Uncompetitive vs. Competitive vs. Non-competitive inhibition

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Competitive, Uncompetitive, and Noncompetitive inhibitors

Competitive inhibitors, uncompetitive inhibitors, and noncompetitive inhibitors are all types of reversible enzyme inhibition. Inhibitors can prevent a substrate from binding, decrease the enzyme’s catalytic activity, or do both. Many drugs are enzyme inhibitors.

Competitive inhibitors

Competitive inhibitors work by binding at the active site on the enzyme. They compete with substrate for the active site and prevent the substrate from binding. Their structure is similar to that of the substrate since they are binding at the same site. The presence of these inhibitors causes an increase in Km of the enzyme, but leaves Vmax unaffected.

Noncompetitive inhibitors

Pure noncompetitive inhibitors are similar to uncompetitive inhibitors. Like uncompetitive inhibitors, they have a separate binding site on the enzyme and can bind the enzyme-substrate complex. However, they can also bind the enzyme when substrate is not bound. Presence of a noncompetitive inhibitor will decrease Vmax and will not affect Km.

Uncompetitive inhibitors

Uncompetitive inhibitors differ from competitive inhibitors in that they have a separate binding site on the enzyme. Also, they only bind to the enzyme when substrate is bound to the enzyme. Uncompetitive inhibitors decrease both Vmax and Km.

What inhibitors tell us about enzyme mechanism

Specific inhibitors can often be used to ID residues critical for catalysis

Review – uncompetitive inhibition

i) only binds when the ES complex is formed –> ii) reduces the amount of ES available, and thus less [S] to create the half of ES –> apprent lower Km iii) reduced the amount of available ES that aren’t bound to the ESI, so at the high [S] concentrations, we have Vmax also lowering

Review – noncompetitive inhibition

i) binds to free or substrate-containing E ii)- reduces Vmax by reducing the number of active enzyme molecules (rather than by diminishing the proportion that have the substrate – as in uncompetitive) iii) noncompetitive is FAR away from the ES site

Review – competitive inhibition

i) less tight bind to Km means that Km INCREASES with more competitive inhibitor… ii) overall efficacy of enzyme is not affected iii) binds only to the active site of the E

Characteristics affecting Vmax

Amount of free E or ES available to complete the reaction at high [S]

Characteristics affecting apparent Km

1) (Un) – When there’s lower ES complex available Why is non-comp not affecting Km? Because you’re not affecting any ES complexes (and thus the dissociation) 2) (Comp) – When there’s a lot of competition for the ES –> "weak binding"

Irreversible inhibitors

1. Group specific reagents 2. Affinity labels (substrate analogs) 3. Suicide inhibitors 4. Transition-state analogs

Group specific reagents

React with specific R groups of AAs

Affinity reagents

React with the active site of the enzye

Suicide inhibitors

Binds to the active site, enzyme proceeds with reaction until it tries to form a transition state – at which point the substrate forms a COVALENT instead of temporary bond…

Transition state inhibitors

a TS analog that shows the type of TS of the ES is.

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