activator
Summary
An activator is a molecule that increases the activity of an enzyme (an allosteric activator binds away from the active site) or the transcription of a gene (a DNA-binding protein that helps recruit RNA polymerase). Most testable: an allosteric activator raises substrate affinity, shifting the curve left.
Detail
Meaning: An allosteric activator binds a regulatory site, not the active site, and stabilizes the high-affinity relaxed form of the enzyme. Substrate then binds more readily, so the rate-versus-[S] curve shifts left (lower K0.5) while Vmax is usually unchanged. AMP and fructose-2,6-bisphosphate activating phosphofructokinase-1 are standard examples. In gene regulation, a transcription activator is a DNA-binding protein that binds an enhancer (eukaryotes) or a site near the promoter (prokaryotes) and helps recruit RNA polymerase.
On the MCAT: A passage shows rate versus [S] with and without a molecule, or a pathway with feedback regulation. The activator gives a higher rate at low [S]. Low-energy signals such as AMP and ADP tend to activate catabolic enzymes, while ATP and citrate inhibit PFK-1. A gene question may ask what happens to transcription when the activator is absent or cannot bind DNA. Expect lower transcription.
Common confusion: An activator is not an inducer or a substrate. In the lac operon, allolactose is an inducer that releases the repressor, while CAP bound to cAMP is the activator that works when glucose is low. An allosteric activator also does not compete with substrate at the active site.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Gatto, Stryer)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.