Enzyme unit

An enzyme unit (U) is the amount of enzyme that converts one micromole of substrate per minute under specified conditions. It measures catalytic activity rather than enzyme mass.

Enzyme: A biological catalyst, usually a protein, that accelerates a chemical reaction without being consumed. An enzyme unit quantifies the catalytic activity of an enzyme preparation.
Michaelis–Menten kinetics: A model describing how initial enzyme reaction rate varies with substrate concentration. It explains why activity measurements depend on substrate concentration.
Katal: The SI unit of catalytic activity, equal to one mole of substrate converted per second. It measures the same kind of activity as U but uses a different scale.
Enzyme activity assay: A measurement of an enzyme’s catalytic rate under controlled experimental conditions. The assay produces the rate from which enzyme units are calculated.
Catalysis: The acceleration of a chemical reaction by a substance that is regenerated rather than consumed. The unit measures how much catalysis an enzyme performs per minute.
Initial rate: The reaction rate measured near the start, before substrate depletion or product buildup substantially changes conditions. Enzyme activity is commonly determined from the reaction’s initial rate.
International unit: A unit of biological effect defined by an agreed reference standard rather than a universal physical quantity. Unlike the enzyme unit’s rate definition, an international unit can be substance-specific.
Enzyme purification: The separation of an enzyme from other cellular or sample components. Total units and specific activity track activity recovery and purification progress.
Reaction rate: The change in concentration of a reactant or product per unit time. One unit is defined by a specified rate of substrate conversion.
Temperature dependence of enzyme activity: The change in an enzyme’s catalytic rate as temperature alters molecular motion and protein stability. Different assay temperatures can yield different measured unit values.

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