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Enzymes and metabolic control definitions

Every term defined in this unit, lesson by lesson, in the same words the lessons use, with a link to the notes where you will see it used.

24 terms

Enzyme action and specificity

Catalyst: A substance that increases the rate of a reaction by lowering its activation energy, and which is not used up in the process.
Activation energy: The minimum energy that colliding particles must have for a reaction to take place.
Enzyme: A globular protein that acts as a biological catalyst, with an active site complementary to its substrate.
Active site: The region of an enzyme, formed by its tertiary structure, to which the substrate binds.
Substrate: The molecule on which an enzyme acts.
Enzyme-substrate complex: The temporary structure formed when a substrate binds to the active site.
Specificity: The property of an enzyme that it catalyses only one reaction, or one class of reaction, because only a substrate complementary to its active site can bind.
Lock-and-key model: A model of enzyme action in which the active site has a fixed shape that is already complementary to the substrate.
Induced fit: A model of enzyme action in which the active site is close to complementary to the substrate and changes shape as the substrate binds, putting strain on the substrate's bonds.

Initial rate: The rate of reaction at time zero, found from the gradient of the tangent to a progress curve at the origin.
Limiting factor: The factor that, of all those affecting a rate, is the one restricting it at that moment.
Optimum: The value of a variable at which an enzyme's rate is highest.
Q10: The temperature coefficient: the rate of a reaction at one temperature divided by its rate at a temperature 10 °C lower. Below the optimum it is about 2 for most enzyme-catalysed reactions.
Vmax: The maximum rate of an enzyme-catalysed reaction, reached when the enzyme is saturated and every active site is occupied.

Competitive inhibitor: A molecule with a shape similar to the substrate that binds to the active site, preventing the substrate from binding.
Non-competitive inhibitor: A molecule that binds to a site other than the active site, altering the enzyme's tertiary structure so that the active site is no longer complementary to the substrate.
Allosteric site: A binding site on an enzyme away from the active site, at which a regulatory molecule binds.
Km: The substrate concentration at which an enzyme-catalysed reaction runs at half its maximum rate. A lower Km means a higher apparent affinity for the substrate.
End-product inhibition: The inhibition of an enzyme early in a metabolic pathway by the final product of that pathway, so that the rate of the pathway matches demand.

Cofactor: A non-protein substance that an enzyme requires in order to function, such as the chloride ion required by amylase.
Coenzyme: An organic cofactor. In the usual A-level convention it binds to an enzyme, carries chemical groups or hydrogen between reactions and is released again; many are made from vitamins.
Prosthetic group: A cofactor tightly bound to an enzyme or other protein, and which may be organic, such as the haem group of catalase, or inorganic, such as the zinc ion of carbonic anhydrase.
Intracellular enzyme: An enzyme that catalyses reactions inside the cell that made it, such as catalase in peroxisomes.
Extracellular enzyme: An enzyme secreted from the cell that made it and catalysing reactions outside it, such as amylase and trypsin in digestion.