Chapter 12: lipids and biological membranes

Fatty acids are key constituents of

Lipidsi

Lipids?

Water-insoluble biomolecules that are highly soluble in organic solvents

Most lipids are

Hydrophobic due to fatty acids

Fatty acids

Long hydrocarbon chains that terminate with carboxylic acid group

Fatty acids vary in

Length and degree of saturation

Fatty acids often referred to in

Their carboxylate form because they are ionized at physiological pH

Fatty acid names are based on

Their parent hydrocarbons

Fatty acid name is derived

From the parent hydrocarbon by substition of oic for the final e

In fatty acids, first number is

The number of carbon atoms, and the second number is the number of double bonds

Numbering fatty acid carbon atoms can be done

2 ways

Chain length and degree of unsaturation affect

Fatty acid properties

Fatty acids in biological systems contain?

• an even number of carbon atoms between 14 and 24 (16 and 18 are most common)

• An unbranched hydrocarbon chain in animals

• A saturated or unsaturated (with double bonds in cis configuration) alky chains

Short chain length and the unsaturation enhance

The fluidity of fatty acids and their derivatives

Biological membranes are composed of

3 common types of membrane lipids

Lipid functions as:

• Fuel molecules

• Highly concentrated energy stores

• Signal molecules and messengers in signal transduction pathways, and components of membranes

• The essential component of biological membranes

Principal lipids in eukaryotic membranes ar

Phospholipids, glycolipids, and cholesterol

Phospholipids are the major class of

Membrane lipids

Phospholipds are composed of

• one or more fatty acids

• A platform to which the fatty acids are attached (examples: glycerol, sphingosine)

• A phosphate

• A alcohol attached to the phosphate

Phosphatidate is the simplist

Phosphoglyceride

Phosphoglycerides

Phospholipds derived from glycerol

In phosphoglycerides,

The -Oh groups at C-1 and C-2 of glycerol are esterfied to the carboxyl groups of the 2 fatty acid chains

Major phosphoglycerides are derived from

• Phosphatidate

An ester bond forms between

The phosphate group of phosphatidate and the hydroxyl group of an alcohol

Sphingomyelin contains

A sphingosine platfrom

Sphingosine

An amino alcohol that contains a long, unsaturated hydrocarbonS

Sphingomyelin

Common membrane phospolipid with a sphingosine bacbone

Glycolipids include

Carbohydrate moieties

Glycolipds

Lipids containing a sphinigosine backbone with 1+ sugars attached to the primary -OH group

In glycolipids, sugar residues are always

On the extracellular side of the membrane

Cerebroside

Glycolipid containing a single glucose or galactose residue

Cholesterol is

A lipid based on a steroid nucleus

Cholesterol

Steroid built from 4 linked hydrocarbon rings

Cholesterol contains

A linked hydrocarbon tain at one end and an -OH group at the other end

Cholesterol oriented

Parallel to fatty acid chains of phospholipids in membranes

In cholesterol, -OH group interacts with

Phospholipid head groups

A membrane lipid is an 

Amphipathetic molecule containing a hydrophilic and a hydrophobic moiety

Amphipathic (amphiphilic) molecules

Molecules that contain both a hydrophobic and hydrophilic moiety

Membrane lipids are

Amphipathic molecules

What is hydrophobi moiety?

Fatty acid tails

What is hydrophilic moiety?

Phosphorylcholine

Phospholipids and glycolipds readily form

Bimolecular sheets in aqueoys media

Membrane formation is a consequence of the

Amphipathic nature of the molecules

Micelle

A globular structure with the polar head groups on the outside surface and the hydrocarbon tails sequestered inside

Lipid bilayer (bimolecular sheet)?

Two lipid sheets

Hydrophobic tails of each sheet interact with

One another, forming a permeability barrier 

Hydrophilic head groups interact with

The aqueous

Lipid bilayer formation is

Spontaneous

Phospholipid and glycolipds, because of the space taken up by their 2 tials,

Do not form small micelles the way single-tailed salts of fatty acids do

Phospholipids and glycolipds spontaneously from

Lipid bilayers in aqueous solutions

Phospholipids and glycolipids spontaneously form lipid bilayers in aqueous solutions, stabilized form:

• Hydrophobic interactions

• Van der Waals interactions between hydrocarbon taisl

• Electostatic and hydrogen-bonding attractions between polar head groups and water molecules

• Langmuir-Blodgett troug

What are the biological consequences of hydrophobic interactions

• Lipid bilayers have an inherent tendency to be extensive

• Lipid bilayers will tend to close on themselves so that there are no edges with exposed hydrocarbon chains (forms compartments)

• Lipid bilayers are self-sealing ( a hole in a bilayer is energetically unfavorable)

Lipid vesicles can be formed from

Phospholipds

Lipid vesicles (lipsomes)

aqueous compartments enclosed by a lipid bilayer

Lipid vesicles are used to study

Membrane permeability or to deliver chemicals to cells

Lipid bilayers are highly

Impermeable to ions and most polar molecules

Lipid bilayer membranes have

Very low permeability for ions and most polar molecules

Permeability of small molecules is correlated with

Their solubility in a nonpolar solvent relative to their solubility in water

Water is an exception due to its

Low molecule wieght

High concentration

Lack of complete charge

Proteins carry out

Most membrane processes

Membrane proteins allow

Transport of molecules and information across a membrane

Membranes vary in

Protein content (<20% to as much as 75%)

The types of membrane proteins in a cell are a reflection of

The biochemistry occurring inside the cell

The lipid rich membranes of myelin form an

Electrical insulator around some nerve fibers

Myelin

Membrane that serves as an electrical insulator around certain nerve fibers

Myelin has

A low content of protein

Myelin plays a

Role in the rapid transmission of nerve impulses

Myelin formed

by Schwann cells wrapping around the axon (the part of the neuron conducting the nerve impulses)

The protein composition of plasma membranes varies with

Cell type

Membrane proteins can be visualized by

SDS-polyactylamide gel electrophoresis

Membranes performing different functions contain

Different types of proteins

What is an integral membrane proteins?

Interact extensively with the hydrocarbon chains of membrane lipids

Integral membrane proteins are released by?

Agents that compete for these nonpolar interactoins

Integral membrane proteins most span

The lipid bilayer

Peripheral membrane proteins?

Bound to membranes primarily by electrostatic and hydrogen-bond interactions with the head group of lipds

Peripheral membrane proteins are disrupted by?

Adding salts or by changing the pH

Peripheral membrane proteins are often bound to

THe surface of integral proteins

Peripheral membrane proteins may be anchored to the

Lipid bilayer by a covalently attached hydrophobic chain

Proteins associate with the lipid bilayer 

In a variety ways

Proteins interact with membranes in a

Variety of ways

Membrane-spanning a helices are

The most common structural motif in membrane proteins

Batceriorhodopsin

Light powered proton pump in archaea

Bacteriorhodopsin are an integral

Membrane portein

Bacteriorhodopsin is composed of?

Almost entirely of a helices

A channel protein can be formed from

Beta strands

Porin?

Protein from the outer membranes of bacteria

Porin is formed from

A single antiparallel B sheet curled up to form a pore or channel

Porin outer surface is

Nonpolar

Inner surface of a porin

Is hydrophobic and filled with water

Embedding part of a protein in membrane can link 

The protein to the membrane surface

Prostaglansin H2 synthase-1

Endoplasmic reticulum membrane-boudn enzyme that promotes inflammation and modulates gastric acid secretion

Prostagladin H2 synthase-1 is a

Homodimer

Prostaglandin H2 synthase-1 lies 

Along the outer surface of the membrane

Prostaglandin H2 synthase-1 bound by

A set of a helices that extened from the bottom of the protein into the membrane

Prostaglandin H2 Synthase-1 is

an integral membra e protein attached to the membrane surface

Prostaglandin H2 synthase-1 is classified

As an integral membrane protein because detergent is required to release the protein

Prosstaglandin H2 synthase-1 catalyzes

The fromation of prostaglandin H2

Arachidonic acid

A hydrophobic molecule generated by the hydrolyis of membrane lipids

Arachidonic acid reaches the prostaglandin H2 synthase-1 acitve site through

A hydrophobic channel

Aspirin inhibits the

Cycloxygenase activity of prostaglandin H2 synthase-1

Aspirin inhibits cyclooxygenase activity by

Transferring its acetyl group to a Ser 530 in a prostaglandin H2 synthase-1

• ser 530 lies along the path to the active site

• Blocks the channel