Enzymology. Abbr. Km. Named after Leonor Michaelis (1875-1945) and Maude Lenore Menten (1879-1960). A dissociation constant that characterizes the binding of an enzyme to a substrate. It is the concentration of the substrate at which half of the maximal velocity of an enzyme-catalyzed reaction is achieved and 50% of the enzyme molecules are bound to the substrate.