Would you expect a fat and sugar molecule to be acted upon by the same
enzyme? Why or why not?

Enzymes are protein molecules which are used by living organisms to either:
a) speed up the breakdown of large molecules 
or
b) speed up the the synthesis of complex molecules.

Enzymes are know to exhibit the property of speceficity, which means
they work on a very specific target.  The speceficity is often
described as a "lock-and-key" configuration since any given key will
only open one lock - just like any one enzyme will act upon only one
specific type of molecule.  Since fats and sugars are distinctly
different molecules with distinctly different conformations (shapes),
it nearly impossible for the same enzyme to act upon these two very
different substances.

Hope that helps.

-d



As such, you would not expect the

First of all, what njbagel has said is true.  But to give a little
more detail, I must say that when you talk about a sugar molecule, it
can consist of many different molecules.  Because sugar is a general
name to describe some particular molecules like glucose.  The same can
be said about fat which consists usually of triacylglycerol and fatty
acids which vary in length and types of bond.

Now, if we take one molecule of glucose, which is one of many sugar
molecules and see what enzymes act upon it, we can quickly see that in
order to get energy from it, you can expect easily over 20 enzymes to
transform it to a point where it ends up in ATP, which is the primary
source of energy for the human body.

As for the different fat molecules, there is also a lot of molecules
that will be needed to transform it to energy, but, at some point
along the transformation, it will get turned into Acetyl-CoA, which is
an intermediate of the glucose metabolic pathway.  This means that
after that point, what was originally a fat molecule of some sort is
now modified by the same enzymes as the glucose molecule.  The same
can also be said about proteins.