Lecture 4

the membrane is made up of a

phospholipid bilayer

• polar surface that interacts with the extracellular environment, hydrophobic interior, and polar surface that interacts with the cytosol

amphipathic lipids are built from

fatty acids

• amphipathic molecules contain a hydrophilic region attracted to water and hydrophobic region excluded from water to form the membrane

• amphipathic fatty acids: one end has a polar carboxyl group, and the rest of the molecule is a nonpolar hydrocarbon tail (acyl chain)

  • the carboxyl group ionizes to carboxylate COO- at physiological pH

• fatty acids differ in length and degree of saturation

saturated

no double bonds (all C atoms are CH2, capped with CH3)

• all C atoms are saturated with H

monounsaturated

• a single cis or trans double bond present

  • all C atoms are not saturated with H, since there is a double bond between carbons

  • trans form: keeps the acyl chain of the fatty acid fairly linear (H atoms on the opposite side of the C=C bond)

  • cis form: makes a 30° kink in the acyl chain of the fatty acid (H atoms on the same side of the C=C bond)

    • keeps membranes fluid because the bent chain cannot tightly pack

there are several different conventions for naming

fatty acids

• first number is the number of C atoms in the chain : number of double bonds in the chain

• saturated fatty acids: higher melting temperature

• monounsaturated fatty acids: lower melting temperature (bend decreases the strength of van der waals interactions and contact area)

• polyunsaturated fatty acids: even lower melting temperature (more non-linear character lowers it and decreases the strength of van der waals interactions)

melting point increases as the chain length becomes

longer because this allows for more surface for van der waals contact with its neighbors, and more contact means more thermal energy is required to pull the chains apart

adding a carboxyl group to an alkane results in an

-oic acid (protonated) or -oate (deprotonated)

• palmitate (ionized form of palmitic acid, most abundant saturated fatty acid in the human body and the first fatty acid produced during fatty acid synthesis, and it is the precursor from which cells build longer chains)

  • hexadecane (16 C atoms)

  • hexadenoic acid is protonated

  • deprotonated is hexadecanoate 16:0

the presence of a double bond makes an

alkane an alkene (ending shifts from -anoic/-oate to -enoic/-enoate)

• start counting from the highest oxidation state carboxyl C atom (C atom bound to the most O atoms), and name the double bond at the first C atom from which it appears

• also add the number of double bonds to the name before the suffix (-oic acid or -oate)

the delta system indicates where

double bonds are found on a hydrocarbon chain

• counted from the most oxidized C atom (carboxyl group)

  • number of C atoms : number of double bonds (∆ position of double bond by the first C atom)

there is a naming convention for

omega fatty acids (omega C atom is the C atom farthest from the carboxyl group, which is the C atom of the terminal methyl CH3 group)

• the omega number of the first double bond from the omega side of the CH3 group is which omega fatty acid it is

  • omega-3 fatty acids: double bond between C3 and C4 relative to the most distal carbon (omega CH3 atom)

    • support membrane fluidity and cell signaling and serve as starting material for the synthesis of certain hormones

  • omega-6 fatty acids: double bond between C6 and C7 relative to the most distal carbon (omega CH3 atom)

there are general rules for fatty acids

• double bonds are usually in the cis configuration

• hydrocarbon length is usually between 12-24 and there are an even number of hydrocarbons (fatty acid synthesis addds 2 C atoms at a time from a 2 C atom donor)

• monounsaturated: double bond is usually between C9-C10 (∆9)

  • cis double bonds introduce rigid 30° kinks, and enzymes are shaped to bind to this specific bend

  • trans fats pack like saturated fats since their chains are nearly linear, which is the basis of their effect on the membrane

• polyunsaturated: double bonds are usually ∆12 and ∆15, spaced 3 C atoms apart

cellular membranes are

mosaic mixtures of lipids and proteins in which proteins are embedded to the lipid bilayer and both the lipids and proteins can move laterally

• boundary between the internal and external environments

• proteins have a redundancy of functions, allowing more than one protein to perform the same function on the bilayer

there are three major types of membrane lipids (fatty acids)

all are amphipathic lipids (cholesterol is weakly amphipathic since the lone OH group is only a small polar feature on the nonpolar molecule) and the backbone, linkages, and head group determine the properties that the lipid brings to the membrane

• glycerophospholipids

• sphingolipids

  • sphingophospholipids and sphingoglycolipids

• cholesterol

glycerophospholipids contain

a glycerol backbone, two fatty acids, a phosphate group, and a polar head group

• two nonpolar hydrocarbon chains (palmitate that is saturated and oleate that is cis-monounsaturated with a bend) bound to a glycerol that has an ester bond to a negatively charged phosphate group, which has another ester bond to polar head group (phosphodiester bond)

  • polar groups make phosphatidyl-serine (+ and -, net -), phosphatidyl-ethanolamine (+, net 0), phosphatidyl-choline (+, net 0), and phosphatidyl-inositol (0, net -)

  • polar head groups determine the overall charge of the glycerophospholipid

• the phosphatidate is just the nonpolar hydrocarbon tails with the glycerol and phosphate group

sphingolipids contain

sphingosine rather than glycerol and divide into two groups

• sphingophospholipids contain one sphingosine which takes the place of one fatty acid and glycerol and one unsaturated fatty acid (bent) and has a phosphate group attached to a polar head group

• sphingoglycolipids contain one sphingosine which takes the place of one fatty acid and glycerol and one unsaturated fatty acid (bent) and has sugars attached (glycan group)

sphingolipids are derived from

sphingosine (18 C chain with a hydroxyl OH group, an amino group, and a single trans double bond near the polar end) and one fatty acid

• sphingosine is an amino-alcohol which has an amide bond at C2 which connects to the fatty acid to make a ceramide (structural parent of every sphingolipid, similar to phosphatidates for glycerophospholipids)

  • polar head group can be added, including phosphocholine / phosphoethanolamine (forms sphingomyelin through a phosphodiester bond), glucose (forms a cerebroside through a glycosidic bond), or a glycan group of sugars which has a negative charge on the sialic acid group (forms a ganglioside)

sphingolipids sit in the

outer leaflet of the membrane with their sugars and polar head groups projecting into the extracellular space, where they take part in cell recognition and signaling

• concentrated in the membranes of nerve cells

cholesterol contains

a rigid four ring steroid structure that is planar

• three 6-membered rings and the first one has a polar head group of OH which orients to the aqueous surface, and one 5-membered ring which has a short hydrocarbon tail which is anchored via van der waals interactions and can insert into the hydrophobic interior of the lipid bilayer

• chemical precursor, and its function occurs from the planarity and rigidity of its rings

cholesterol is the key

metabolic precursor in steroid hormone and bile acid biosynthesis

• it can also be modified by removing the hydrocarbon tail to make steroid hormones and bile acids

  • progesterone: cell signaling

  • glycocholate: bile acid for digestion (adds more polar OH groups)

membrane lipids spontaneously form

bilayers in water, which are flexible and functional (thermodynamically favored formation of the membrane)

• hydrophilic polar head groups are on the surface (outer and inner leaflets) and hydrophobic hydrocarbon tails are in the interior shielded from water

  • the driving force for this is the hydrophobic effect since the water surrounding the membrane gains entropy from the hydrophobic tails clustering together in the interior of the bilayer

the bilayer is held together by

hydrogen bonds and electrostatic interactions which hold the polar head groups against the surrounding water, and van der waals interactions hold the packed hydrocarbon tails against one another

• these weak noncovalent forces allow for the lipids to remain free to move within the plane of the membrane, which gives the bilayer its fluidity and flexibility

• also allow for the membrane to self-seal as the exposed tails are driven back out of contact with water

membrane lipids are

asymmetrically distributed

• the two leaflets are not symmetrical and the membrane proteins and lipids on them depend on the function, but are both have the polar heads facing the aqueous environment and hydrophobic interior

• there are different lipids on each leaflet

  • outer leaflet includes choline-containing lipids and glycolipids, including sphingomyelin, phosphatidylcholine, and ganglioside

  • inner leaflet includes phosphatidylinositol, phosphatidylethanolamine, and phosphatidylserine

  • cholesterol is present in both leaflets

trans bilayer movement requires

catalysis (flip-flop diffusion)

• lateral diffusion: moving positions on the same leaflet is energetically favorable and occurs readily and fast

  • the polar and hydrophobic contacts are maintained throughout the move

• transverse diffusion: movement of a lipid from one leaflet to another, not energetically favorable

  • the lipid must drag its polar head group through the hydrophobic core, and un-catalyzed flip-flop diffusion is slow

  • the enzyme speeds up the rate, but does not change the favorability of the process

flippase

P-type ATPase

• uses energy from the hydrolysis of ATP to ADP moves aminophospholipids PE and PS from the outer leaflet to the inner cytosolic leaflet

  • creates space and makes conditions favorable for it to move

• build and hold the asymmetry at the cost of ATP

floppase

ABC transporter

• uses energy from the hydrolysis of ATP to ADP moves phospholipids from the inner cytosolic leaflet to outer leaflet

• build and hold the asymmetry at the cost of ATP

scramblase

moves lipids in either direction toward equilibrium (down the concentration gradient)

• no ATP is used and it collapses asymmetry

membrane fluidity is affected by

temperature

• fluidity describes how freely the lipids move, either by lateral diffusion, rotation, or flexing of their tails, and depends on both temperature and lipid composition of the membrane

• heat produces a thermal equilibrium motion of side chains

  • liquid-ordered state Lo: chains are extended and packed tightly with little motion

  • liquid-disordered state Ld: thermal energy makes the chains more fluid and at constant motion with no regular arrangement

    • Ld state does not hold together well enough to be a membrane and act as a barrier

there is a certain transition temperature

Tm (melting temperature for the transition between lipid-ordered Lo and lipid-disordered Ld states)

• Lo states below the Tm value, Ld states above the Tm value

• with increasing temperature, there is some more flexibility, but with too much temperature, the membrane will become too fluid

• sigmoidal graph which is flat at low temperatures, and then rises steeply as the temperature passes through Tm, and then becomes flat at high temperatures

  • functioning near the Tm value keeps the membrane both intact and flexible

fluidity can be maintained at different temperatures by

altering the membrane lipid composition

• all membranes have a mixture between saturated and unsaturated fatty acids, where they can be packed tight enough to still be a barrier but loose enough to still have flexibility

  • saturated fatty acids have straight chains that stack against one another with maximal van der waals contact, which raises the Tm and makes the membrane more ordered

  • unsaturated fatty acids have kinks that keep the neighboring chains from packing closely, which lowers the Tm and makes the membrane more fluid

• the fluidity of the membrane can also be adjusted at lower or higher temperatures by incorporating shorter / longer and unsaturated / saturated fatty acids

membrane fluidity is affected by

cholesterol

• polar OH head group, rigid steroid ring structure of cholesterol stiffens the hydrophobic region of the membrane, and nonpolar hydrocarbon tail is more flexible and fluid

• modify the fluidity of the membrane to make it optimal (fluidity buffer that opposes either extreme)

  • for a membrane that is fluid and above its Tm, cholesterol restrains and orders the mobile chains by filling space and reducing their motion, so the membrane becomes less fluid

  • for a membrane that is rigid and below its Tm, cholesterol wedges between the tightly packed chains and prevents them from settling into an ordered gel so the membrane becomes more fluid

cholesterol buffers

membrane fluidity at temperature extremes

• with cholesterol, the graph transitions from a sigmoid to a linear curve

  • at temperatures lower than the Tm value, the cholesterol makes it more flexible and fluid (above the normal curve)

  • at temperatures higher than the Tm value, the cholesterol makes it less flexible and stabilized (below the normal curve)

nonpolar molecules cross the lipid bilayer by

simple passive diffusion: diffusion of hydrophobic molecules across the membrane down their concentration gradient (requires no energy)

• ex. small nonpolar molecules such as CO2 and O2 can diffuse freely through the hydrocarbon interior of the bilayer into and from the red blood cells down its concentration gradient

• larger nonpolar molecules can also cross passively but more slowly due to their size being a limiting factor

• small polar but uncharged molecules such as H2O and ethanol can cross well but more slowly due to their polarity rather than size

• large polar molecules such as glucose and ions are blocked, because they cannot shed their favorable interactions with water via the hydration layer and enter the low-dielectric core

  • nonpolar molecules > small polar molecules > large polar molecules and ions

diffusion is driven by a

concentration gradient (move from higher to lower concentration) and molecules move toward equilibrium

• the net change in movement is stopped at equilibrium, but there will always be movement of the molecules across the membrane after the concentrations have been equalized

• simple diffusion is thus a spontaneous process because it raises entropy

passive diffusion has a

free energy change (∆G is the strength of the diffusion driving force, which grows with temperature and the magnitude of the concentration gradient)

• ∆G = RT ln (C2 / C1)

• calculation of concentration gradient (negative if spontaneous, down the concentration gradient if C2 is less than C1, positive if nonspontaneous, against the concentration gradient if C1 is less than C2)

  • for an uncharged molecule of solute:

    • R = 0.008314 kJ/mol K

    • T = 310 K (physiological conditions at 37°C)

    • C1 = concentration at starting point

    • C2 = concentration at ending point

the ∆G of ion transport depends on both the

concentration and electrical gradients (an ion moves down both gradients at once, so it is pulled to the side with opposite charge and where its concentration is lower)

• ∆G = RT ln (C2 / C1) + ZF∆V

  • first part is due to the concentration gradient, second part is due to the electrical gradient (valence of the atom)

  • Z = ion charge

  • F = 96.5 kJ/V mol (Faraday’s constant)

  • ∆V = potential across the membrane in volts

both concentration and electrical gradients influence the

free energy of ion movement

• if the concentration / electrical gradient is favorable / unfavorable, this can change the equation

• if both components are negative, ∆G will be very negative and favor the ion movement (both conditions are favorable)

• if both components are positive, ∆G will be very positive and will not favor the ion movement (both conditions are unfavorable)

polar molecules require a

transport protein to diffuse across the membrane

• there is normally a high activation energy barrier, but it is decreased with a carrier to go through the hydrophobic core

  • the activation energy barrier dictates the rate of reaction, whereas the ∆G value dictates whether it will proceed

in order for a polar molecule to pass through the membrane, it would have to

give up the water molecules stabilizing it by the hydration shell, and diffuse across the hydrophobic hydrocarbons in which it is insoluble, and this raises the free energy and creates a large activation energy barrier

• membrane transport proteins lower the activation energy barrier by creating a pathway lined with hydrophilic amino acid side chains so the polar solute travels through a polar or charged channel rather than dissolving in the lipid

  • binding occurs through weak noncovalent interactions which compensate for the loss of water reactions from dehydration, keeping the solute in a low-energy state

transport proteins facilitate the

diffusion of polar molecules down a concentration gradient - passive transport

• each transporter has an interior that is sized to accommodate the transported molecule

• each interior is lined with amino acids that facilitate transport by providing an energetically favorable environment, substituting for the hydration layer that the solute must give up

• because the fit and lining are specific, a given transport protein interacts with a particular solute rather than any polar molecule at random

transport proteins only

speed up the crossing of the solute by lowering the activation energy barrier, and the overall process remains spontaneous since the solute is moving from a high to low concentration, and thus the ∆G value is negative

• free energy change says whether a process is favorable, but not about how quickly it will run

• transports alter the rate of movement of solutes without altering the favorability and spontaneity of the process

active transporters drive molecules

against their concentration gradient - active transport

• net direction of movement from a low to high concentration has a +∆G value, and energy input is required through ATP hydrolysis which releases energy to make the process favorable through coupling (primary active transport)

  • low C1 concentration, high C2 concentration