enzyme inhibitors

what are the two main types of inhibitors?

irreversible

reversible

how do irreversible inhibitors work?

• inhibitor deactivates enzyme by forming strong covalent bond

• removes molecule from enzyme from availability

how do reversible inhibitors work?

inhibitor molecule binds non-covalently to an enzyme, temporarily stopping or slowing activity

what are the three main types of reversible inhibitor?

• competitive

• non-competitive

• uncompetitive

how do competitive inhibitors work?

• competes with substrate for binding to active site

• inhibitor resembles substrate in structure

• EI complex is formed

• reduces the amount of free enzyme for substrate binding

competitive inhibitor diagram

how does competitive inhibition impact K_m and V_max?

• increases K_m

  • more substrate is required in order to outcompete inhibitor

• does not change V_max

when does competitive inhibition have an effect?

when the substrate concentration is low

how can competitive inhibition be overcome?

increase substrate concentration (substrate is more likely to bind to the active site

direct plot and lineweaver burk plot of competitive inhibitor

what is an uncompetitive inhibitor?

• bind to ES complex at location other than active site (only binds to ES complex)

• binding site is only accessible after substrate binding

• binding site can be on enzyme or substrate

how does uncompetitive inhibitor work?

• forms enzyme-substrate-inhibitor (ESI) complex

• locks substrate in place, reducing the rate of product formation

• reduces ES complex concentration so drives E+S —> ES reaction

how does uncompetitive inhibition affect K_m and V_max?

V_max decreases

• substrate turnover decreases

• ESI complex cannot form product

K_m increases (higher affinity)

• inhibitor removes ES complex, driving formation of more E+S —> ES (Le Chatelier’s principle)

• therefore it appears as though enzyme has higher affinity for substrate

when is uncompetitive inhibition most effective?

when there is a high substrate concentration - increase the concentration of ES complexes

how can uncompetitive inhibition be overcome?

it can’t - increasing substrate concentration does not do anything

direct plot and lineweaver burk plot of uncompetitive inhibitor

what is a non-competitive inhibitor?

• binds to enzyme’s allosteric site

• binds to enzyme or enzyme-substrate complex with the same affinity

how does a non-competitive inhibitor work

• EI and ESI complexes formed

• distortes the shape of the active site

• reduces activity of enzyme and rate of reaction

when does non-competitive inhibition have an effect?

at low and high substrate concentrations

effects cannot be overcome by increasing substrate

how does non-competitive inhibition affect V_max and K_m?

V_max decreases

• dysfunctional enzymes decrease turnover of S —>P

K_m stays the same

• enzyme affinity for substrate is the same

direct plot and lineweaver burk plot of non-competitive inhibitor

examples of drugs that are competitive inhibitors

• ibuprofen - inhibitor of cyclo-oxygenase (COX) 1 and 2 which are used for signalling/inflammation

• statins - used to treat hypercholesterolemia. inhibits 3-hydroxy-3-methylglutaryl coenzme A reductase

examples of drugs that are non-competitive inhibitors

• cyanide: inhibits cytochrome oxidase

• heavy metals: bind to -SH groups and alter enzyme structure

examples of drugs that are uncompetitive inhibitors

very rare

examples of drugs that are irreversible inhibitors?

β lactam antibiotic penicillin: covalent inhibitor of penicillin binding proteins