Module 4: V6 - V10 Flashcards
What roles do membrane proteins play?
- permit selective entry and exit of molecules from cell e.g. via transporters
- provide recognition signals e.g. receptors for growth factors
- provide structural support to the cell
How do proteins interact with the membrane?
most membrane proteins are free to diffuse laterally in lipid matrix
What are two types of protein that interact with the membrane?
integral membrane protein and peripheral protein
What are integral membrane proteins?
proteins which are firmly attached / embedded in the membrane
What are peripheral membrane proteins?
proteins which associate with outside surfaces
How do peripheral membrane proteins associate with membranes?
by ionic interactions and H-bonding with polar head groups of lipids and integral membrane proteins
How do integral membrane proteins interact with membranes?
by hydrophobic interactions with acyl chains of membrane lipids
Which reagents can be used to disrupt ionic interactions and release peripheral membrane proteins?
high salt, change pH, chelating agent
Which reagents can be used to disrupt hydrophobic reactions and release integral membrane proteins? How does this process work?
detergents such as SDS
dissolves membranes and maintains extracted protein in solution by interacting with hydrophobic regions of proteins
How can we determine orientation and arrangement of membrane proteins?
treating these proteins with an enzyme that chops them up will determine which parts are exposed to the protease and which parts aren’t
What does trypsin do?
trypsin cleaves on the carbonyl side of lysine and arginine, but only has access to the “outside” part of the protein of an intact cell
What does analysis of remaining protein after trypsin digestion of cells identify?
identifies the domains of the protein buried in the bilayer and/or exposed on the inner surface
Based on sequence can we predict transmembrane protein domains?
yes, genome sequencing allows the sequences of all proteins to be predicted
What information do we know about proteins which we can use to predict transmembrane protein domains?
sequence will consist of hydrophobic amino acids
conformation will be an alpha helix
span (segment of protein) equal to width of membrane
How many hydrophobic residues are required to span a 3 nm membrane?
20 hydrophobic residues
How is hydrophobicity measured? How is this measure derived?
the hydropathy index which is derived from the free energy change required to move an organic solvent to water
therefore, (+) values for hydrophobic amino acid residues as this process is not favourable and (-) values for hydrophilic amino acid residues as this process is favourable
What is indicative of a transmembrane region on a hydropathy plot?
a region in which 20 or more amino acid residues are hydrophobic (positive)
Which molecules are permeable?
oxygen and carbon dioxide
What are transport proteins in membranes responsible for?
transferring small water soluble molecules across the lipid bilayer
Do transport proteins transfer more than one type of molecule?
no, they are specific
What are the two classes of membrane transport proteins?
carriers (transporters) and channels
What are the different types of carriers and what type of transport do they mediate?
channel-mediated carriers which allow for passive transport across the membrane
carrier-mediated carriers which allow for both passive and active transport depending on the concentration gradient of the molecule
How do carriers prevent molecules from constantly passing through?
they are closed on one side and opened on the other and they undergo a conformational change during the transport process
What type of protein is GLUT1 and what process does it facilitate?
GLUT1 is a transporter found in membranes of many cells and it facilitates passive transport (facilitated diffusion) of glucose down a concentration gradient
How does GLUT1 manage glucose transport?
exists in two conformations (T1 and T2)
binding of glucose (from blood plasma) may induce a conformational change from T1 to T2
explains net transport of solute down its concentration gradient
What are active transporters used for? How do they do this?
transport solutes against a concentration gradient
use ATP hydrolysis mediated by P-type ATPases to pump ions across membranes
Why is an Na and K gradient in animal cells important?
to maintain cell volume and create transmembrane electrical potential
What are the three roles of sugars in biology?
carbohydrates are an important source of energy/stored fuels, provide structure to cells and organisms and play a significant role in cell biology
Why are sugars important in cell biology?
major component of the cell surface
important in influencing the function of proteins and important in specific recognition interactions
How do sugars convey information?
carbohydrates have enormous structural diversity
What are cell surface sugars important in?
cell-cell adhesion
bacteria adhesion
virus attachment to host cells
binding of toxins to cell surface
What are monosaccharides?
are aldehydes or ketones that have two or more hydroxyl groups
What does aldose mean?
contains an aldehyde group
Do most naturally occurring sugars belong to the D- or L-series?
the D-series
What is a Fischer projection?
a diagram which provides a clear and simple view of the stereochemistry at each carbon centre
How many carbon atoms do monosaccharides have?
≥ 3 carbon atoms
What are the most common monosaccharides in nature?
hexoses (6C)
Which carbon is C1 in a monosaccharide?
the first carbon after the aldehyde group
What are sugars that differ only in the configuration around one carbon atom?
these sugars are called epimers
Are hexoses mostly open chains or cyclised?
open chains are mostly cyclised into rings (pyranose rings)
How does glucose become cyclised?
the C1 aldehyde reacts with the C5 hydroxyl group to form an intramolecular hemiacetal
What is the conformation of monosaccharides?
chair form (boat form is not stable)
Membrane rafts can be up to 0.9 nm thicker than the surrounding membrane. What could you say about the phospholipid acyl chains of the lipids found in membrane rafts, compared to the rest of the membrane?
the phospholipid acyl chains of the lipids found in membrane rafts are longer compared to the rest of the membrane
Do you think the membrane proteins found in membrane rafts will be the same or different to those in the rest of the membrane?
the membrane proteins will be different to those in the rest of the membrane
What can you say about the number of residues you would expect in the transmembrane alpha-helices of membrane raft proteins? Could you put a number on this difference?
approximately 26 amino acid residues. 20 residues to span 3nm and then an extra 6 residues to span the extra 0.9nm
What exactly is a carbohydrate?
any of a large group of organic compounds occurring in foods and living tissues and including sugars, starch, and cellulose
they contain hydrogen and oxygen in the same ratio as water (2:1) and typically can be broken down to release energy in the animal body
How can monosaccharides be both straight chain and cyclic molecules?
monosaccharides are able to cyclise into rings as a result of the reaction between the C1 aldehyde and the C5 hydroxyl group
monosaccharides are also able to form open chains because they are not bound by glycosidic linkages
Why are naturally occurring sugars called ‘D’ sugars?
this is because the “D” designates the configuration of the asymmetric carbon furthest from the aldehyde group
Which aldohexoses are common in biology and how are they related?
mannose, glucose and galactose
they share the same overall structure, but they differ in the configuration around one carbon atom
