What is an enzyme? 1
A catalyst that breaks down
large molecules
2
A catalyst that speeds up a
reaction
3
A biological catalyst / substance
that speeds up chemical
reactions

ZBWCYAX

Explanation:

Equation for the intermediates: Z↔W↔Y↔X

1) B catalyzes the breakdown of Z, therefore inhibiting B leads to the build up of Z.

2) If enzyme C is inactive then the organism will not  receive Y to grow, thus Y is a by-product of C.

3) If A is inhibited then W, Y and Z build except for X, thus it make sense that A catalyzes a reaction that leads to X as a by-product.

4) Inhibiting C leads to the reaction toward making Z and W, thus it catalyzes the breakdown of C directly and Z indirectly.