11.3: Membrane Transport

The movement of a solute through a permeable barrier along its concentration gradient until equilibrium is reached.

The movement of solutes through a permeable barrier, aided by a membrane protein without the usage of ATP, along its concentration gradient until equilibrium is reached.

Active transport requires ATP, because it involves moving solutes against their concentration gradient. Passive transport does not require ATP; it involves moving solutes along their concentration gradient.

The energetically unfavorable movement of solutes against their concentration gradient, using the hydrolysis of ATP as their source of energy.

ATPase transports and ABC transporters

P-type, V-type, and F-type

P-type ATPase is an integral transmembrane multi-domain polytopic protein. Its is reversibly phosphorylated by ATP and its function is the transport of CATIONS across membranes.

The critical Asp is phosphorylated, which generates a conformational change that drives membrane transport.

SERCA: uniport transporter of Ca2+ ions. Na+,K+: antiport transporter of Na+ and K+ ions.

V-type ATPases are integral transmembrane multi-domain polytopic proteins that are involved with the movement of PROTONS across a membrane.

The V stands for vacuolar, because usually intracellular compartments (such as vacuoles) need to be acidified for proper function.

F-type ATPases are extremely similar to V-type ATPases in both structure and function. However, the F relates to "energy-coupling Factors," meaning that it couples the movement of protons to the hydrolysis and synthesis of ATP.

F-type ATPases, when working in their reverse direction (all depending on proton gradients), they actually catalyze the synthesis of ATP.

ATP-Binding-Cassette ABC transporters actively move many types of molecules across membranes, including amino acids, polypeptides, salts, and lipids.

ABC transport proteins have two ATP-binding domains, and two transmembrane domains. Each intermembrane domain is comprised of 6 transmembrane helices.

The mechanism works via the interconversion between two conformations, dependent on ATP hydrolysis. Either conformation has the binding site facing either the inside or outside of the cell.

The energetically unfavorable movement of solutes against their concentration gradient, coupled to a very energetically favorable reaction to have a net negative delta G.

The sodium-glucose symporter. Works by coupling the energetically favorable movement of Na+ into a cell with the energetically unfavorable movement of glucose into a cell.

ΔG = RT * ln(C2/C1) Where: R = ideal gas constant, 8.314 J/mol*K T = temperature, K C2/C1 = ratio of final conc : initial conc

ΔG = RT * ln(C2/C1) + ZF(ΔV) Where: R = ideal gas constant, 8.314 J/molK T = temperature, K C2/C1 = ratio of final conc : initial conc Z = charge of ion F = Faraday's constant, 96480 J/molV ΔV = membrane potential, V

Remember Michaelis-Menten equation for enzyme kinetics. Vo = Vmax * [Sout] / (Kt + [Sout])

1. High rates of diffusion down a concentration gradient.

2. Saturability: conformational changes occur.

3. Stereospecificity: there is a very specific binding site.

Channels have much higher rates of diffusion, because they are only gated and do not depend on conformational changes. Thus, they are not saturable. It is like a bottomless water cup with a lid as a "gate."

Transporters on the other hand have lower rates of diffusion, because they have a capacity; that is to say, they are saturable. This is because they rely on conformational changes that occur when their substrate binds to move across the membrane.

Chloride-bicarbonate exchanger ionophore aquaporin ion channels ion transporters GLUT(1-4)-transporters

Function: Antiport passive exchange of anions across a plasma membrane. Specifically, bicarbonate (HCO3-) and Chloride (Cl-) are moved antiport to one another, both along their concentration gradient.

Method: Tissues

CO2 from metabolism enters the cell. Cell doesn't want CO2 bubbles, so it converts CO2 to HCO3-. (via carbonic anhydrase) HCO3- needs to travel to the blood because it is a pH buffer. HCO3- travels to Chloride-Bicarbonate Exchange protein. Coupled with H+, HCO3- exchanges with Cl-

A small, amphipathic, and cyclic peptide that binds small ions. It masks the ion's charge due to its charged interior, but it has an uncharged exterior. Because it's a pretty small molecule, it effectively converts the ion to a nonpolar molecule and it's able to simply diffuse across membranes.

Aqua = water, Porin = pore = tunnel Water tunnel; aquaporins are essentially just channels across membranes that allow the free flow of water molecules across, reaching rates of transfer that approach free diffusion Rate: 10^9 s^-1

GLUT1 is a passive transporter that moves glucose into blood cells from blood plasma. "Supplies glucose to erythrocytes" -the book It also is important for moving glucose across the blood brain barrier.

It is specific to D-glucose

Structurally, it's comprised of 12 hydrophobic transmembrane segments. These segments are hydrophobic on their exterior (where they contact the nonpolar lipid interior of the membrane) but hydrophilic on the interior (the tunnel where glucose travels through).

Overall, the entire structure is stabilized by the hydrophobic effect.

GLUT2 is is a passive uniport transporter that facilitates the downhill diffusion of glucose in intestinal, liver, and kidney cells.

Na+-glucose transporter: Secondary active transport GLUT2: Passive transport (facilitated diffusion) Na+K+ ATPase: P-type ATPase (Primary active transport)

From the intestinal lumen, glucose enters the cell via the Na+-glucose symporter. The glucose leaves the cell and into the blood plasma via GLUT2, down its concentration gradient. The Na+ leaves the cell via the Na+K+ ATPase, exchanging 2Na+ out for 3K+ in (leaving the inside of the cell more acidic)