Enzyme Regulation Flashcards
Feedback inhibited enzymes
Excessive build up of end products inhibits enzyme activity
Covalently modified enzymes
Reversible attachment of small molecules that inhibit activity (phosphorylation, methylation and ribosylation)
Allosteric enzymes
Conformational changes by reversible non covalent binding of co-factors and regulatory sub-units
Zymogens and proenzymes
Become activated by proteolytic cleavage of inactive precursor molecules
Enzyme specificity- Absolue
Defined to one substrate in its one define stereoisomeric state
Enzymes specificity- Group
Enzymes can recognise compounds belonging to a group e.g phosphates and kinases
Enzyme specificity- Promiscuity
Enzymes that can act upon several substrates regulation is very important
Specific inhibition is hard
Allosteric Regulation
Causes changes (inhibitor or activator) to active site so enzyme can bind or not bind
Homotropic regulation
The substrate and regulator bind to the same site
regulatory site=active site
Heterotrophic regulation
The regulator binds to site other than the active site
Co-operative regulation
Binding of the first substrate molecule to the active site makes it easier for more substrate to bind
Test for co-operability
The hill plot can be used to estimate the degree of cooperation between subunits
Log(theta/1-theta) = nlog [S]- nlogKA
Theta = fraction of bound enzyme [S] = ligand concentration KA= [S] at 50% occupation
Aspartate Transcarbamoylase
Catalyses the first step in pyrimidines
Precisely regulated to produce the right amount of CTP
Negativity allosterically regulated by CTP
T state (less active)= favoured by CTP
R state (more active)= favoured by substrate binding
Apo-enzyme
Just protein
Holo-enzyme
Protein and co-factor or co-enzyme
Cofactors and Coenzymes
Many enzymes require prosthetic groups that are non amino acid in nature
Where do the Co’s come from
Most cannot be synthesised
A precursor molecule must come from the diet
Co-enzymes
Organic: Carry groups between enzymes
Most are vitamin derived
Co-factors
Inorganic- Metal ions
Required for enzyme activity
Co-enzymes and Co-substrates
Co-enzymes often donate chemical groups such as C, H and NH2
These are consumed by the reaction and are hence called Co-substrates
Common co-enzymes
Biotin- carboxylation
Coenzyme A - Acyl transfers
FAD and NAD- Oxidation and Reduction
Aspartate Transcarbamoylase continued
6 regulatory and 6 catalytic subunits
Catalytic sites show normal MM kinetics and are unresponsive to CTP
Regulatory subunits have no catalytic ability but binds to CTP
CTP binding
Stabilises the T state
Inhibition changes the quaternary structure which makes is harder for substrate to bind
More substrate is required to achieve reaction rate
ATP and aspartate carbamolylase
Positive allosteric effector
Competes with CTP for site (alleviating inhibition)
Makes substrate binding easier and shifts equilibrium to the R state
Two reasons:
Signals high energy for mRNA synthesis
Maintaining the balance or pyrimidine and purine rings
