enzymes Flashcards
what are enzymes and why
biological catalysts
they catalyse metabolic reactions - both at a cellular level (respiration) and for the organism as a whole (digestion)
enzymes can affect structures in an organism (involved in production of collagen) as well as functions (respiration)
enzyme action can be intracellular - within cells, or extracellular - outside cells
enzymes are proteins
why are enzymes highly specific
due to their tertiary structure
what do enzymes do the activation energy
they lower it
the amount of energy needed to be supplied to chemicals before the reaction can start is activation energy
enzymes lower this, often making the reactions happen at a lower temperature, speeding up the rate of reaction
why do enzymes affect the activation energy
if two substrate molecules need to be joined, being attached 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 a strain on bonds in the substate, so the substrate molecule breaks up more easily
induced fit model
As the substrate binds to the active site, the active site changes its shape slightly, which causes force to be placed on the substrate, causing products to be released
enzyme properties relate to their tertiary structure
enzymes are very specific - they usually only catalyse one reaction, eg maltase only breaks down maltose, sucrase only breaks down sucrose - this is due to only one complementary substrate fitting into the active site
the active site’s shape is determined by the enzyme’s tertiary structure (which is determined by the primary structure)
each different enzyme has a different tertiary structure and so a different shaped active site - if the substrate shape doesn’t match the active site, an enzyme-substrate complex won’t be formed and the reaction won’t be catalysed
what happens if the tertiary structure is altered
the shape of the active site will change - this means that the substrate won’t fit into the active site, an enzyme-substrate complex wont be formed and the enzyme will no longer be able to carry out it’s function
why would the tertiary structure be altered
the tertiary structure of the enzyme may be altered by changes in pH or temperature
the primary structure (amino acid sequence) of a protein is determined by a gene. If a mutation occurs in the gene, it could change the tertiary structure of the enzyme produced
temperature and enzyme activity
the rise in temperature makes the enzyme’s molecules vibrate more
if the temperature goes above a certain point (optimum temp), the vibration breaks some of the bonds that hold the enzyme in shape
the active site changes shape and the enzyme and substrate no longer fit together
at this point, the enzyme is denatured - it no longer functions as a catalyst
pH and enzyme activity
enzymes all have an optimum pH value
most human enzymes - pH 7
pepsin - pH 2 - found in stomach
above and below the optimum pH, the H+ and OH- ions found in acids and alkalis can mess up the ionic and hydrogen bonds that hold the enzyme’s tertiary structure in place
this makes the active site change shape, so the enzyme is denatured
enzyme concentration and rate of reaction
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 - so increasing the concentration of the enzyme 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 enzyme has no further effect
substrate concentration and rate of reaction
only affects the rate of reaction up to a point
the higher the substrate conc, the faster the reaction - more substrate molecules means a collision between substrate and enzyme is more likely so more active sites will be used - this is only true up until a ‘saturation’ point though - after that, there are so many substrate molecules that the enzymes have about as much as they can cope with and adding more makes no difference
substrate conc decreases 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
competitive inhibitors
molecules that have a similar shape to that of a substrate molecules
they compete with the substrate molecules to bind to the active site but no reaction takes place
instead they block the active site, so no substrate molecules can fit it
how much the enzyme is inhibited depends on the relative concentrations of the inhibitor and the substrate
- if there’s a high concentration of the inhibitor, it’ll take up nearly all of the active sites and hardly any of the substrate will get to the enzyme
- if there’s a higher concentration of substrate, then the substrate’s chances of getting to an active site before the inhibitor increase - so increasing the concentration of substrate will increase the rate of reaction
non-competitive inhibitor
bind to the enzyme away from the active site - allosteric site
this causes the active site to change shape so the substrate molecules can no longer bind to it
they don’t ‘compete’ with the substrate molecules to bind to the active site because they are a different shape
increasing the concentration of substrate wont make any difference to the reaction rate - enzyme activity will still be inhibited
