Michaelis constant

Michaelis constant is the substrate concentration at which the reaction velocity is equal to half the maximal velocity of the reaction.

K_m = Michaelis constant

K_m = the value of substrate concentration[S] that causes V= ½ V_max *

The units of K_m are M, concentration.

K_m indicates the affinity of the enzyme for its substrate and thus the stability of the Enzyme-Substrate Complex.

Velocity is related to K_m through the Michaelis & Menten equation: v = (V_max [S])/(K_m + [S]).

• V_max = maximum rate of reaction achievable for the enzyme under given conditions, only occurs at infinite substrate concentration.

[S] = substrate concentration.

A reaction with a low Michaelis constant value indicates a low binding affinity, where as a large value indicates a high binding affinity. 

References

Michaelis, L ; Menten, M L ; Johnson, K A ; Goody, R S. The original Michaelis constant: translation of the 1913 Michaelis-Menten paper,
Biochemistry, 04 October 2011, Vol.50(39), pp.8264-9