Membrane Transport

(Question 1) 

I. Energetics- 

a. When Vm is high, ion movement is opposed

b. When Vm is low, ion movement is favored

c. When Vm=0, there is no movement across a membrane 

d. Chemical gradient-different in solute concentration

e.Vm and the chemical gradient make up the electrochemical gradient

*2nd Law of Thermodynamics- Molecules like greatest randomness and lowest energy

* When Vm and the concentration gradient are low there is energetic favor

II.  Transporters (2 kinds)

a.[ficilitated diffusion] Go with concentration gradient-ionophonre-mediated ion channel, ion channel and facilitated diffusion 

b. [active transport] Go against concentration gradient-primary active transport, secondary active transport 

I. Carriers-bind their substartes with high stereospecificity, catalyze transport at rates well below the limits of free diffusion and are saturable in the same sense as are enzymes

II. Channels-Generally allow transmembrane movement at rates several orders of magnitude greater than those of typical carriers, rates approaching a limit of unhindered diffusion.

Channels typically show less stereospecificity than carriers and are usually not saturable

a. uniport-transports one solute in one direction

b. cotransport-an anion exchanger typical of all systems that simultaneously carries two solutes across a membrane

1) Symport-carries two solutes in the same direction

2) Antiport -carriers two solutes in opposite directions

1)      Tell the “big picture” story of why membrane transport is needed and how it happens

I.                   Importance of membrane transport-

a.       Most things do not cross membranes without a transporter because they increase movements of molecules by magnitudes

b.      Polar molecules have a high energy barrier and cannot cross the nonpolar environment easily without a transporter

c.       Polar or charged compounds and ions require a

transporter

d.      In some cases a membrane protein simply facilitates the diffusion of a solute down its concentration gradient, but transport often occurs against a gradient of concentration, electrical charge or both, in which case solutes must be “pumped” in a process that requires energy

e.       The energy may come directly from ATP hydrolysis or may be supplied in the form of movement of another solute down its electrochemical gradient with enough energy to carry another solute up its gradient

f.       Ions may also move across membranes via ion channels formed by proteins, or they may be carried across by ionophores, small molecules that mask the charge of the ions and allow them to diffuse through the lipid bilayer

g.      With very few exceptions, the traffic of small molecules across the plasma membrane is mediated by proteins such as transmembrane channels, carriers, or pumps.

I.                   Forces that drive simple diffusion-

a.       When two aqueous compartments containing unequal concentrations of a soluble compound or ion are separated by a permeable divider (membrane), the solute moves by simple diffusion from the region of high concentration, through the membrane, to the region of lower concentration, until the two compartments have equal solute concentrations

b.      In another case, ions of opposite charge may be separated by a permeable membrane.  Now there is a transmembrane electrical gradient, or membrane potential (Vm). 

                                                              i.      This membrane potential produces a force opposing ion movements that increase Vm and and drive ion movements that movements that reduce Vm (why?)

                                                            ii.      Therefore, the direction in which a charged solute tends to move spontaneously across a membrane depends on both the chemical gradient (the difference in solute concentration) and the electrical gradient (Vm) across the membrane.

                                                          iii.      Together, these two factors are referred to as the electrochemical gradient/potential.  This behavior of solutes is in accord with the second law of thermodynamics: molecules tend to spontaneously assume the distribution of greatest randomness and lowest energy

c.       To pass through a lipid bilayer, a polar or charged solute must first give up its interactions with the water molecules in its hydration shell, then diffuse through a solvent (lipid) in which it is poorly soluble.  The energy used to strip away the hydration shell and move the polar compound from water into and through lipid is regained as the compound leaves the membrane on the other side is rehydrated

d.      An activation barrier must be overcome in this case to reach the intermediate stage.  The energy of activation (delta G double dagger) for translocation of a polar solute across the bilayer is so large that pure lipid bilayers are virtually impermeable to polar and charged species.

II.                Forces that drive facilitated diffusion-

a.       Membrane proteins lower the activation energy for transport of polar compounds and ions by providing an alternative path through the bilayer for specific solutes

b.      Proteins that bring about this facilitated diffusion (Passive transport), are not enzymes in the usual sense; their “substrates” are moved from on compartment to another but are not chemically altered (molecules pass through the transporters without changing)

c.       Membrane proteins that speed the movement of a solute across a membrane by facilitating diffusion are called transporters or  permeases

d.      When molecules move through transporters, the transporters bind to them with stereochemical specificity through multiple, weak, noncovalent interactions

e.       The activation energy is lowered when transporters are involved because the negative free-energy change associated with the weak interactions formed counterbalance the positive free energy it takes to remove the hydration shell of a molecule (loss of water of hydration).

f.       In general, transporters span the membranes several times, forming a transmembrane channel lined with hydrophilic amino acid side chains: The channel provides an alternative path for a specific substrate to move across the lipid bilayer without its having to dissolve in the bilayer, further lowering the activation energy for transmembrane diffusion (passage of molecules)

g.      The result is an increase of several orders of magnitude in the rate of transmembrane passage of a substrate (molecule)

3)      Compare and contrast simple diffusion and facilitated diffusion

I.                   Comparison of simple and facilitated diffusion-

a.       In simple diffusion and facilitated diffusion, the ultimate goal is to have a molecule or solute (ion, charged, polar) transported across a membrane

b.      Activation energy and hydration of a solute molecule’s shell happen in both cases

c.       There is an electrochemical gradient in both cases

II.                Contrast of simple and facilitated diffusion-

a.       Simple diffusion deals with equilibrating solutes across a permeable membrane from higher solute concentration to lower

b.      Facilitated diffusion uses transporters that lower the activation energy through weak interactions.

c.       Facilitated diffusion is much faster

4)      Compare and contrast facilitated diffusion with enzyme catalysis

I.                   Comparison of facilitated diffusion and enzyme catalysis

a.       Charge development is unfavorable and can be circumvented by donation of a proton by H3O+ (specific acid catalysis) or HA (general acid catalysis) where HA represents any acid.  Similarly, charge can be neutralized by proton abstraction by OH- (specific base catalysis) or B: (general base catalysis), where B: represents any base

b.      In both cases, charges are bad and something is being done to neutral, mask or make up for the charge in the reactions

I.                   Need for active transport-

a.       In passive transport, the transported species always moves down its electrochemical gradient and is not accumulated above the equilibrium concentration. 

b.      Active transport, by contrast, results in the accumulation of a solute above the equilibrium point.  Active transport is thermodynamically unfavorable (endergonic) and takes place only when coupled (directly or indirectly) to an exergonic process such as the absorption of sunlight, an oxidation reaction, the breakdown of ATP, or the comitant flow of some other chemical species down its electrochemical gradient.

c.       In primary active transport, solute accumulation is coupled directly to an exergonic chemical reaction, such as conversion of ATP to ADP+Pi.

d.      Secondary active transport occurs when endergonic (uphill) transport of one solute is coupled to the exergonic (downhill) flow of a different solute that was originally pumped uphill by primary active transport

6)      Compare and contrast active transport and facilitated diffusion

I.                   Comparison of active transport and facilitated diffusion-

a.       In passive transport, the transported species always moves down its electrochemical gradient and is not accumulated above the equilibrium concentration. 

b.      Active transport, by contrast, results in the accumulation of a solute above the equilibrium point.  Active transport is thermodynamically unfavorable (endergonic) and takes place only when coupled (directly or indirectly) to an exergonic process such as the absorption of sunlight, an oxidation reaction, the breakdown of ATP, or the comitant flow of some other chemical species down its electrochemical gradient.

c.       Main difference: one happens above equilibrium; one requires an exergonic process to occur

7)      Discuss the properties of carriers and channels and recognize examples of each based on descriptions of the transporters properties

I.                   Carriers-

a.       Carriers display enzyme-like kinetics; they are saturable and much slower than channels because they’re enzymes

b.      Carriers bind their substrates with high stereospecificity, catalyze transport at rates well below the limits of free diffusion, and are saturable in the same sense as are enzymes; there is some substrate concentration above which further increases will not produce a greater rate of activity

c.       Among the carriers, some simply facilitate diffusion down a concentration gradient; they are the uniporter superfamily.  Others (active transporters) can drive substrates across the membrane against a concentration gradient, some using energy provided directly by a chemical reaction (primary active transporters) and some coupling uphill transport of one substrate with the downhill transport of another (secondary active transport)

II.                Channels-

a.       Channels generally allow transmembrane moment at rates several orders of magnitude greater than those typical of carriers, rates approaching the limit of unhindered diffusion.  Channels typically show less stereospecificity than carriers and are usually not saturable.  Most channels are oligometric complexes of several, often identical, subunits, whereas many carriers function as monomeric proteins.

8)      Identify a transporter as antiporter, symporter, or uniport

I.                   Antiporter-

a.       Anion exchanger that is typical of all systems, called contransport systems, that simultaneously carry two solutes across a membrane.  When, as in this case, the two substrates move in opposite directions, the process is antiport.

II.                Symporter- In symport, two substrates are moved simultaneously in the same direction

III.             Uniport- Transporters that only carry one substrate as uniport systems

9)      Describe the GLUT family of transporters and explain their function based on general principles of transport

I.                   GLUT family of transporters-

a.       Energy-yielding metabolism in erythrocytes depends on a constant supply of glucose from the blood plasma.  Glucose enters the erythrocyte by facilitated diffusion via specific glucose transporter, at a rate about 50,000 times greater than the uncatalyzed diffusion rate

b.      The glucose transporter of erythrocytes (called GLUT1 to distinguish it from related glucose transporters in other tissues) is a type III integral protein with 12 hydrophobic segments, each of which is believed to form a membrane spanning helix.  The detailed structure of GLUT1 is not yet known but one plausible model suggests that side-by-side assembly of several helices produces a transmembrane channel lined with hydrophilic residues that can hydrogen-bond with glucose as it moves through the channel

c.       When [S]=Kt, the rate of uptake is ½ Vmax; the transport process is half-saturated.  The concentration of blood glucose is about three times Kt, which ensures that GLUT1 is nearly saturated with substrate and operates near Vmax; process is fully reversible

d.      As [S]in approaches [S]out the rates of entry and exit become equal.  Such a system is therefore incapable of accumulating the substrate (glucose) within a cell at concentrations above that in the surrounding medium; it simply achieves equilibration of glucose on the two sides of the membrane much faster than would occur in the absence of a specific transporter

e.       Thus GLUT1 shows three hallmarks of passive transport:

                                                              i.      High rates of diffusion down a concentration gradient

                                                            ii.      Saturability

                                                          iii.      Specificity

f.       In liver, GLUT2 transports glucose out of hepatocytes when liver glycogen is broken down to replenish blood glucose

                                                              i.       GLUT2 has a Kt of about 66 mM and can therefore respond to increased levels of intracellular glucose (produced by glycogen breakdown) by increasing outward transport

g.      Sketetal muscle and adipose tissue have yet another glucose transporter, GLUT4 which is distinguished by its stimulation by insulin: its activity increase when release of insulin signals a high blood glucose concentration, thus increasing the rate of glucose uptake into muscle and adipose tissue                       

I.                   Three types of ATP driven transport-

a.       The family of active transporters called P-types ATPases are ATP-driven cation transporters that are reversibly phsophorylated by ATP as part of the transport cycle; phosphorylation forces a conformational change that is central to moving the cation across the membrane.