You have isolated four mutants in amino acid metabolism of the mold N. crassa. Mutant 1 requires two compounds for growth, X and Z. Mutant 2 requires only X. Mutants 3 and 4 require only Z. Mutant 3 accumulates a compound, W, that supports the growth of Mutant 4 but not that of Mutants 1 or 2. Mutant 4 accumulates a compound, Y, that alone supports the growth of Mutant 1.
(a) Diagram the biosynthetic pathway connecting compounds W, X, Y, and Z, indicating the step at which each mutant is blocked.
(b) According to the diagram in (a), what is the first committed step in the synthesis of Z?