C2006/F2402 '08 OUTLINE FOR LECTURE #5  Last updated 02/05/08 02:42 PM

(c) 2008 Dr. Deborah Mowshowitz , Columbia University, New York, NY

Handouts:   5A-- Transport of glucose through body, & 5B -- Models for Active Transport  (You may also want to refer to 4A.)

I. Measurement of Transport of Small Molecules -- Review of Essentials

    A. How is Transport Measured? See top of handout 4A; discussed in detail last time.

    B.   Curve #1 vs Curve #2.  For both curves, you are considering the reaction X_out ↔ X_in. So what's the big difference?

1. In Curve #1, you are looking at how the concentration of X_in varies with time (starting with a fixed  concentration of X_out, and no X inside_.).

a. (Initial) Slope of the curve = rate of uptake (with time as the variable).

b. Plateau value  =  yield =  final value of [X]_in when curve #1 levels off.

c. Note that curve #1 ALWAYS levels off.

2. In Curve #2, you are looking at how the rate of uptake (flux) -- initial slope of curve #1 -- varies for different starting concentrations of X_out.  This curve levels off if a protein must bind to X and/or change conformation significantly in order to move X. (Same idea as a V vs S curve for an enzyme catalyzed reaction.)

II. Kinetics and Properties of each type of Transport -- How you tell the cases apart.

All the cases below refer to the reaction [X]_in ↔ [X]_out.  All the important features are summarized in the table on handout 4A.

    A. Simple Diffusion (Case 1)

1. Curve #1 (concentration of substance X inside plotted vs. time) plateaus at [X]_in = [X]_out.

2. Curve #2 (rate of uptake of X plotted vs concentration of X added outside) does not saturate.

3. Energy: Rxn ( X _in ↔ X _out) is strictly reversible. (K_eq = 1; Standard free energy change (ΔG^o) = 0; at equil. [X]_in = [X]_out).
Actual free energy change (ΔG) and direction of transport depends on concentration of X. If [X] is higher outside, X will go in and vice versa.

4. Importance. Used by steroid hormones, some small molecules, gases. Only things that are very small or nonpolar can use this mechanism to cross membranes. Materials (usually small molecules) can diffuse into capillaries by diffusing through the liquid in the spaces between the cells. (The cells surrounding capillaries do not have tight junctions, except in the brain.) 

    B. Carrier mediated Transport = Facilitated Diffusion using a carrier protein (Case 3). Note we are deferring case 2.

1. Curve #1 same as above (case 1)

2. Curve #2 saturates. See Becker fig. 8-6, or Sadava fig. 5.12 (5.11)

3. Mechanism: Carrier acts like enzyme or permease, with V_max, K_m etc. See Becker fig. 8-8. 

        Carrier can be considered an enzyme that catalyzes:

X_out   ↔  X_in

        Carrier is specific, just like an enzyme. Will only catalyze movement of X and closely related compounds.

4. Energy as above (case 1) -- substance flows down its gradient, so transport is reversible, depending on relative concentrations in and out.

5. Regulation: Activity of transport proteins can be regulated at least 3 ways -- methods a-c below. Methods a & b are common to many proteins and are primarily listed here for comparison (more details elsewhere). Method c is unique to transmembrane proteins.

Notes: (1). This discussion is about the regulation of the activity of pre-existing protein molecules. Regulation of the amount of protein by adjusting rates of synthesis, degradation, etc., will be discussed later.
            (2). Method c applies to cases 2 & 3, not just case 3.

a. allosteric feedback -- inhibition/activation of carrier proteins

b. covalent modification (reversible) of the carrier proteins  --  common modifications are

(1). Phosphorylation  -- addition of phosphate groups -- catalyzed by kinases.

    Kinases catalyze: X + ATP → X-P + ADP

(2). Dephosphorylation -- removal of phosphate groups --  catalyzed by phosphatases.
   
    Phosphatases catalyze: X-P + H₂O → X + P_i

                        P (bold) = phosphate group;  P_i = inorganic phosphate (in solution)

c. removal/insertion of carrier into membranes.

(1). Newly made membrane proteins are inserted into the membrane of a vesicle, by a mechanism to be discussed later.

(2). Vesicle can fuse with plasma membrane; process is reversible.

(a). Fusion of the vesicle with the plasma membrane inserts transport protein into plasma membrane where it can promote transport.

(b). Budding (endocytosis) of a vesicle back into the cytoplasm removes the transport protein and stops transport. 

(3). Some channels and/or carrier proteins are regulated in this way -- channel or carrier proteins can be inserted into the membrane (or removed) in response to the appropriate hormonal signals. Examples --

        (a). GLUT4 -- the insulin sensitive glucose transporter. Insulin promotes insertion of the transporter into the plasma membrane of some cells, allowing increased glucose uptake. Details next time.

        (b). Water channels (aquaporins) in kidney cells.  The hormone ADH (anti-diuretic hormone) promotes insertion of the channels into the plasma membrane. Therefore water is recovered, not lost. More details when we get to kidney.

To see how you analyze uptake, try problem 2-1. To summarize everything so far, try 2-4.

    C. Active Transport (Cases 4 & 5)

1. What's the same? Curve #2 saturates as in previous case.

2. What's different? Curve #1: when it plateaus, [X]_in greater than [X]_out  -- because movement of substance linked to some other energy releasing reaction. (This assumes we are following the reaction X_out → X _in).

3. Mechanism -- An enzyme-like transport protein is involved as in previous case.

a. X moves up its gradient. Protein acts as transporter or pump catalyzing movement of X up its gradient. Therefore transporter action must be powered, directly, or indirectly, by breakdown of ATP.

b. Most primary active transport involves movement of cations. (But see ABC transporters below.) Gradient of cations can then be used to do work, such as secondary active transport.

4. Energy: Not readily reversible; K_eq not = 1 and standard free energy (ΔG^o)  not = zero. Overall reaction usually has large, negative ΔG^o because in overall reaction, transport of X (uphill, against the gradient) is coupled to a very downhill reaction. The downhill reaction is either

a. Splitting of ATP (in primary active transport), or

b. Running of some ion (say Y) down its gradient (in secondary active transport).

5.  Secondary (Indirect) Active Transport -- How does ATP fit in? Process occurs in 2 steps:

a. Step 1. Preparatory stage: Splitting of ATP sets up a gradient of some ion (say Y), usually a cation (Na⁺ or H⁺). 

b. Step 2. Secondary Active Transport Proper: Y runs down its gradient, and the energy obtained is used to drive X up its gradient. See Becker fig. 8-10.

c. Overall: Step (1) is primary active transport; step (2) is secondary and can go on (in the absence of ATP) until the Y gradient is dissipated.

Note that step (1) cannot occur at all without ATP but step (2) can continue without any ATP (for a while).

Try problem 2-2 & 2-10.

6. Some Examples  & Possible mechanisms (models are discussed below). Click on links for animations.

  D. Channels (Case #2)

1. Curve #1 for neutral substances -- Same as cases #1 & #3 above -- with the following important exceptions:

a. Very high rate of transport -- Initial slope of Curve #1 very steep. 

b. Channels often conduct ions. Curve #1 plateaus with  [X]_in = [X]_out  only if X is neutral or there is no electric potential. 

2. Curve #2: Shape like simple diffusion (linear, no saturation) at physiological concentrations. Curve plateaus only at extraordinarily high concentrations, so we are assuming no saturation.

3. Gating/regulation

a.  Most Channels are gated = % time any particular channel/gate is open is controlled (but each individual gate is either open all the way or shut).

(1). Ligand gated -- opens or shuts in response to ligands (= chemicals that bind to substance under discussion). Typical substances that open ligand gated channels are  hormones, neurotransmitters, etc. For a picture see Sadava fig. 5-10 (5.9).

(2). Voltage gated -- opens or shuts in response to changes in voltage. Allows transmission of electrical signals as in muscle and nerve -- see Becker figs. 13-8 & 13-9 (9-9 & 9-10).

(3). Mechanically gated -- opens or shuts in response to pressure. Important in touch, hearing and balance. 

b. Some channels are open all the time (ungated); An example = K⁺ leak channels. These allow a little K⁺ to leave or "leak out" of cells, causing cells to have a slight overall negative charge. This is critical to conduction of impulses by nerve and muscle as will be explained in detail later. Why do leak channels only allow "a little" K⁺ to leave? Why isn't the concentration of K⁺ on both sides of the membrane the same? See below.

c. Other means of regulation -- some channels are regulated by insertion into the membrane or removal from the membrane as explained above for aquaporins in kidney.

4. Curve #1 for ions. Most channels are ion channels -- transport charged particles, not neutral molecules. This raises new energy considerations:

a.  Role of charge: If X is charged, need to consider both chemical gradient & voltage (charge gradient). These can both "push" ions the same way or push in opposite directions.

b. Result of charge: K_eq not usually equal to 1 here --  Curve #1 plateaus when chemical gradient and voltage are balanced (not necessarily at [X]_out = [X]_in). Example:  K⁺ ions stop leaking out of the cell and you reach equilibrium for K⁺ when the charge difference across the cell membrane (which pushes K⁺  in) balances out the concentration difference across the membrane (which pushes K⁺ out).

5. Mechanism. 

a. High Capacity: Lack of saturation and high rate of transport indicate that max. capacity of channel is very large and is not easily reached. This is assumed to be because of one or both of the following:

(1). Binding of ion to channel protein is weak (Km >> 1), and/or

(2). No major conformational change of channel protein is required for ion to pass through.

b. Specificity: Channels are very specific -- each channel transports only one or a very small # of related substances.

c. One model (FYI). How to explain the combination of high speed (& capacity) and high specificity? Mechanism of specificity has been recently figured out for one channel. For pictures see Sadava fig. 5.11 (5.10) & Becker fig. 13-8 & 13-9. For more, see Nobel Prize in Chemistry for 2003, or an interview with Rod MacKinnon about the channel. This is a current hot topic of research, and may be discussed again when we get to nerve function.

See Sadava fig. 44.6 (44.5)for comparison of ion pumps and ion channels;  Becker p. 203 (209)  for comparison of carrier and channel proteins. 

6. Terminology. Diffusion through a channel is usually called "facilitated diffusion" because a protein is needed (as a "facilitator" to form the channel) for transport across the membrane. (As in your texts, and on handout 4B.) However, diffusion though a channel is also sometimes called "simple diffusion," because the rate of transport as a function of [X] is generally linear, as for simple diffusion, as explained in point #2 above. (See handout 4A, case 2.) In other words, the kinetics of passage through a channel are linear (at physiological concentrations of X), like simple diffusion --  not hyperbolic, as in carrier mediated transport or standard enzyme catalyzed reactions. Perhaps transport through a channel should be called "channel mediated diffusion," or "facilitated diffusion through a channel."

See problem 2-6, A. Can you rule out transport through a channel?

III. Active Transport -- How does it work? See above for links to animations & references to pictures in texts.

    A. Examples of Models

1.  An example of primary active transport: Na⁺/K⁺ pump found in all eukaryotic cells (See handout 5B).

a. Pump/enzyme has 2 major forms: one faces in (E₁), one faces out (E₂).

b. Forms have different affinities for K⁺ and Na⁺. (See handout)

c. Role of Phosphorylation:  addition/removal of phosphate switches the pump from one form to the other.

d. Role of kinase & phosphatase activities of pump

(1). This is an example of the use of kinases & phosphatases (for addition/removal of phosphates) to regulate protein activity by reversible covalent modification.

(2). Result of enzyme action in this case:

• Kinase activity (catalyzes phosphorylation) flips pump "out" ( E₁ → E₂)

• Phosphatase activity (catalyzes removal of phosphate) flips pump "in" (E₂ → E₁)

e. Role of Ion binding: activates appropriate enzyme, kinase or phosphatase

f. Location of Enzymes: kinase and phosphatase are part of the pump itself. Not separate proteins.

g. How one cycle goes:

• E₁ → E₂: Binding of Na⁺ on inside activates kinase. Kinase phosphorylates pump. Flips pump out, dumps Na⁺, picks up K⁺.

• E₂ → E₁: Binding of K⁺ on outside activates phosphatase. Phosphatase de-phosphorylates pump. Flips pump in, dumps K⁺, picks up Na⁺.

g. Stochiometry: 3 Na⁺ out per 2 K⁺ in per ATP hydrolyzed. Some of the charge differential is balanced by anion transport. Cells are negative on inside relative to outside, but most of the charge imbalance is NOT due to the pump. (Most charge imbalance is due to K⁺ leak channels as explained above.)

h. Reversibility -- this pump is reversible in the sense that all reactions are reversible: high Na⁺ and K⁺ gradients can be used to make ATP from ADP and P_i , although this never happens in vivo (see 4 below). However it is irreversible in that ATP cannot be used to pump Na⁺ in and K⁺ out. It can only be used to pump K⁺ in and Na⁺ out. (In contrast to the situation with Na⁺/Glucose co-transport.)

2.  An example of secondary active transport: Na⁺/Glucose co-transport

a. Transporter Protein has 2 forms with different affinities for glucose. Either form can face in or out.

b. Role of Na⁺: Binding of Na⁺ switches the protein from one form to the other; alters affinity for glucose.

c. Conformational  Change: Binding of both glucose & Na⁺ probably flips protein so it faces the "other" way; loss of both glucose & Na⁺ does the same -- flips protein so it faces the "other" way.

d. Direction of glucose transport & reversibility: Either form (with or w/o Na⁺) can face in or out. Therefore this pump is reversible in the sense that Na⁺ can theoretically be used to pump glucose into or out of a cell. However, in normal cells, glucose always goes into the cell with Na⁺. Why?

    B. Other Features of Active Transport

1. ABC Transporters

a. Large super family of transporters.  Wide range of substances, not just cations, can be transported. Different proteins transport different classes of substances. All ABC transporters are similar in structure.

b. Type of primary active transport. ATP binds to a cytoplasmic protein domain called an ATP binding cassette. ATP hydrolysis drives movement of substance being transported.

c. Examples:

    (1). MDR protein = multiple drug resistance protein. Pumps many different hydrophobic drugs out of cells; causes lack of response to many cancer treatments. First eukaryotic ABC transporter to be discovered.

    (2). CFTR: Protein defective in cystic fibrosis (CFTR) belongs to same super family as ABC transporters and is similar in structure, but acts as a channel (for Cl^-), not a transporter. ATP probably opens and closes channel.  See Becker Box 8B.

2. Are pumps reversible?

a. Theoretically, all pumps (like Na⁺/K⁺ pump) are reversible -- a pump can break down ATP and use the energy to drive ions up their gradient, or (if ion gradient is large enough) ions running down their gradient can provide enough free energy to drive phosphorylation of ADP to ATP. Therefore, proteins that catalyze active transport are sometimes called "ATPases" or pumps, whether their normal function is to hydrolyze ATP or to synthesize ATP.

b. Practically speaking, inside cells, most pumps are irreversible. Most (but not all) individual "pump" proteins work only one way in cells, because the standard free energy for the "usual" direction is very negative. Therefore it takes very high concentrations of products (very high ATP or very high ion concentrations, depending on the reaction) to push the reaction in the "reverse" direction. The concentrations needed to reverse the reaction are not reached in cells, but can be achieved in test tubes (by adding ATP, setting up ion gradients, etc.). So in vitro (in test tubes), but not in vivo (in living cells), you can make the pumps run in either direction. Two examples of important pumps that are reversible (in vitro), but usually run in one direction (in vivo):

(1). In the inner membranes of mitochondria and chloroplasts, chemical or light energy is used via electron transport to set up a proton gradient, which then runs down; driving phosphorylation of ATP. So these systems almost always act to make ATP while ions run down their gradient. (Diff. proteins are used in the two organelles.)

(2). The Na⁺/K⁺ pump in the plasma membrane almost always uses up ATP --  this system drives ions up their gradients at the expense of ATP.

For more examples, see Becker table 8-3.

Now try problems 2-3 & 2-5.

IV. Putting all the Methods of Transport of Small Molecules Together or What Good is All This? 

    A. How glucose gets from lumen of intestine → muscle and adipose cells. An example of how the various types of transport are used. (Handout 5A and Becker fig. 11-22 in 5th ed; not in 6th.) Steps in the process:

1. How glucose exits lumen. Glucose crosses apical surface of epithelial cells primarily by Na⁺/Glucose co-transport. (2^o act. transport)

2. Role of Na+/K+ pump. Pump in basolateral (BL) surface keeps Na⁺ in cell low, so Na⁺ gradient favors entry of Na⁺. (1^o act. transport)

3. How glucose exits epithelial cells. Glucose (except that used for metabolism of epithelial cell) exits BL surface of cell (and enters interstitial fluid) by facilitated diffusion = carrier mediated transport. (Interst. fluid = fluid in between body cells.)

4. How glucose enters and leaves capillaries -- by simple diffusion through spaces between the cells. Note: this is NOT by diffusion across a membrane.

5. How glucose enters body cells -- by facilitated diffusion (= carrier mediated transport). Carrier is only "mobilized" that is, inserted into membrane (by fusion of vesicles as explained previously) in some cell types (adipose & muscle) in presence of insulin. Carrier is permanently in cell membrane in other cell types (brain, liver). See below on GLUT transporters.

6. Role of glucose phosphorylation. Conversion of G → G-6-phosphate traps G inside cells. 

1. Role of Active transport -- Needed to get glucose from lumen to inside of epithelial cell.

        a. Primary active transport -- Na⁺/K⁺ pump keeps intracellular [Na⁺] low.

        b. Secondary active transport -- Glucose enters epithelial cells by Na⁺/Glucose co-transport

2. Role of Passive Transport & Phosphorylation

a. Passive Transport -- Used  to move glucose the rest of the way -- out of epithelial cells, in & out of capillaries, and into body cells. 

b. Phosphorylation of glucose -- Used in the body cells to keep the free glucose level at the "end of the road" low, and ensure that the glucose gradient is "downhill" from epithelial cells to capillaries to body cells.

3. Role of Diffusion: Glucose and other small molecules (but not macromolecules) diffuse in and out of capillaries through the liquid filled spaces between the cells, not by diffusing across the cell membrane. Note that proteins are too big to enter or leave capillaries this way.

4. Role of GLUT transporters (another protein/gene family)

a. GLUT proteins are responsible for carrier mediated transport of glucose. All passive glucose transport across membranes depends on a family of proteins called GLUT 1, GLUT 2, etc. This family of genes and transport proteins is responsible for all carrier mediated transport of glucose. 

b. Different family members (genes and proteins) are expressed in different cell types. GLUT 1 protein is found in plasma membrane of RBC & most other cells, GLUT 2 protein on BL surface of intestinal epithelial cells, GLUT 4 protein in muscle and adipose tissue, etc. (Note all genes for all proteins are present in all these cell types -- DNA is the same!)

c.  All the genes and corresponding proteins are similar, but have significant structural and functional differences. This is another example of a gene/protein family. All the proteins have a similar overall  structure -- 12 transmembrane segments, COOH and amino ends on intracellular side of membrane, etc. For a picture click here;  For a diagram and table click here.

d. Position & Action of GLUT 4 is insulin dependent. GLUT 4 is the only insulin dependent member of the family. Insulin triggers insertion of GLUT 4 protein into the plasma membrane, by triggering vesicle fusion, as explained above. All the other proteins are located constitutively in their respective membranes. 

e. Direction of transport. Note that one member of this family (GLUT 2) is responsible for ferrying glucose OUT of epithelial cells; different members are responsible for helping glucose ENTER most other cells. All family members bind glucose on one side of the membrane, change conformation and release glucose on the other side of the membrane. Which way the glucose goes (overall) depends on the relative concentrations of glucose on the two sides of the respective membrane, not on which GLUT protein is used.

Try problem 2-9.

V & VI Next Time.

V. Ways that Big Molecules Enter Cells --  Types of Endocytosis. In all cases net effect is that cell membrane folds in and pinches off, forming a vesicle in the cytoplasm that contains material from the outside.

    A. Pinocytosis = bulk phase endocytosis; no receptor. Cells take in random samples of surrounding fluid containing a random selection of extracellular substances. See Becker fig. 12-13. 

    B. Phagocytosis -- in specialized cells only -- extensions of cells (pseudopods) reach out and engulf solids. See Becker fig. 12-14. Vesicle that is formed is called a phagocytic vesicle (or vacuole) or phagosome. 

    C. RME = receptor mediated endocytosis. Cells take in specific substances from surrounding fluid using a receptor. See Becker fig. 12-15 (diagram) & 12-16 (micrograph). Different cell types have different combinations of receptors.

VI.  RME -- Receptor Mediated Endocytosis

        A. General and/or important Features. 

1. Receptors -- Need specific receptor for each substance (or class of closely related substances) to be transported.

2. Concentrates substances transported -- moves them up their gradient.

3. Requires energy -- multiple stages in process use ATP or GTP. Energy must be required because substances move against their gradients. Energy is required to form the vesicles and to process and/or transport the vesicles inside the cell.

4. Role of clathrin -- A peripheral membrane protein is needed to deform membrane and allow vesicles to form -- provides a coat. (See Becker figs. 12-15 to 2-18 and/or Sadava fig. 5.17 (5.16.)  Other proteins are required as well, but will not be discussed.

a. Clathrin is coat protein for vesicles forming from plasma membrane and trans-Golgi. (trans side of Golgi = "far end" = side away from nucleus and ER = last part that proteins travel through as they are processed in Golgi. Also called "TGN" for "trans Golgi Network." See Sadava fig. 4.11 (4.12)  or Becker fig. 12-8 for labeled pictures.)

b. Budding of other membranes involve different "coat" proteins. Best known are COPI & COPII which are involved in ER-Golgi transport. (See Becker for details if interested. Types of coats are summarized in table 12-2.) 

5. It's a cycle --  Exocytosis balances endocytosis so cell surface area stays the same. See Sadava fig. 5.16 (5.15) or Becker fig. 12-15. For LDL receptor, it takes about 10-20 minutes for one "round trip." 

6. Topology -- material can enter and/or exit cell without being in contact with cytoplasm. Material can remain inside a vesicle or outside cell at all times.

7. Possible fates of endocytosed material -- Where does vesicle go? Where do receptor &/or ligand end up? (Specific Examples next time.)

a. Transcytosis --  vesicle crosses cell and fuses with opposite cell surface.

b. Degradation -- vesicle fuses with lysosomes and contents are degraded.

c. Return to surface --  material recycles -- vesicle fuses with plasma membrane.

d. Sorting -- not everything in the vesicle may go to the same place. Vesicle may fuse with endosome (sorting vesicle), and different parts of the endocytosed material may be directed to different destinations.

    B. Details of RME Cycle -- Next Time.

Anything we don't get to above will be done next time. Then details of how large molecules cross membranes.  (Some specific examples -- fates of LDL, EGF, and Fe-transferrin.) How do large molecules get in and out of cells? How do newly made proteins get to the right place?