What is the effect of substrate concentration on enzyme activity?

Enzyme activity is directly proportional to substrate concentration, but only up to a certain point. 

Higher substrate concentration increases the rate of enzyme activity. This is because at higher substrate concentrations there are more substrate molecules colliding with enzyme molecules. As the substrate concentration increases, so does the enzyme activity rate. However, this trend does not continue forever. Enzyme activity increases only up to a certain point, reaching an optimum rate at the enzyme’s optimum substrate concentration. 

After the enzyme’s optimum substrate concentration is reached, any increase in concentration will not have any effect on the reaction rate. This is because at optimum concentration, all of the enzymes will effectively be saturated, unable to bind to any more substrate even though there is plenty available. At this point, substrate concentration will no longer be a limiting factor and a continued increase in substrate concentration will produce no significant change in the reaction rate.