The Michaelis-Menten equation illustrates in mathematical terms the relationship between initial reaction velocity vi and substrate concentration . Vi=Vmax /Km+ The Michaelis constant Km is the substrate concentration at which vi is half the maximal velocity (Vmax/2) attainable at a paicular concentration of the enzyme. Ref: Kennelly P.J., Rodwell V.W. (2011). Chapter 8. Enzymes: Kinetics. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper's Illustrated Biochemistry, 29e.