Consider a metabolic pathway for production of some product (P) that is known to exist in organisms A, B, C, D, and E. This pathway could be represented as a sequence of four consecutive enzymatic steps (E1, E2, E3, and E4) .
Step 1 In organisms A and B, substrate S1 is used as a precursor for a product P; in organisms C, D, and E, S2 is used as a substrate. S1 is metabolized by an enzyme E1a (in organisms A and B) and substrate S2 by an enzyme E1c (in organisms C, D, and E).
Step 2: Catalysis is performed by an enzyme E2 in all organisms under consideration.
Step 3: Organisms A and B use enzyme E3a, organisms C and D use enzyme E3c, and organism E uses enzyme E3e to perform the third step of this pathway. These alternatives are reflected on a module diagram.
Step 4: Catalysis is performed by an enzyme E2 in all organisms under consideration.