5.3 Inhibition and Regulation

5.3 Inhibition and Regulation

Where the inhibitor binds

Enzyme inhibition means a molecule lowers an enzyme's activity. Separate competitive inhibition from noncompetitive inhibition by the binding site and by what extra substrate can do.

A competitive inhibitor binds the active site. It is similar enough to the substrate to occupy the pocket, so substrate and inhibitor compete. Enough substrate can outcompete the inhibitor, and the block is overcome. Competitive inhibition raises apparent Km. Km is the substrate concentration that gives half the maximum rate, so a higher apparent Km means more substrate is needed to reach that halfway rate. Vmax does not fall if enough substrate is added. The enzyme can still hit its original top speed once substrate, not inhibitor, fills the active sites. Competitive inhibitors do not bind the allosteric site, a regulatory spot away from the active site.

A noncompetitive inhibitor binds elsewhere, at an allosteric site. Binding changes the enzyme's shape, so the active site works poorly even if substrate is present. Extra substrate does not fully overcome the inhibition, because the inhibitor is not waiting in the same pocket. Vmax falls. While the inhibitor is bound, the enzyme cannot reach its original maximum rate. In this simple pattern, the problem is not a higher Km from competition at the active site. The problem is a less effective catalyst.

Feedback, allosteric signals, and cooperativity

Feedback inhibition uses an end product as the inhibitor of an earlier enzyme, commonly the enzyme for the first committed step. When the product is scarce, the early enzyme works. When the product accumulates, it shuts that step down. The pathway does not keep making intermediates that would pile up, and the cell does not keep spending resources on a molecule it already has.

Follow a pathway from A to B to C to D, where D is the final product and A to B is the first committed step. If D builds up, D binds the enzyme that converts A to B and slows it. Less B forms, so less C and less D follow. When the cell uses D, the end product lets go, and the early enzyme works again. A bacterial case is isoleucine synthesis: isoleucine inhibits the first committed enzyme of its own pathway. No dose numbers are required. The logic is the lesson.

Allosteric regulation is the wider idea. A regulator binds an allosteric site and changes activity. An allosteric inhibitor stabilizes a less active shape. An allosteric activator stabilizes a more active shape. Competitive inhibition is not this pattern, because a competitive inhibitor uses the active site.

Enzymes with several subunits can show cooperativity. Substrate binding at one subunit changes how readily the other subunits bind substrate, so the rate responds more sharply as substrate increases. Hemoglobin is a well-known protein example of cooperativity, but hemoglobin is not an enzyme. For catalysis, picture a generic allosteric enzyme with multiple subunits: binding at one active site shifts the others toward a form that binds substrate more easily.

Reading the outcome, not a formula

If extra substrate restores the original maximum rate, the inhibition is competitive: active site, higher apparent Km, Vmax unchanged at high substrate. If extra substrate cannot restore that maximum, the inhibition is noncompetitive: allosteric site, lower Vmax. If the inhibitor is the pathway's own end product acting on an early enzyme, the control is feedback inhibition. If a signal molecule increases activity by binding away from the active site, the control is allosteric activation.

A useful contrast is two test tubes with the same enzyme and a moderate amount of substrate. In the first tube a competitive inhibitor occupies some active sites, and the rate is low. Pour in much more substrate and the rate climbs toward the original Vmax, because substrate displaces the inhibitor. In the second tube a noncompetitive inhibitor has shifted enzymes into a less active shape. Pour in the same extra substrate and the rate stays below the original Vmax, because the empty active sites were not the problem. The competitive molecule never needed an allosteric site. The noncompetitive molecule never needed to mimic the substrate and sit in the pocket.

The pile-up that feedback prevents

Imagine a cell building an amino acid only when proteins need it. While the amino acid is being used, the first committed enzyme stays active. If protein building slows, the amino acid accumulates and binds that enzyme at an allosteric site. The committed step slows, so the pathway stops pulling material into unused product. When proteins start being made again, the amino acid level falls, the allosteric sites empty, and synthesis resumes. Isoleucine control in bacteria follows this pattern. The final product accumulates and shuts off the first committed step, which prevents a pile-up of intermediates.

Allosteric activators and inhibitors can act on the same pathway, and cooperativity can make the switch steeper, so a small change in signal produces a large change in rate. None of this rewrites delta G. Regulation changes how many enzyme molecules are in a working shape. It does not make an endergonic reaction exergonic, and the inhibitor is not the substrate being consumed.

Sort stems by the site and the signal. A molecule that resembles the substrate and blocks the pocket is a competitive inhibitor. A molecule that binds away from the pocket and lowers the rate ceiling is a noncompetitive inhibitor. The pathway's own product turning off an earlier enzyme is feedback inhibition. A sigmoidal oxygen curve for hemoglobin is cooperativity in a protein that is not an enzyme.