Allosteric Enzymes, Cooperativity and the Sigmoid Curve
Positive cooperativity can make substrate binding or enzyme rate follow an S-shaped curve. Binding at one site favours further binding at other sites. Allosteric effectors alter enzyme activity, sometimes by shifting the substrate concentration needed for a response.
What a learner can do afterwards
- Explains how positive cooperativity can produce a sigmoid response rather than a hyperbolic one.
- Predicts affinity-based left and right response shifts in a model that holds curve shape and maximum rate fixed.
- Links a steep response to pathway regulation without treating steepness as a measure of maximum catalytic rate.
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Positive cooperativity means binding at one site favours further binding elsewhere. It can produce a sigmoid rate-versus-substrate curve: shallow beginning, steep middle and plateau. Independent binding often gives a hyperbolic response. Negative cooperativity makes further binding harder.
Allosteric effectors bind at regulatory sites distinct from the active site. In our model, curve shape and maximum rate stay fixed. Increased apparent affinity shifts the response left: less substrate gives the same rate below the plateau. Decreased affinity shifts it right, lowering rate at fixed substrate. Other effectors can change shape or maximum rate.
One model describes lower-affinity and higher-affinity states whose populations change with ligand binding. Haemoglobin shows cooperative oxygen binding but is a carrier, not an enzyme. In feedback inhibition, a pathway's end product inhibits an early regulatory enzyme, reducing further production.
Near a sigmoid curve's steep middle, a small substrate change can cause a large rate change and regulate pathway flow. This response is graded, not a literal on-off switch. Steepness measures sensitivity to substrate concentration, not maximum catalytic rate.
Positive cooperativity can give a steep sigmoid response. Predict affinity-based left or right shifts only in the stated fixed-shape, fixed-maximum-rate model.
Lesson, questions and flashcards written for this dataset.
Readings and flashcards written for this dataset. Video resources by Khan Academy.
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