enzymes Flashcards
what are enzymes?
biological catalysts in living cells which speed up chemical reactions whilst remaining unchanged- essential for survival
what do enzymes do to the equilibrium?
- DO NOT alter the equilibrium
- just accelerate the time taken to reach the equilibrium
what is enzyme alkaline phosphatase involved in?
mineralisation of tissue and bone
what is enzyme amylase involved in?
found in saliva, converts starch into sugars
what is enzyme maltase involved in?
found in saliva, breaks sugar maltose into glucose
what is enzyme lysozyme involved in?
antimicrobial, breaks down the peptidoglycan layer in the cell wall of bacteria
what is enzyme pepsin involved in?
digestion in stomach, breaks down proteins into smaller peptides
what is enzyme trypsin involved in?
found in small intestine, breaks down proteins into amino acids
what is enzyme acetylcholinesterase involved in?
breaks down the neurotransmitter acetylcholine in nerves and muscles
what is enzyme DNA polymerase involved in?
synthesises DNA from doexyribonucleotides
what are the two ways in which an enzyme can work?
- lock and key
- induced fit
what is lock and key?
Enzyme active site complementary in shape to that of substrate
Active site precisely shaped to hold specific substrates
what is induced fit?
Active site and substrate do not fit together exactly
Enzyme changes shape when substrate binds
Active site has a shape complementary to the substrate only after the substrate has bound
what is induced fit?
Active site and substrate do not fit together exactly
Enzyme changes shape when substrate binds
Active site has a shape complementary to the substrate only after the substrate has bound
what kind of molecule are most enzymes?
proteins (exceptions are catalytic RNA molecules)
what affects the nature of an enzyme?
pH
temperature
what is the optimum?
the conditions at which the enzyme works best at
what happens to the enzyme at extreme conditions?
it may lose its 3D structure- denature, and become totally inactive
what is the standard free energy change of a reaction? (G)
Difference between energies of reactants (initial state) and products (final state) is the
what is the activation energy of a reaction? (Ea)
the energy input required to initiate a reaction
what effect do enzymes have on activation energy?
enzymes lower activation energy (enzymes cannot alter free energy)
what are factors which can affect the rate of an enzyme reaction?
Temperature
pH
Enzyme concentration
Substrate concentration
Inhibitors and activators
Covalent modification
what will an increase in enzyme concentration do to a reaction?
increase the reaction rate
what effect does increasing substate concentration have on the rate of an enzyme reaction?
originally, increasing S will increase reaction rate, until all active sites are occupied whereafter it will remain constant
what is Vmax?
the maximum rate of reaction, where the enzyme is saturated with substrate
what does the relationship between reaction rate and substrate concentration depend on?
affinity (Km) of an enzyme for its substrate
(Km is the concentration of substrate at which the rate of reaction is half its max value)
what is the difference between a low Km and high Km?
Low Km: enzyme is normally saturated with substrate, acts at a fairly constant rate regardless of variations in substrate concentration
High Km: enzyme is not normally saturated with substrate, its activity will vary as substrate concentration varies – rate of product formation depends on substrate availability
what happens to substrate concentration overtime?
decreases
what are the two types of enzyme inhibitors?
- irreversible
- reversible
describe irreversible inhibitors
- damage enzymes beyond repair
- generally cause covalent modification of the enzyme
describe reversible inhibitors
- full enzyme activity is regained when the inhibitor is removed
- most natural enzyme inhibitors fall into this category
what is feedback inhibition?
- When the product of a metabolic pathway builds up in the cell, inhibition of the pathway often occurs
- This is often done by the product of the pathway acting as an inhibitor of an earlier step in the pathway
- This is known as feedback inhibition
- An example of this is the pathway that forms isoleucine from threonine
what types of inhibitors are there?
- competitive
- non-competitive
what is a competitive inhibitor?
Substrate and inhibitor compete for same active binding site – structural analogues, prevent substrate from binding
Inhibition can be overcome (reversible) with large quantities of substrate – substrate will ultimately occupy all binding sites, enzyme fully operative
what is non-competitive inhibition?
Bind to an allosteric site on the enzyme i.e. one other than active site
Structurally unrelated to substrate
Inhibition cannot be reversed by adding large quantities of substrate
what is an allosteric enzyme?
- Allosteric enzymes possess multiple subunits
- In addition to active sites they also possess regulatory (allosteric) sites to which non-substrate modulators bind, creating conformational change in subunits
- Subunits interact such that the binding of a substrate, inhibitor, or activator to one subunit alters the conformation of all the subunits:
- positive modulators (increase affinity for substrate at active site)
- negative modulators (decrease affinity for substrate at active site)
- Many important regulatory enzymes are allosteric
what is a common enzyme modification?
addition of a phosphate group to serine, threonine, tyrosine or hydroxyl group
