The key to biochemical pathways in living cells is catalysis of vital chemical reactions by enzymes.  Enzymes can only catalyze a chemical reaction when the shape of its active site is complementary to the shape of its substrate. 
 

Shown below is an example of the complementary shapes of the active site of an enzyme (tyrosine hydroxylase) and its substrate (tyrosine):
 
 
   

UPPER LEFT:   This is a wire frame diagram of the amino acid tyrosine.  As discussed on the previous page, tyrosine is simply a phenylalanine which has been hydroxylated at the para position of the phenyl ring by phenylalanine hydroxylase.  Also shown is the amino group (the blue line represents the nitrogen atom).  On the carboxyl group, the keto oxygen is to the right, and the hydroxyl group, with the acidic hydrogen, is to the left.
 
 

UPPER RIGHT:   This is a ball and stick model of the same tyrosine molecule, in the same orientation.  The red ball at the far right is the oxygen of the para hydroxyl group on the phenyl ring.  The amino group and the carboxyl group are readily recognizable to the left.
 
 

LOWER RIGHT:   This is a stereo view of the same tyrosine molecule in the same orientation.  To see it in 3-Dimensions, stare at it from a distance of approximately 2 feet, with your eyes unfocused.  Concentrate on the middle image.  When it starts to come into 3-D focus, it may help to move your head slowly in or out, or up and down slightly, or from side to side slightly.  If you cannot see it in 3-D, see Dr. Barnes for a STEREOPTICON 707.

note:  tyrosine is the substrate for tyrosine hydroxylase.  The 3-Dimensional SHAPE of this molecule gives it a complementary fit to the active site of tyrosine hydroxylase!
 
 
 

LOWER LEFT:  This is a stereo view of a space filling model of the active site of tyrosine hydroxylase.  It is very important to see it in 3-Dimensions.   Stare at it from a distance of approximately 2 feet, with your eyes unfocused.  Concentrate on the middle image.  When it starts to come into 3-D focus, it may help to move your head slowly in or out, or up and down slightly, or from side to side slightly.  If you cannot see it in 3-D, see Dr. Barnes for a STEREOPTICON 707.

note:  the "cavity" which is formed by the active site (the entire protein is much larger and is therefore not shown).  If you are seeing it correctly there will be a "porch roof" extending out towards you and a "wall" on the right.  The shape of the tyrosine hydroxylase active site is complementary to the shape of its substrate, tyrosine.  When tyrosine fits into the active site of tyrosine hydroxylase the enzyme add a second hydroxyl group to make DOPA (dihydroxyphenylalanine).