LSM2106 Chap.7 Enzymatic Regulation
Enzymatic Regulation
Making catalytic capacity conditional
The seventh lecture covers regulation. The previous chapter described an enzyme working at whatever rate its substrate concentration dictates, and a cell cannot operate that way: pathways compete for the same intermediates, demand changes within seconds, and some reactions do damage when they run at the wrong moment.
Regulation is the set of devices that make catalytic capacity conditional.
Four mechanisms are worth holding in order of speed. Binding a small molecule at a site away from the active site changes activity within milliseconds and reverses as soon as the molecule leaves.
Covalent modification, typically adding or removing a phosphoryl group, acts within seconds, needs one enzyme to install it and another to remove it, and therefore behaves as a switch a signal can throw. Proteolytic activation, in which an inactive precursor is cut to release the working enzyme, is fast and permanent, because nothing rejoins a cut chain.
Changing how much enzyme exists takes minutes to hours and is what a cell uses for a sustained change in demand. The timescale in a question usually settles the mechanism before any chemistry is considered.
Cooperativity, thresholds and feedback
An allosteric enzyme has several subunits and several binding sites, and binding at one alters the others.
The kinetic signature is a sigmoid rather than hyperbolic plot of velocity against substrate. That shape buys a threshold. A non-cooperative enzyme needs an eighty-one-fold change in substrate concentration to move from a tenth of maximum rate to nine tenths, while a strongly cooperative one makes the same journey over roughly a threefold change.
The cell gains a switch positioned wherever the constant sits, and it gains something to tune, since activators lower the threshold and inhibitors raise it without changing the ceiling.
Feedback inhibition is the commonest use. The end product of a pathway binds allosterically to an enzyme catalysing the first committed step and shuts the pathway down as it accumulates.
Placing control at the first committed step avoids committing intermediates that have no alternative destination. A caution worth carrying: an allosteric inhibitor and a competitive inhibitor both move a curve to the right, so a single velocity measurement cannot separate them.
A competitive inhibitor is outcompeted at high substrate, while a cooperative enzyme does not produce a straight double-reciprocal plot in the first place.
What this chapter covers
- 01
Why an always-active catalyst is a liability for a cell
- 02
Four regulatory mechanisms ordered by the speed at which they act
- 03
Reversibility as the property that distinguishes them experimentally
- 04
Cooperative binding and the sigmoid velocity curve
- 05
Thresholds, activators and inhibitors that move the steep region
- 06
Feedback inhibition and the logic of the first committed step
One experiment that separates two candidate mechanisms
- 2Name the property that differs between the two candidates.
- 2Choose a procedure acting only on that property.
- 2State both outcomes before running it.
Key terms
- Allosteric site
- A binding site distinct from the active site at which a small molecule binds non-covalently and changes the activity of the enzyme.
- Cooperativity
- Interaction between binding sites on a multi-subunit protein, so that binding at one site alters the readiness of the others and the response curve becomes sigmoid.
- Covalent modification
- Regulation by attaching or removing a chemical group, usually a phosphoryl group, which requires separate enzymes to install and to reverse it.
- Proteolytic activation
- Conversion of an inactive precursor into an active enzyme by cutting the chain, an irreversible step often used to confine activity to a particular location.
- Feedback inhibition
- Inhibition of an early enzyme in a pathway by the pathway's own end product, so that flow slows as the product accumulates.
- First committed step
- The earliest reaction whose product has no destination other than the pathway in question, and therefore the economical place to put regulation.
Enzymatic Regulation FAQ
How do I tell which regulatory mechanism a described experiment shows?
Start with the timescale and with reversibility. Effects complete and reversed within a second are allosteric. Effects that persist after the stimulus is withdrawn but can be undone by adding another enzyme are covalent. Effects that cannot be undone at all are proteolytic. Effects taking an hour and surviving a wash are changes in the amount of enzyme present.
Why is regulation usually placed at the first committed step of a pathway?
Because material that has passed that step has nowhere else to go. Controlling entry avoids accumulating intermediates that cannot be used elsewhere, whereas blocking a late step leaves everything upstream piling up. The shared precursor before the committed step remains available to other pathways, so nothing is wasted.
Why would a cell use an irreversible mechanism at all?
Because irreversibility can be the point. An enzyme that must never act inside the cell that made it is safest as an inactive precursor cut only after it reaches its destination, since a reversible switch could be thrown by a stray signal in the wrong compartment. Tying activation to location rather than to a signal is the design goal.
Exam move
Build a four-row table of mechanism, timescale, reversibility and the experiment that would show it, and fill it in from memory rather than copying. Then practise writing a two-outcome prediction for each experiment, because these questions award the prediction and not the technique. Revisit the cooperativity curve alongside the oxygen chapter, since the two use one idea.
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