Enzyme kinetics refers to the rate of reaction that occurs as the environment of the enzyme changes. This concept is studied using a Michaelis-Menten plot, a plot showing velocity on the y-axis versus substrate concentration on the x-axis.
The graph grows exponentially till it reaches a maximum point. The Vmax on the graph is the maximum speed of the reaction no matter how much substrate is added. The 50% velocity point is referred to as 1/2 Vmax. The 1/2 Vmax point helps us to locate an important factor known as the Michaelis-Menten constant, or Km on the x-axis.
You may consider Km as the concentration of the substrate when the velocity of the reaction is half its maximum. The km value increases when there is less affinity of the substrate to the enzyme. Consider the red graph in the diagram above. Notice that it has the same Vmax as the graph in black but the Km is higher. Hence you can conclude that the red graph represents a substrate with a lower affinity to the enzyme. Notice too that the red graph takes a longer time to reach Vmax. That also tells you that the substrate affinity is lower.
