Which substrate binds to the enzyme with the highest affinity

This point is reached when there are enough substrate molecules to completely fill saturate the enzyme's active sites.

The maximal velocity, or V max , is the rate of the reaction under these conditions. V max reflects how fast the enzyme can catalyze the reaction. Click on the image at right to see how high V max and low V max enzymes compare. V max is given by the asymptote to the velocity curve as the substrate concentration is extrapolated to infinity.

Notice that K m stays constant for the two enzymes described here. Affinities of enzymes for substrates vary considerably, so knowing KM helps us to understand how well an enzyme is suited to the substrate being used.

Measurement of KM depends on the measurement of Vmax. This is, of course not true. The value of KM is inversely related to the affinity of the enzyme for its substrate.

High values of KM correspond to low enzyme affinity for substrate it takes more substrate to get to Vmax. Low KM values for an enzyme correspond to high affinity for substrate.

Kevin Ahern and Dr. As soon as the catalytic site is empty, more substrate is available to bind and undergo reaction. The rate of formation of product now depends on the activity of the enzyme itself, and adding more substrate will not affect the rate of the reaction to any significant effect. The rate of reaction when the enzyme is saturated with substrate is the maximum rate of reaction, Vmax. The relationship between rate of reaction and concentration of substrate depends on the affinity of the enzyme for its substrate.

This is usually expressed as the Km Michaelis constant of the enzyme, an inverse measure of affinity. For practical purposes, Km is the concentration of substrate which permits the enzyme to achieve half Vmax.

An enzyme with a high Km has a low affinity for its substrate, and requires a greater concentration of substrate to achieve Vmax. The Km of an enzyme, relative to the concentration of its substrate under normal conditions permits prediction of whether or not the rate of formation of product will be affected by the availability of substrate. An enzyme with a low Km relative to the physiological concentration of substrate, as shown above, is normally saturated with substrate, and will act at a more or less constant rate, regardless of variations in the concentration of substrate within the physiological range.

An enzyme with a high Km relative to the physiological concentration of substrate, as shown above, is not normally saturated with substrate, and its activity will vary as the concentration of substrate varies, so that the rate of formation of product will depend on the availability of substrate. If two enzymes, in different pathways, compete for the same substrate, then knowing the values of Km and Vmax for both enzymes permits prediction of the metabolic fate of the substrate and the relative amount that will flow through each pathway under various conditions.