|MadSci Network: Biochemistry|
Enzymes are typically quite specific as to the reaction they catalyze, and as to what molecules (substrates) they act on. Thus, most enzymes that act on sugars will not act on a fat like a triglyceride, or vice versa. [And in fact, an enzyme that acts on one sugar will often not act on another. -- SM]
It does indeed have to do with their shapes and sizes - i.e. of both the enzyme and the substrate. The enzyme has a binding site (or sites) on its surface that serve to specifically bind the substrate molecule(s), in such a manner that the reaction can then be catalyzed. The reaction will take place only if the substrate molecule binds in the proper way, with its chemical groups in the correct orientation relative to the enzyme. Thus enzymes can typically distinguish between potential substrates, even if they differ in structure in only small diferences in their structure.
Sugars and fats are for the most part very different in structure, and are thus easily differentiated by their shape, size, and nature of the chemical groups making up these molecules
For more information, I would suggest you look in any biochemistry text, such as those by Lehninger, Stryer, Zubay, or Voet and Voet.
Try the links in the MadSci Library for more information on Biochemistry.