Proteins and cellular transport Flashcards
Folding of protein
Done after translation. Physical properties of proteins dictate how it folds
Effect of protein folding on function
Tertiary and quaternary structure most important for function. E.g. if a receptor is the wrong shape a hormone is not able to bind. If the structure of the protein is not correct it will not function properly leading to diseas
Clustering
Occurs when there are polar and non-polar side chains. Polar side chains will be on the outside, as they are hydrophilic and the non-polar side chains will cluster in the centre because they are hydrophobic
Primary structure
linear sequence of amino acids in the polypeptide, amino acids will have different properties
Secondary structure
Local structures within the peptide chain, caused by hydrogen bonds. Classically they are alpha-helix (spiral structures) and beta pleated sheet (ribbon structures).
Tertiary structure
The folding of the secondary structure to form a 3d shape. Include hydrogen bonds, electrostatic interactions, van der waals attraction, hydrophobic interaction. Individually weak but collectively strong
Quaternary structure
Formation of a protein complex with two or more polypeptide chain. Can be a globular protein.
Quality control mechanism of the secretory pathway
1) The signal recognition particle (SRP) will bind to the ribosome and pause translation. The ribosome will have a specific N-terminal signal sequence.
2) The SRP bound ribosome binds to the SRP receptor on the surface of the RER.
3) The SRP drops off, the protein is now being fed into the lumen of the RER via the protein translocator, translation continues.
4) The protein is now in the secretory pathway.
5) In the RER the polypeptide chain will fold to form its tertiary structure and where post translational modifications will occur. Disulphide bonds will form.
Where do proteins synthesised from free ribosomes end up?
Cytosol, mitochondria, nucleus and peroxisome. Because proteins can not move back from the ER to these organelles.
Protein destination
Protein destination is dependent on the location of the ribosome during translation. Proteins are normally translated in the ribosomes in the cytosol unless signalled to go to the RER (if they have an N-terminus)
How do proteins travel from the RER to the organelle they are needed in
Proteins are transported in vesicles from the RER to the golgi apparatus. =They are then packed in vesicles, the protein signals where it needs to go. Part of the membrane of the organelle bud off into the vesicles containing our protein. The vesicles then fuse and release their cargo proteins into the destination organelle.
Exocytosis
Vesicles fuse with the plasma membrane and release their cargo outside the cell
Endocytosis
vesicles form at the plasma membrane to capture proteins and move them into the cell
What is intracellular fluid?
Fluid within cells. Accounts for two thirds of fluid
What is extracellular fluid?
Fluid outside cells. Accounts for a third of our body fluid, divided between interstitial and plasma fluid
What is plasma fluid?
The extracellular components of the blood present in the heart and blood vessels (intravascular compartments)
What is interstitial fluid?
Bathes the non-blood cells of the the body
What is transcellular fluid?
The fluid between epithelial lined spaces i.e. joint fluid
Movement between fluid compartments
Water can move freely between the different compartments but solutes can not because of cell membranes
Contents of intracellular fluid
High concentrations of K+ and Mg+ driven by Na+/K+ ATPase pump
Contents of extracellular fluid
Na+ is the major cation, Cl- and HCO2- are the major anions. Interstitial is similar to plasma but less proteins
Osmolarity in fluid compartments
Electrolytes (ionised substances) contribute the most to solute concentration (osmolality). Normally osmolality is the same in each compartment and they are said to be in equilibrium. If there is a change in one compartment, water will move between the compartments and restore this equilibrium. Fluid composition is driven by cell membranes and transport proteins
Channel proteins
Integral membrane proteins which allow direct access to the cell and facilitate uncoupled transport of a solute down a concentration gradient i.e. aquaporins.
Gated channels
Integral membrane proteins that open in response to a stimuli and facilitate uncoupled transport of a solute down a concentration gradient. The stimuli which allow them to open can be a voltage, mechanical (if the cell is stretching) or ligand binding. If the gate is closed a solute cannot move even if there is a concentration gradient.
Carrier mediated (uniporters)
Solute moves due to an electrochemical gradient i.e. glucose transporters. A carrier opens into the extracellular space, the solute then binds to the open transporter which will cause a change in shape. The outer gate will then close and the inner gate will open, allowing the solute to move through the membrane
Pros and cons of carrier mediated (uniporters)
pro- greater flux then passive diffusion
con- reaches saturation at lower concentration because there is a definitive number of carriers. System can be poisoned by a similar looking solute
Secondary active transporters
They move one solute down its concentration gradient which provides the energy to move another solute against theirs. Both solutes are bound to the transporter. Two types symporter and antiporters
Symporters
Secondary active transporters which move two ions in the same direction e.g. sodium and glucose transporter
Antiporters
Secondary active transporters that move two ions in opposite directions. I.e. sodium/ proton acceptor
Primary active transporters
Uses the energy from the hydrolyses of ATP to move solutes against their concentration gradient. The transporter protein has a domain which can bind to ATP and catalyse its hydrolysis to ADP and inorganic phosphate. The energy released is used to change the shape of the transporter
Types of primary active transporters
P-type ATPase, F-ATPase, V-ATPase and ABC transporters. The first three are ATPases and the final one is an ATP binding cassette transporter.
Passive diffusion
Permeable substances move from an area of high concentration to an area of low concentration down an electrochemical gradient through a plasma membrane. As substances get bigger and more heavily charged they become less permeable.
How permeability effects transport
Solutes with high permeability can move straight through the plasma membrane, those with low permeability will need a transport protein.
