Michaelis constant
Say it: mih-KAY-lis KON-stunt
The Michaelis constant (Km) is the substrate concentration at which an enzyme-catalyzed reaction runs at half its maximum rate.
Also: Km, Michaelis-Menten constant
Definition
What is the Michaelis constant?
The Michaelis constant, written Km, is a value in the Michaelis-Menten equation, which describes how the rate of an enzyme reaction depends on the amount of substrate. In that equation, v = Vmax[S] / (Km + [S]), v is the initial rate, [S] is the substrate concentration, and Vmax is the top rate reached when every enzyme molecule is busy. Setting [S] equal to Km gives exactly half of Vmax, which is the usual working definition. Km is a concentration, so it is reported in units such as millimolar or micromolar.
Km depends on both the enzyme and the substrate. A low Km means the enzyme reaches half its top speed at a low substrate concentration, which is often read as high affinity. Strictly, though, Km combines the rate constants for substrate binding, release and conversion to product, and it equals the true dissociation constant only in special cases.
Students meet Km in kinetics practicals and in questions on enzyme inhibition: a competitive inhibitor raises the apparent Km but leaves Vmax unchanged. It is named after Leonor Michaelis and Maud Menten, who published their analysis in 1913.
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Terms that go with it
- EnzymeAn enzyme is a biological catalyst, usually a protein, that speeds up a specific chemical reaction without being used up in the process.
- Active siteThe active site is the pocket or groove on an enzyme where the substrate binds and the chemical reaction is catalyzed.
- Competitive inhibitionCompetitive inhibition is enzyme inhibition in which a molecule resembling the substrate occupies the active site and keeps the substrate out.
- ProteinA protein is a large molecule made of one or more chains of amino acids, folded into a specific shape that determines its job in the body.
- MetabolismMetabolism is the full set of chemical reactions in a cell or organism, both those that break molecules down for energy and those that build them up.
Sources
Checked against
Written for biofideX in plain words, after checking these references:
- LibreTexts, Cells: Molecules and Mechanisms (Wong), 3.3 Enzyme Kinetics
- LibreTexts, CSU Chico Biochemistry I, Analysis of the General Michaelis-Menten Equation
- Johnson KA, Goody RS. The Original Michaelis Constant. Biochemistry, 2011
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