Say you are give a substance (call it substance "X"). X is some type
of drug which is metabolyzed in the liver (most likely using the
cytochrome P450 system). You know the initial structure of X, and you
know the chemical structures of the enzymes. How would one go about
predicting what will happen [chemically] to X once it reaches the
liver?

[Note: I'm aware this question will probably require a long
explanation, so even a link to websites or the names of books would
do]