Enzymes Flashcards
What does catalysis acctually mean
Catalyse making and breaking covalent bonds between substrates at faster reaction rates
Reactions occur at a lower conditions eg ph or temp
How do you calculate keq equilibrium constant
K+1 (conc of 2 substrates)
/
K-1 (conc of products in reversible reaction)
Eg 100x100 units
/
2 x 2 units - the units left in reversible reaction
What is Keq correlated to
With energy released by the reaction
GIBBS FREE ENERGY - change from substrate to products
What does the Gibbs free energy value need to be for substrates to become products
-ve
Substrate must be at a higher energy level than end products
What is transition state
A form of substrate which is at high energy
If the transition state isn’t lowered eg by enzymes the reaction won’t reach equilibrium
Why don’t reactions happen spontaneously without enzymes
They need enzymes to lower activation energy to lower the transition state of the substrate
What does the ES complex reduce in order for reaction to reach equilibrium
Reduces transition state
This increases speed equilibrium is reached
Do active sites change after reaction?
No they are unaltered catalysts
There are 5 ways enzymes catalyse reactions - name them
1- proximity- substrates closer together
2- orientation- align correct bonds in substrates
3- strain - in induced fit the AS puts strain on bonds in substrate
4- acid base catalysis
5- covalent catalysis
How do enzymes use acid base catalysis to catalyse a reaction
Protons (H+) or hydroxyls (OH-) get donated
They are added into active site (introduced to charged side chains)
This overcomes the low concentration of reactive molecules
What is covalent cataclysms
When enzyme active sites temporarily bond covalently to substrates
There are 4 ways enzymes can be specific in their reactions - explain them
1- absolute - only work with 1 substrate
2- bond- the substrates need a specific bond eg oxygen bonds in esterase
3- group- only work on Eg hexose sugars
4- stereo - DISTINGUISH OPTIMAL ISOMERS eg L or D amino acids
There are 6 types of enzyme reaction, briefly explain them
1- oxidase/reductase - OIL RIG
2- transferase - transfer other groups to the substrates eg phosphates
3- lyases- rearrange bonds in substrates
4- hydrolases- insert H20 for hydrolysis eg of peptide bonds
5- isomerases - change the position of atoms
6- ligases/syntheses - synthesis of new molecules
What is the turnover number (Kcat)
Number of substrates converted by 1 enzyme in 1 second
Name an enzyme with very high Kcat
A and B amylase
Why do you always measure initial rate of reaction
Because the substrate conc is known
What increases rate of reaction
Increased substrate concentration- more ES complexes
What does V proportional to S mean
Velocity /rate of reaction depends on substrate concentration
What is it called when eventually all Es complexes are formed
Saturation of enzymes
What is the Michaelis menton equation for velocity
V = Vmax x (s) / km + (s)
What is v max
Maximum velocity shown when enzymes have reached saturation - infinite substrate concentration
V max could never be found so who do scientists find it
Plot the graph and the concentrations they did get to and then use computer to predict v max
Inhibitors can be irreversible and reversible, explain how some are irreversible
They bind to active site via covalent bonds
This prevents substrate binding
They bind to side chains such as cysteine or serine (reactive)
What side chains do irreversible inhibitors bind to in active site via covalent bond
Reactive chains like serine or cysteine
Explain how penecillin is an irreversible inhibitor
Penecillin binds to serine on glycopeptidetranspeptidase (PBP)
This inactivates the enzymes
Stops synthesis of cell walls in bacteria
Penecillin is a B lactam antibiotic , what does this mean
B lactam is a complex of rings which will block the active site via covalent bonds
How do bacteria become resistant to B lactam antibiotics
They have an enzyme called B lactamase which degraded the B lactam before it binds to PBP
Why are competitive inhibitors most useful drugs
They bind to active site very tight so dosage can be lower
How do competitive inhibitors change v max and Km
V max will be unchanged - the Es complex will still occur eventually
Km will be increased(more substrate concentration needed) for affinity
How can you overcome a competitive inhibitor
By adding more substrate to outcompete
Give an example of a competitive inhibitor
Methotrexate - anti cancer drug
Malonate- binds instead of succinate so no fumarate produced
Explain the effect of non competitive inhibitors when bound to allosteric sites on vmax and km
V max will be decreased - they interfere will the ES complex and reaction reaching equilibrium
Km doesn’t change- the affinity to bind to substrate is the same because they bound to allosteric site
Give example of a non competitive inhibitor
Deoxycycline used to treat gum disease
What is an UNCOMPETITIVE inhibitor
Inhibitors which only bind to enzymes with multisubstrates
They bind INTO THE ES COMPLEX
Explain the effect of uncompetitive inhibitors on km and vmax
Vmax is lowered as they prevent catalysis in the ES complex so lower reaction rate
Km is acctually lowered (higher affinity) because substrates bind to active site more efficiently (enzyme binds to allosteric site)
When ph isn’t at optimum, what part of the active site is altered
The side chains which have charged molecules
Such as aspartate (0-) and lysine (NH3+)
What is the optimum ph for enzymes
7.4
There are enzymes with different optimum ph due to evolution. Give an example
Pepsin due to it being in stomach acid - 2 is optimum
What happens in enzymes when they move out of optimum ph
If lower ph (acidic) they will remove COO- carboxylates and at high alkali conditions (OH-) they remove NH3+ side chains
What happens at high and low ph to side chains
Protonation at low ph eg of histidine
At high ph carboxylate loses a proton and deprotonation occurs eg of glutamate and aspartate
