Some enzymes don't yield a simple hyperbolic plot but rather a sigmoidal plot. This indicates coopertivity between catalytic sites that can only occur with a protein containing multiple catalytic subunits. Vmax is the fastest an enzyme can go. It is the product of two values, k2, the turnover number for a mole of enzyme molecules, and the enzyme concentration. I. Enzyme activities can be altered by pharmacological or by physiological molecules.
Competitive inhibitors compete for the same binding site as the enzyme substrate.
They make the enzyme appear to bind substrate less well, appear to increase Km, without altering the Vmax. This is because adding large amount of substrate swamps out the inhibitor. Noncompetitive inhibitors don't alter substrate binding but prevents catalysis. They are recognized by their decrease of Vmax without an effect on Km. Physiological inhibitors are normally of the allosteric type.