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Description of the speed with which a protein effects catalysis or its potential to catalyze a specific reaction.
Enzyme activity is a function of substrate concentrations, product concentrations (since too much product can inhibit the generation of additional product and particularly so given the catalysis of reversible reactions), and enzyme turnover rate. The latter is how fast a reaction can proceed once all necessary substrate molecules are available in an enzyme's active site and given the presence of no product to interfere with the resulting forward reaction.
Enzyme activity can be affected by enzyme inhibitors and activators. Turnover rate in particular can be affected by the presence of allosteric inhibitors or activators whereas competitive inhibitors have he effect of reducing the effective concentration of substrate (since more substrate is required to gain equivalent access to an enzyme's active site as one sees absent the competitive inhibitor). Enzyme activity can also vary as a function of osmolarity, pH, temperature, and even also the physical state of an enzyme which in turn can vary as a function of enzyme deterioration over time or due to mutations in the genes encoding an enzyme.
The study of enzyme kinetics has at its basis determinations of enzyme activity. The typical such determination involves measurement of rates or product formation given a defined amount of enzyme and as a function of substrate concentration (see substrate saturation curve).
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