohols sphingosine and dihydrosphingosine.

·        Sphingophospholipids

¨   Sphingomyelins (= sphingophospholipids;  see Fig. 9-7)

¯   Ceramides bearing either a phosphocholine or a phosphoethanolamine.

¯   The most common sphingolipid.

¯   The membranous myelin sheath that surrounds and electrically insulates many nerve cell axons is particularly rich in sphingomyelin.

·        Sphingoglycolipids

¨   Cerebrosides

¯   The simplest sphingoglycolipid.

¯   The –CH2-OH group is derivatized by a single sugar residue, such as galactocerebroside in neuronal cell membranes and glucocerebrosides.

¨   Gangliosides

¯   Most complex, are ceramide derivatized by oligosaccharides containing at least one sialic acid.

¯   Primary components of cell surface membranes.  Their complex carbohydrate head groups act as specific receptors for hormones, bacterial toxins, and also function in cell-cell recognition.

 

E. Cholesterol  (Fig. 9-10)

·        A major component of animal plasma membranes.

·        Metabolic precursors of steroid hormones (e.g. testostorone, estrogens)

·         Tend to decrease membrane fluidity.  Its rigid steroid ring system interferes with the motions of FA’s side chain.

 

MEMBRANES AND FUNCTIONS OF BIOLOGICAL MEMBRANES

 

Compartmentation

·        Cells use plasma membrane as an envelope.

·        Subcellular organelles – nuclei, mitochondria, chloroplasts, endoplasmic reticulum, Golgi apparatus, etc.

 

Control of the passage of materials

·        Control the flow of nutrients, waste products, ions, etc.

·        Contain “pumps” and “gates.”

 

Many fundamental biochemical processes occur on or in a membranous scaffolding

·        Oxidative phosphorylation

·        Photosynthesis

 

Processing of information

·        Sensory stimuli

·        Intercellular communication

·        Nerve impulses

·         Hormonal actions

 

 

ARTIFICIAL MEMBRANES

I.   Lipid Monolayer

·        Low concentration of lipids tend to form a monolayer on the surface of water.

·        Polar heads of these amphiphilic molecules (see Fig. 9-4) are immersed in the water, and the hydrophobic tails extend into the air.

 

II.  Micelles 

 

·        Globular aggregates whose hydrocarbon groups are out of contact with the water medium.

 

Single-hydrocarbon-tail lipids (Fig. 9-13):  e.g. soap, detergents. 

·        Form micelles once their concentrations are above the Critical Micelle Concentration (CMC).

·        For small single-tail lipids, e.g. dodecylsulfate, CMC » 1 mM.

Double-tail lipids (Fig. 9-14):

·        Tend to form flattened micelles, part of them are lipid bilayer in structure.

·        For most biological double-tail lipids, CMC < 10-6 M.

 

 

III. Lipid Bilayer and Liposomes (Fig. 9-15)

·        Liposomes are closed, self-sealing, solvent-filled vesicles that are bounded by a single Lipid Bilayer (LB).

·        LBs are impermeable to most polar substances.

·        Permeabilities of LB increase with solubilities in nonpolar solvents.

·        LBs are appreciably permeable to water despite its polarity.  Its small size makes water significantly soluble in the hydrocarbon core of LBs.

·        LBs are two-dimensional fluids.

Transverse Diffusion:  Flip-flop.  extremely slow.  (Fig. 9-16)

Lateral Diffusion:  Much faster.  Lipids are highly mobile in the plane of the bilayer.  (Fig. 9-16)

 

IV. Bilayer Fluidity varies with

·        Lipid Composition:  More fluid with shorter chains, more unsaturated fatty acids, and less cholesterol.

·        Temperature:  More fluid above Transition Temperature. (Fig. 9-18)

 

 

BIOLOGICAL MEMBRANES

 

·        Composed of proteins associated with a lipid bilayer matrix.

·        Specific proteins occur only in particular membranes.

 

I.   Peripheral or Extrinsic Proteins

·        Can be dissociated from membranes by relatively mild procedures that leave the membrane intact.  e.g. cytochrome c.

 

II.  Integral or Intrinsic Proteins  (Figs. 9-19, 9-26)

·        Tightly bound to membranes by hydrophobic forces.

·        Can only be separated from membranes by treatment with agents that disrupt membrane structure.

·        All biological membranes contain integral proteins.

·        Asymmetrically oriented amphiphilic molecules.

·        No integral proteins are completely buried in a membrane.

·        For many integral proteins, the hydrophobic segments anchor the active region of the protein to the membrane.

·        The transmembrane parts are highly hydrophobic and, in some cases, are simple alpha helices.

 

III. Asymmetric Orientation of Membrane Proteins  (Figs. 9-20, 9-26)

 

·        Some membrane proteins (integral or peripheral) are located on or exposed to only a specific surface of a membrane.

·        Other integral proteins (known as transmembrane proteins) span the membrane.  These proteins are oriented in only one direction with respect to the membrane.

 

 

FLUID MOSAIC MODEL

 

·        A unified theory of membrane structure proposed by Jonathan Singer and Garth Nicholson in 1972.

·        Integral proteins resemble “icebergs” floating in a fluid 2-dimensional lipid “sea.”

·        Has been experimentally verified.

·        Example 1 (Fig. 9-28):  Photobleaching and recovery

·        Example 2 (Fig. 9-27):

¨   Mouse cells – labeled with mouse protein-specific antibody-green dye conjugate.

¨   Human cells - labeled with human protein-specific antibody-red dye conjugate.

¨   Both cell types were fused by treatment with Sendai virus.

¨   Initially, the green and red colors were polarized on a given cell surface.  After ~ 40 min, the two color were completely mixed.