Michealas Constant (Km ) is the affinity between an enzyme and a substrate. It is the halfway point of Vmax , or Vmax/2.
Km is inversely proportional to the affinity between an enzyme and a substrate. The higher the Km, the lower the affinity. 
- ↑ ↑ Berg J., Tymoczko J. and Stryer L. (2012) Biochemistry, 7th Edition, New York: W.H. Freeman.