Inhibitor 4e2x18

Inhibition • The rate of reaction can be slowed down or inhibited by competitive and noncompetitive inhibition • The two types of inhibitions can be determined by plotting the rate of enzyme activity with the inhibitor and without the presence of inhibitor

Competitive inhibition • The competitive inhibition increases the K. without affecting Vmax • Competitive inhibitor and substrate compete for the same active site of the enzyme. • A higher substrate concentration is needed to overcome the competitive competition to achieve the same velocity reached in the absence of the competitor. • Vmax can be reached if there is sufficient substrate. • One half Vmax would require a higher substrate concentration, thus KM is larger.

Non-competitive inhibition • The non-competitive inhibition decreases Vm without affecting iç. • Non-competitive inhibition cannot be overcome by increasing the substrate concentration. • Thus the Vm of the reaction decreases. • The non-competitive inhibitor and substrate bind to different sites on the enzyme. • The enzyme therefore shows the same KM in the presence or absence of the non-competitive inhibitor

Inhibitor • is a substance that binds to an enzyme, thus, decreasing or stopping its activity. • Enzyme inhibitors can be grouped into two types: • (a) Competitive inhibitors • (b) Non-competitive inhibitors

Competitive inhibitors • increases KM without affecting Vmax • Competitive inhibitors have a shape similar to the natural substrate. • They can fit temporarily into the active site of the enzyme-preventing substrate binding to it. • The competitive inhibitor and natural substrate compete for the same active site of the enzyme. • Entry of a competitive inhibitors or substrates would depend on their relative concentration. • Competitive inhibition is reversed by increasing the substrate concentration

Examples of competitive inhibitors • Malonate. • It competes with succinate for the active site on the enzyme succinate dehydrogenase

Non-competitive inhibitors • decreases Vmi KM is unchanged • Non-competitive inhibitors have no structural similarities to the natural substrate. • They do not attach to the active site but bind with the enzyme at another site (allosteric site) • The binding causes a change in conformation of the enzyme molecule and its active site- prevents the substrate from binding to its active • The non-competitive inhibitor and the substrate are not competing for the active site. • Therefore an increase in substrate concentration does not affect the rate of reaction

• The rate of reaction decreases with increasing inhibitor concentration. • An example of a non-competitive inhibitor is : • cyanide which attaches itself to the copper prosthetic group of enzyme cytochrome oxidase and inhibiting respiratory reactions.

End-product inhibition • The end-product of a reaction can act as a non-competitive reversible inhibitor and regulates the metabolic pathway. • Example: ATP is produced by cellular respiration.

• When the concentration of ATP is high it acts as an allosteric inhibitor and inhibits biochemical reactions. • When ATP concentration falls, ATP leaves the allosteric site and cellular respiration is no longer inhibited

Riversible inhibition

• Competitive inhibition is reversible as the inhibitor binds temporarily the active site. It can be overcome by increasing the relativeconcentration of the substrate • Some non-competitive inhibitions are reversible, that is, if the inhibitor binds temporarily and loosely to the allosteric site

• Some inhibitors bind very tightly, often, by forming covalent bonds with the enzyme. • The nerve gas DIPF is an irreversible inhibitor. It binds permanently with acetylcholinesterase, altering its shape. • The enzyme cannot bind with and break down its substrate acetylcholine (neurotransmitter). • Acetylcholine molecules accumulate in the synaptic cleft. • Nerve impulses cannot be stopped causing continuous muscle contraction. This leads to convulsions, paralysis and eventually, death.

• Many pesticides such as organophosphate pesticides act as irreversible enzyme inhibitors. • Exposure to the pesticides can produce harmful effects to the nervous and muscular systems of humans. • Heavy metal ions such as Hg2, Ag, As and iodine-containing compounds which combine permanently with sulphydryl groups in the active site or other parts of the enzyme cause inactivation of enzymes.

• • • • •

reversible (rapid binding/release from enzyme in an equilibrium) or irreversible (very tightly bound to enzyme, either covalently or noncovalently, but effectively don't come off) Reversible inhibitors – type of inhibition diagnosed by effect of inhibitor on Km and Vmax • – effects/"diagnosis" obvious on double reciprocal plot. • – basis for drugs' actions • – research tools in figuring out enzyme chemical catalytic mechanisms •

• • • • •

Reversible Inhibition: –competitive –uncompetitive, or –noncompetitive – defined operationally by their effects on enzyme kinetic parameters, Km and/or Vmax.

• • • • • • • • • • • •

Reversible Enzyme Inhibitors, continued • Decreases Km and reduces Vmax by same factor so slope of 1/Vo vs. 1/[S] doesn’t change • binds only to ES complex • Has no effect on Km (no effect on S binding) • reduces Vmax (reduces kcat) “Pure” B ES Complex • Prevents S from binding, so increases Km