L2 - Membrane Proteins

Peripheral membrane proteins

Globular; Associates with membrane through electrostatic interactions and H-bonding

Peripheral membrane proteins are released by

mild detergent/interfering with charge/H-bonds

Integral membrane protein

Strongly associated with bilayer

Integral membrane protein are released by

Hydrophobic agent/Very harsh detergents

Monotopic membrane proteins

Has small hydrophobic domains that interact with only a single leaflet of membrane

Bitopic membrane proteins

Spans bilayer once, extending on either surface; single hydrophobic sequence somewhere in the middle

Polytopic membrane proteins

Crosses the membrane several times; multiple hydrophobic sequences of at least 20 residues that each cross the membrane when in the α-helical conformation

Amphitropic proteins

Associates reversibly with membranes, found in both membranes and the cytosol

Type I protein

one transmembrane segment, amino terminus outside and carboxyl terminus inside

Type II protein

one transmembrane segment, amino terminus inside, carboxyl terminus outside

Type III protein

several transmembrane segments, single polypeptide chain, terminus locations will vary

Hydropathy Index expresses free energy change associated with

Measures the relative hydrophobicity of residues; the movement of an amino acid side chain from a hydrophobic environment to water - ranges from highly exergonic to highly endergonic

In the hydropathy index, Delta G < 0 is

hydrophilic

In the hydropathy index, Delta G > 0 is

hydrophobic

Overall hydropathy estimated by summing the

free energies of transfer for the residues in the sequence

Hydropathy plots

average hydropathy index plotted against residue number

Hydropathy index y axis

average hydropathy for a window

Hydropathy window

segment of given length

Hydropathy residue number x axis

the residue in the middle of the window

Hydropathy plots can help us determine the Type I and II proteins but not

Type III

___ and ___ side chains serve as membrane interface anchors

Tyr and Trp

Positive inside rule

positively charged Lys and Arg residues in the extramembrane loop of membrane proteins occur more commonly on the cytoplasmic face (inside of cell)

Transport proteins reduce the energy of activation for diffusion by

forming noncovalent interactions with the dehydrated solute and providing a hydrophilic transmembrane pathway

Ion channel is a

single gate that is unsaturateable.

Transporter (pump) have alternating gates that can never ____________________  and can pump against gradient. Is saturateable. Concentration at which the rate of transport will plateau similar to a V_max graph.

open at the same time

Transporter classes

Uniport: Only one molecule is moved

Cotransport: Two molecules are involved

1. Symport: Both molecules go the same way

2. Antiport: Molecules go in the opposite way of each other

Saturable vs non saturable transport

Saturable transporters perform similar to enzymes kinematics (V_max)

Non naturable transporters are influenced by diffusion and concentration, not similar to enzyme kinetics