Note that V max is the maximum velocity - which is approached as substrate concentration approaches infinity. This is called scaling of a variable. Non-competitive inhibitors decrease the apparent V max while K M is unchanged. The black curve represents the relationship between reaction velocity and substrate concentration for an enzyme which obeys Michaelis-Menten kinetics. The red curve represents the reaction kinetics in the presence of a non-competitive inhibitor and the green curve represents the reaction kinetics in the presence of a competitive inhibitor.
Some enzymes are multisubunit.
ATCase catalyzes a reaction which leads to the synthesis of the pyrimidine ring of C, U, and T nucleotides. CTP, one of these products, is an inhibitor of this enzyme and ATP is an activator.