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enzymes (factors affecting enzyme activity (investigating the effect of…
enzymes
factors affecting enzyme activity
temperature
the rate of reaction increases when the temperature is increased
more heat means more kinetic energy so molecules move faster
this makes the enzymes more likely to collide with the substrate molecules
the energy of these collisions also increase, which means each collision is more likely to result in a reaction
but if the temp gets too high the reaction stops
how an enzyme is denatured
1) the rise in temp causes enzymes molecules to vibrate more
2) if the temp goes above a certain level the vibrations break the bonds that hold the enzyme in shape
3) the active site changes shape and the enzyme and the substrate will no longer fit together
temp coefficient
shows how rate changes with temp
Q10= rate at higher temp / rate at lower temp
optimum for humans is 37
pH
all enzymes have an optimum pH ( most in humans is 7)
above and below the optimum pH the H+ and OH- ions found in acids and alkalis can mess up ionic bonds and hydrogen bonds which hold the enzymes tertiary structure in place
this makes the active site change shape, denaturing the enzyme
enzyme concentration
the more enzyme molecules there are in a solution, the more likely a substrate molecule is to collide with one and form an enzyme substrate complex
increasing concentration of enzymes, increases the rate of reaction
but if the amount of substrate is limited there comes a point when there's more than enough enzyme molecules to deal with all the available substrate
so adding more enzymes will have no further effect
substrate concentration
the higher the substrate concentration, the higher the rate of reaction
more substrate molecules means a collision between substrate and enzyme is more likely and so more active sites will be used
this is only true up until a 'saturation pint' - where all the enzymes active sites are being used so adding more substrate will make no difference to the rate of reaction
substrate concentration increases with time during a reaction so if no other variables are changed, the rate of reaction will decrease over time too.
this makes the initial rate of reaction the highest rate of reaction
measuring the rate
inverted measuring cylinder
measures how fast the product of reaction appears
its easy to collect volume of oxygen produced
spotting tile
measures how fast reactant is used up
starch and amylase
add iodine and measure how long it takes to disappear (become maltose)
investigating the effect of temp on catalase activity
IV= temp
C= pH, enzyme concentration, substrate concentration
DV= volume of oxygen produced
1) set up boiling tubes containing the same vol and conc of hydrogen peroxide and add equal vols of buffer solutions
2) set up apparatus (inverted measuring cylinder or gas syringe)
3) put each boiling tube in a water bath set at a different temp e.g. 10, 20, 30, 40 along with another tube containing catalase
4) wait for 5 minutes
5) use a pipette to add the same vol and conc of catalase to each boiling tube
6) record how much oxygen is produced in the first 60s of the reaction using a stopwatch
7) repeat the experiment at each temp 3 times and then calculate the mean volume of oxygen produced at each temp
8) calculate the mean rate of reaction by dividing the volume of oxygen produced by the time taken
cofactors and enzyme inhibition
cofactors and coenzymes are essential for enzymes to work
cofactors = non proteins, inorganic molecules or ions which help the substrate and enzyme bind together- not used up or changed (some enzymes require one to work)
organic cofactors are coenzymes
coenzymes= participate in reaction and are changed by it. they often act as carriers moving chemical groups between different enzymes. they are continuously recycled
if a cofactor is tightly bound to an enzyme it is known as prosthetic group- permanently present
enzyme activity can be inhibited
competitive inhibition
competitive inhibitors have a similar shape to that of substrate molecules
they compete with the substrate molecule to bind to active site- but no reaction takes place
instead they block the active site so no substrate molecules can fit in it
how much the enzyme is inhibited depends on relative concentrations of inhibitor and substrate
if theres a high conc of inhibitor it will occupy nearly all the active sites and hardly any of the substrate will get to the enzyme
if theres a high conc of substrate then the substrates chances of getting to the active site before the inhibitor increase- so increasing substrate conc increases rate of reaction
non competitive inhibition
non competitive inhibitors bind to the enzymes allosteric site
this causes the active site to change shape so the substrate molecules can no longer bind to it
increasing the conc of substrate wont make a difference to the rate, enzyme activity will still be inhibited
inhibitors can be reversible or non reversible
if the inhibitor has strong covalent bonds= the inhibition is irreversible
if the inhibitor has weak ionic or hydrogen bonds = the inhibition is reversible
some drugs and metabolic poisons are enzyme inhibitors
some antiviral drugs inhibit the enzymes reverse transcriptase which catalyses the replication of viral dna - prevents viruses from replicating
some antibiotics e.g. penicillium inhibits the enzyme transpeptidase which catalyses the formation of proteins in bacterial cell walls. this weakens the cell wall and eventually causes the cell to burst
cyanide is an irreversible inhibitor of cytochrome c oxidase which catalyses respiration- causes cells to die
arsenic inhibits the action of pyruvate dehydrogenase another enzyme that catalyses respiration reactions
enzyme inhibition can help protect cells
enzymes are sometimes synthesizes as inactive precursors in metabolic pathways to prevent them causing damage to cells
e.g. some proteases are synthesised as inactive precursors to stop them damaging proteins in the cell in which theyre made
part of the precursor molecule inhibits its action as an enzyme . once this part is removed the enzyme becomes active.
metabolic pathways are regulated by end product inhibition
a metabolic pathway is a series of connected metabolic reactions. the product of the first reaction takes part in the second reaction etc. each reaction Is catalyzed by a different enzyme
many enzymes inhibited by the product of the reaction they catalyze (product inhibition)
end product inhibition= when the final product in the metabolic pathway inhibits an enzyme that acts earlier on in the pathway
end product inhibition controls the amount of end product made
they are reversible reactions
actions of enzymes
enzymes are biological catalysts
they speed up chemical reactions
they catalyze metabolic reactions at cellular level and for the organism as a whole
enzymes can effect structures in an organism (collagen) as well as functions (respiration)
intracellular enzyme example (catalase)
hydrogen peroxide is the toxic by product of several chemical reactions, if built up it can kill cells
catalase works inside cells to catalyse the breakdown of hydrogen peroxide into oxygen and water
extracellular enzyme example (trypsin and amylase)
amylase and trypsin both work outside the cells in digestive system
amylase is found in saliva which is secreted by cells in the salivary glands. it catalyses hydrolysis of starch into maltose in the mouth
trypsin catalyzes the hydrolysis of peptide bonds (large polypeptides into smaller ones). it is produced in pancreas and secreted in small intestine
enzymes reduce activation energy
activation energy= a fixed amount of energy needed to be supplied to the reactants in order to initiate a reaction
enzymes REDUCE the amount of activation energy required (often making reactions happen at lower temps)- speeds up rate of reaction
when a substance binds to an enzymes active site an enzyme substrate complex is formed- it is this formation which lowers the activation energy because...
if 2 substrate molecules need to be joined attaching to the enzyme holds them close together reducing any repulsion between the molecules so they can bond more easily
if the enzyme is catalysing a breakdown reaction fitting into the active site puts strain on bonds in substrate . this strain means substrate molecules break up more easily
the 'lock and key' model
the 'induced fit' model
