Allosteric Regulation Flashcards
Allosteric effectors and modulators
Binding of a ligand to one binding site affects the binding of another ligand at another binding site
- > homo or heteroallosteric effectors
- > positive or negative allosteric effectors
may bind to each oligomeric subunit
Allosteric enzymes
may be activated by substrate and other positive modulators
may be inhibited by product or other negative modulators
allosteric effectors are often of some physiological relevance, relating the role of the enzyme/ pathway to the needs of the cell
allosteric effectors, and thus disobey michaelis menton kinetics
- > sigmoidal binding curve
- > can be made to appear to follow MM kinetics under special conditions in vivo i.e. allosteric activators
Allosteric enzymes catalyze reaction far from equillibrium
far from equillibirum reactions are irrelversible, and thus there exists a reverse reaction catalzyed by a seperate enzyme. Regularion prevents a futile cycle and regulates flux through a pathway
there is no point to regulating an enzyme that controls a reaction that generally operates at equilibrium because you would shut down or activate both the forward and reverse reactions
Two models of allosteric regulation
the concerted (symmetry model)
the sequential model
What are some common features to allosteric enzymes
Kinetics do not follow MM kinetics
- hyperbolic not a sigmoidal curve
- we use K0.5 instead of km
the regulatory molecules are generally structurally distinct from the substrates or the products of the relevant enzyme-catalyzed reactions
generally they are oligomeric, and regulatory effects on activity induce change in the conformation of the enzyme
R state
high affinity for substrate
T state
does not have a high affinity for substraye
The concerted/ symmetry model of allosterism
many enz follow this
each oligomer can exist in two conformational states that are in equalibrium (R and T)
The molecular symmetry of the protein is conserved during the conformational change. all subunits change at once => ALL OR NONE
The conformational change althers the affinity for the ligand
Conformational change alters the affinity for the ligand in the concerted model of allosterism
T state predominates in the absence of S
S has a much higher affinity for the R state that for the T state
allosteric effectors bind at sites seperate from substrate binding sites
positive allosteric effectors have a higher affinity for the R state than the T state
Negative allosteric effectors have a higher affinity for the T state thatn the R state.
The sequential model of allosterism
compare and contrast to the symmetry model
in the absence of a ligand, the protien exists in one conformational state (as opposed to the two in the symmetry model)
Ligand binding induces a conformational change in the subunit to which it binds. cooperative interactions arise through the influence of those conformational changes on neighbouring subunits
The conformational changes occur sequentially as more ligand binding sites are occupied (as opposed to a concerted fashion)
interactions between the subunits can be of a positive or negative type, so that binding of a second (and later) molecules of ligand can be more of less favourable
(In contrast the symmetry model allows only positive cooperativity in substrate binding)
Which model of allosterism does PFK-1 follow
the symmetry model
PFK1
what binds to the T and R states
tetrameric enz
cat F6P + ATP => F16BP + ADP
is a key regulatory enz of glycolysis
FBP substrate binds preferentially to the R state, the T has low affinity
ATP is both a substrate and an allosteric inhibitor
- > two binding sites for ATP
- > SUbstrate site binds ATP equalliy well in R or T
- > allosteric site binds ATP almsot exclusivly in the T state
ADP, AMP, and F26BP reverse the inhibiton by ATP, that is, they are allosteric activators
Allosteric changes in PFK1
bidnig sites for effectors are at teh interface btwn the subunits
phospho group of F6P forms favourable electrostatic interactions with Arg162 in the R state.
In the T state F6P is repelled by a -ve charge on Glu161, and Arg 162 swings inwards
All or none stabalization since the hydrogen bonds at the interface need to switch to either water or the other subunit
Why are ADP and AMP important in PFK1 regulation
metabolic flux through glycolysis may vary 100x or more, and ATP only varies 10% between rest and vigourus exercise.
Adenylate kinase can tranfter phosphate groups betwenn AxP moiteies
Small changes in ATP lead to large changes to AMP and ADP, and these are much more powerful modulators
All or none conformational changes in R and T PFK1 subunit interfaces
T state subunits H bond each other
R state subunits form favourable H bonds with water
Need to replace all of the H bonds at once otherwise it would be too unstable
