It was Michaelis and Menten, in 1913, who proposed a successful explanation for the effect of substrate concentration on the enzyme activity. According to them, enzyme (E) and substrate (S) combine rapidly to form a complex, the enzyme substrate complex (ES) or Michaelis constant. This complex then breaks down relatively slowly to form the product (P) of the reaction. (The formation of ES complex is central to all current thinking on the mechanism of enzyme action.)

Where K₁, K₂, K₃, and K₄ are the rate constants for each reaction.

When the substrate concentration reaches maximum, all the E will be converted to ES form. Now the reaction becomes rate limiting and the overall reaction rate becomes “insensitive” to further increase in substrate concentration.