% of genome coding for membrane proteins

25 percent

variability of chem composition of membranes

all contain plasma + proteins

range from myelin (axon signal transmission), with 20% proteins to inner mitochondrial membrane (ETC), with >70% proteins

major lipid components of membranes

cholesterol

and

phospholipids (glycerophospholipids and sphingophospholipids)

phosphatidic acid (1,2,-diacylglycerol-3-phosphate)

parent compound for series of glycerophospholipids

unsaturated fatty acid chain

double bonds

usually found on C2 of glycerophospholipid

saturated fatty acid

all single bonds

found on C1 of glycerophospholipids

rotation around single bond-->flexible

configuration of fatty acid double bonds

 almost always cis

results in rigid 30 degree bend in hydrocarbon chain (kink)

reduces van der Waals, reduces melting point and therefore memb fluidity

more unsat cis-->greater fluidity

more sat and unsat trans-->greater rigidity

amphipathic

characteristic of glycerophospholipids

polar, hydrophilic head

apolar, hydrophobic tail

self-association of amphipathic lipids in aqueous solution

micelle--1 layer

bilayer

liposome

major groups attached to glycerol (polar head) by phosphodiester bridge (5):

choline (+)

ethanolamine (+)

serine (+)

glycerol

inositol (ring)

2 most common glycerol-phospholipids

phosphatidylcholine (lecithin)

phosphatidylethanolamine (cephalin)

polar heads with no net electric charge

phosphatidylinosital and phosphatidylserine and phosphatidylglycerol phosphogliceride (diphosphatidylglycerol or cardiolipin)

negatively charged

cardiolipin

found in abundance in cardiac muscle

contains two phosphatidic acids linked by a glycerol

mitochondrial inner membranes

sphingolipids

abundant in nervous tissue

--NH2 attachment site

mores stable(slow turnover) than diacylglycerols (rapid membrane turnover)

ceramide

sphingosine with sat or unsat long chain fatty acyl group in amide linkage on the amino group

similar in structure to diacylglycerol

sphingomyelin

ceramide + phosphorylcholine

most abundant sphingolipid in mammalian tissue

cholesterol and membranes

doesn't cross membrane

equal representation on inner and outer membrane leaflets

increase in cholesterol-->dec in memb fluidity

membrane composition plasma memb vs intracellular memb

plasma: neutral lipids and sphingolipids

intracellular: glycophospholipids

flipase

enz facilitating transverse (flip-flop) exchange of inner/outer leaflet memb lipids

viscosity throughout membrane bilayer

middle=highly viscous

surface=much less fluid because of cholesterol

asymmetric lipid distribution

aside from cholesterol, inner/outer leaflets are not symmetrical in lipid distribution

ex: phosphatidylethanolamine and phosphatidylserine almost exclusively found in inner leaflet, phosphatidulcholine and sphingomyelin mostly found in outer leaflet

effect of short-chain fatty acids on memb fluidity

more short-chain fatty acids--> increased fluidity

effect of unsaturation on memb fluidity

inc unsaturated-->inc fluidity

effect of cis double bond kinks

prevent tight packing

create pockets containing water and small ions

calcium ion effect on fluidity

more calcium ions decrease fluidity bc of interaction with neg charged phospholipids

lipid rafts

microdomains enriched in cholesterol and sphingolipids (usually assoc'd with intermemb protein)-->'rafts' of dec fluidity, move easily

can dimerize with eachother-->new binding capabilities

depleted cholesterol-->dissociation of rafts