BIOL 304 Chapter 12: Lipids and Biological Membranes Part 1

What are lipids?

water-insoluble biomolecules that are highly soluble in organic solvents

Why are most lipids hydrophobic?

due to the presence of fatty acids

What are fatty acids?

long hydrocarbon chains that terminate carboxylic acid groups

True or False: fatty acids can vary in length and degree of saturation

True! can be unsaturated or saturated

What are saturated fatty acids?

single bonded fatty acids

What are unsaturated fatty acids?

double or triple boned fatty acids

Why are fatty acids referred to by their carboxylate form?

they usually exist in this form since they're ionized at physiological pH

How are fatty acids named?

names are derived from the parent hydrocarbon by substitution of "oic" with final "e"

Where does numbering of carbon start in fatty acids?

at the carboxyl terminal carbon atom

How are fatty acids numbered?

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

What is the second carbon of a fatty acid called?

the alpha carbon

What is the third carbon of a fatty acid called?

the beta carbon

What is the methyl carbon atom at the distal end of the fatty acid chain called?

the omega (ω) carbon

What is the first way that fatty acids are structured within biological systems?

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

What is the second way that fatty acids are structured within biological systems?

an unbranched hydrocarbon chain

What is the third way that fatty acids are structured within biological systems?

a saturated or unsaturated (with double binds in cis configuration) alkyl chain

What enhances the fluidity of fatty acids?

short chain length and unsaturation

What is the first function of lipids?

fuel molecules

What is the second function of lipids?

highly concentrated energy stores

What is the third function of lipids?

signal molecules and messengers in signal-transduction pathways, and components of membranes

What is the fourth function of lipids?

the essential component of biological membranes

What are the principal lipids in eukaryotic membranes?

phospholipids, glycolipids, and cholesterol

What are membranes?

sheetlike structures that are two molecules thick and form closed boundaries; they are asymmetric, non covalent assemblies that are fluid

What are membranes made of?

mainly lipids and proteins linked by carbohydrates

How are lipid bilayers formed?

through the hydrophobic and hydrophilic moieties of lipids

True or False: membranes contain proteins that are embedded in lipid bilayers with distinct functions

True!

True or False: membranes tend to be electrically polarized

True!

What are phospholipids composed of?

one or more fatty acids, a platform to which the fatty acids are attached, a phosphate, and an alcohol attached to the phosphate

What are phosphoglycerides?

phospholipids derived from glycerol

How is phosphatidate created?

the OH groups at carbon 1 and carbon 2 of glycerol are esterified to the carboxyl groups of the two fatty acid chains

What are phosphatidates used for?

to form phosphoglycerides

How do phosphatidates form phosphoglycerides?

an ester bond forms between the phosphate group of phosphatidate and the hydroxyl group of an alcohol

What is sphingosine?

an amino alcohol that contains a long, unsaturated hydrocarbon chain

What is sphingomyelin?

common membrane phospholipid with a sphingosine

What are glycolipids?

lipids containing a sphingosine backbone with 1+ sugars attached to the primary -OH group

Where are the sugar residues of glycolipids located?

on the extracellular side of the membrane

What is cerebroside?

glycolipid containing a single glucose or galactose residue

What is cholesterol?

steroid built from 4 linked hydrocarbons

How is cholesterol structed?

has no fatty tails and has a more distinct ring structure; allows for more fluidity with the lipid membrane (lipid membrane will not be as densely packed)

How are the ends of cholesterol structured?

one end has a linked hydrocarbon tail and the other end has an OH group

How is cholesterol oriented in the membrane?

it is parallel to fatty acid chains if other phospholipids

What is the purpose of the OH end of cholesterol?

interacts with phospholipids head groups in a membrane

What are amphipathic (amphiphilic) molecules?

molecules that contain both a hydrophobic and hydrophilic moiety

True or False: membrane lipids are amphipathic molecules

True!

What is the hydrophobic moiety in membrane lipids?

fatty acid tails

What is the hydrophilic moiety in membrane lipids?

phosphorylcholine

Why are membrane arranged in a certain way?

their formation is caused by the amphipathic nature of the molecules

What are micelles?

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

What happens if phospholipids or glycolipids are surrounded by water?

they readily organize themselves to form a micelle

What is the lipid bilayer?

bimolecular sheet; two lipid sheets

How are the hydrophobic tails arranged in the lipid bilayer?

are inside the bilayer; the tails of each sheet interact with each other, forming a permeability barrier

How are the hydrophilic heads arranged in the lipid bilayer?

are outside the bilayer; the heads groups interact with the aqueous medium

Why do phospholipids and glycolipids not form micelles in the lipid bilayer?

there is no space to form it since the space is being taken up by their two tails

What interactions help stabilize the lipid bilayer in aqueous solutions?

hydrophobic, van der Waals, electrostatic, and hydrogen-bonding interactions

Where do van der Waals interaction occur in the lipid bilayer?

between hydrocarbon tails

Where do electrostatic and hydrogen-bonding interactions occur in the lipid bilayer?

between polar head groups and water molecules

How does the lipid bilayer form compartments?

lipid bilayer will tend to close on themselves so that there are no edges with exposed hydrocarbon chains

Why is the lipid bilayer self-sealing?

a hole in a bilayer is energetically unfavorable, which means the lipid bilayer seals up minor holes quickly

What are liposomes (lipid vesicles)?

aqueous compartments enclosed by a lipid bilayer

What are lipid vesicles used for?

to study membrane permeability or to deliver chemicals to cells

How are liposomes made?

formed by suspending a lipid in aqueous medium and sonicated (agitated by high-frequency sound waves)

How are ions and molecules able to be trapped within liposomes?

within a medium, vesicles form around these ions trapping them in aqueous compartments

How can membrane proteins be embedded into liposomes?

by solubilizing the proteins in the presence of detergents and then adding them to the phospholipids

True or False: lipid bilayer membranes have a very low permeability for ions and most polar molecules

True!

Why is water able to pass through the lipid bilayer easily?

it has a low molecular weight, high concentration, and lack of complete charge

What allows for ions and other molecules to pass through the membrane?

membrane proteins

True or False: membranes vary in protein content

True! every cell type has different types of proteins and different levels of proteins in their membrane; type of proteins in membrane are a reflection of biochemistry occurring within the cell

What is myelin?

a membrane that serves as an electrical insulator around certain nerve fibers

Why does myelin have a low concentration of proteins?

allow for it to stretch better, which is ideal for nerve impulses

What is the main role of myelin?

they play a crucial role in the rapid transmission of nerve impulses

What type of cells make up myelin?

Schwann cells

How can membrane proteins be visualized?

by using SDS-polyacrylamide gel electrophoresis

How are we able to tell what cell type we are looking at in electrophoresis?

membranes performing different functions contain different types of proteins, this allows us to tell what cell we're looking at since these proteins differ

What are integral membrane proteins?

proteins that interact extensively with the hydrocarbon chains of membrane lipids

What are peripheral membrane proteins?

proteins bound to membranes primarily by electrostatic and hydrogen-bond interactions with head groups of lipids

What conditions can disrupt peripheral membrane proteins?

adding salts or by changing the pH

What are peripheral membrane proteins usually bound to?

integral proteins or covalently attached hydrophobic chains

What is the most common structural motif in membrane proteins?

membrane-spanning alpha helicases

What is bacteriorhodopsin?

a light-powered proton pump in archaea; they are a integral membrane protein composed of alpha helicases

Why do the membrane-spanning helicases of bacteriorhodopsin contain mostly nonpolar amino acids?

nonpolar residues are in contact with the hydrocarbon core of the membrane or with one another, the core is nonpolar so these residues also need to be nonpolar

Where are the polar and charged residues of the membrane-spanning helices found?

in the cytoplasmic and extracellular regions, since they interact with the environment outside of the membrane

What is porin?

a protein from the outer membrane of bacteria

How is porin formed?

from a single antiparallel beta sheet curled up to form a pore or channel

The outer surface of porin is nonpolar, while the inner surface is...

hydrophilic and filled with water

How is the antiparallel beta sheet in porin structured?

it has alternating hydrophobic and hydrophilic amino acids along each beta strand

What is prostaglandin H2 synthase-1?

endoplasmic reticulum membrane-bound enzyme that promotes inflammation and modulates gastric acid secretion; a homodimer

Where is prostaglandin H2 synthase-1 located?

lies on the outer surface of the membrane

How is prostaglandin H2 synthase-1 bound to the membrane surface?

through proteins, specifically, by a set of alpha helicases that extend from the bottom of the protein into the membrane

Why is prostaglandin H2 synthase-1 considered an integral membrane?

it is attached to the membrane surface and also a detergent is required in order to release it

What is arachidonic acid?

a hydrophobic molecule generated by the hydrolysis of membrane lipids

Why is arachidonic acid important?

it interacts with prostaglandin H2 synthase-1 to catalyze the formation of prostaglandin H2

How does aspirin inhibit prostaglandin H2 synthase-1?

it inhibits the cyclooxygenase activity by transferring its acetyl group to a Ser 530

Why does inhibition of Ser 530 lead to inhibition of cyclooxygenase activity?

Ser 530 lies along the path to the active site, the transfer results in a blockage of the channel

True or False: some proteins associates with membranes through covalently attached hydrophobic groups

True! soluble proteins can associate with membranes if hydrophobic groups in certain conditions

What is the first type of hydrophobic group that allows for proteins to associate with membranes?

a palmitoyl group attached Cys by a thioester bond

What is the second type of hydrophobic group that allows for proteins to associate with membranes?

farnesyl group attached Cys at the carboxyl terminus

What is the third type of hydrophobic group that allows for proteins to associate with membranes?

glycosylphosphatidylinositol (GPI, a glycolipid) anchor attached to the carboxyl terminus

What is lateral diffusion?

a process by which lipids and many membrane proteins are constantly in lateral motion

How can lateral diffusion be visualized?

through the technique of florescence recovery after photobleaching (FRAP)