Membrane protein Flashcards
what is the function of a cell membrane (phospholipid bilayer)
- plasma membrane/cell surface membrane separates intracellular contents from extracellular environment
*Intracellular membranes further separate the contents of organelles from the rest of the cell.
No membranes no concentration gradients no energy!
function of phospholipid bilayer is to allow the cell to be selectively permeable, meaning it only lets certain things in and out. The phospholipid bilayer prevents large molecules or charged molecules like ions from diffusing directly across the membrane without the use of a channel protein
membrane proteins are amphiphillic what does this mean?
Membrane proteins are Amphiphilic (both hydrophobic and hydrophilic domains)
Proteins molecules possess both:
Hydrophilic domains:
“Water-loving” domains that interact with acqueous intra/extracellular environment.
Hydrophobic domains:
“Water-fearing” domains that interact with the non-acqueous core of the phospholipid bilayer
These domains allow membrane proteins to interact with membranes + serve as an interface between:
* the aqueous hydrophilic intra/extracellular environment &
*the hydrophobic lipid-based membrane defining the perimeter of cells + organelles.
most transmembrane proteins such as G-protein linked receptors cross the membrane with ____ ____
most transmembrane proteins such as G-protein linked receptors cross the membrane with alpha helices
An alpha helix (or α-helix) is a sequence of amino acids in a protein that are twisted into a coil (a helix)
Extracellular domains are often glycosylated and have disulphide bonds.
Disulphide bonds formed between cysteine residues in the extracellular oxidising environment stabilise the protein fold.
What is meant by the term glycosylation?
Extracellular domains are often glycosylated
and have disulphide bonds
Disulphide bonds formed between cysteine residues in the extracellular oxidising environment stabilise the protein fold.
Glycosylation (i.e. enzyme-mediated addition of sugar groups) to, added in the lumen of the endoplasmic reticulum decorates many proteins with polysaccharide chains. Important to protect the protein from attack by extracellular proteases, and can mediate cell-cell communication.
define the following
*passive transport
*simple diffusion
*facilitated transport
*active transport
Passive transport of solute molecules across a phospholipid membrane occurs spontaneously along a concentration gradient or electrochemical gradient - either
- directly through the lipid membrane [simple diffusion] OR
- through membrane transport proteins (channel proteins and transporter proteins) [facilitated transport/diffusion].
Simple diffusion is the passive movement of materials through a membrane without the help of proteins.
Facilitated transport is the passive movement of materials through a membrane with the help of membrane transport proteins (ions channels and transporters).
Active transport involves the energy driven, transporter-mediated movement of molecules across a membrane against a concentration or electrochemical gradient
explain the difference between facilitated vs simple diffusion
Facilitated transport is a type of passive transport. Unlike simple diffusion where materials pass through a membrane without the help of proteins, in facilitated transport, also called facilitated diffusion, materials diffuse across the plasma membrane with the help of membrane proteins
what is primary vs secondary active transport
active transport:
* high affinity solute binding site exposed to cytosol
* solute binding induces ATP binding/lysis
*conformational change exposes binding site to extracellular space
* net flux is dependent on an ATP-driven primary active pump/transporter, and is always low to high (against electrochemical/conc gradient)
primary active transport= dependence on ATP hydrolysis
Uphill solute translocation is possible if coupled to ATP hydrolysis.
e.g. The Na+/K+ ATPase that effluxes sodium to generate a powerful gradient.
SERCA, the Sarcoplasmic/Endoplasmic Reticulum Ca2+ P-type ATPase transporter
secondary active transport= powered by an ion gradient
Uphill solute translocation is possible if coupled to the downhill movement of an ion.
e.g. SGLT1 the Sodium/Glucose Linked Transporter of the intestinal epithelium
Glucose is absorbed in the small intestine via transcellular movement across intestinal epithelial cells; explain how
at LOW gut glucose concentrations [<30 mM]:
1)Secondary active transport of glucose, mediated by SGLT1 at the apical membrane of epithelial cells.
2)SGLT1 harnesses the energy of Na+ ion gradients* (via Na+/K+ pump to facilitate symport (taking up in same direction) of glucose into epithelial cells, against a concentration gradient. {*Generated previously by ATP-driven primary active transport of Na+ ions out of epithelial cells by Na+/K+ ATPases present on the basolateral membrane.}
3)Glucose crosses the epithelial cell cytoplasm and is then exported, via GLUT2-mediated facilitated diffusion at the basolateral membrane (down a concentration gradient) into the extracellular fluid where it can travel to the portal vein.
At high gut glucose concentrations [>30 mM] after meal
1)GLUT2 protein inserted in apical membrane.
2)Mediates facilitated diffusion of glucose into epithelium.
3)SGLT1-mediated secondary active transport is ongoing.
=>Regulates blood glucose levels
Na+/K+ ATPase pump is an example of what kind of active transport?
Primary active transport. Sodium-potassium pump, which moves Na+ out of cells, and K+ into them. Because the transport process uses ATP as an energy source, it is considered an example of primary active transport
what are the 2 main types of membrane transport proteins + examples
Two main classes of Membrane transport proteins:
Channel Proteins,
Transporter/carrier Proteins.
Channel Proteins (e.g. Na+, K+ , Cl- ion channels) are specific for particular solutes; open and close (in a regulated manner) allowing bulk flow of solutes down a concentration gradient.
Transporter/carrier proteins (e.g. GLUT2) mediate facilitated transport/diffusion of solutes across biological membranes by binding to the solute on one side of the membrane and releasing it on the other side to ‘aid’ its diffusion.
how would you describe the part of the protein inside the membrane?
hydrophobic
what type of uses an electrochemical gradient to move solutes against the concentration gradient?
secondary active transporter
which protein mediates facilitated diffusion of glucose from the epithelial cell into the portal vein?
GLUT2
