PKU can result from defects in phenylalanine hydroxylase (PAH), an enzyme that converts phenylalanine into amino acid tyrosine. It has been hypothesized that phenylalanine is both the substrate and an allosteric regulatory molecule for PAH. If this hypothesis is true, complete two models to show how PAH would appear if phenylalanine concentration were low (inactive enzyme) or high (active enzyme).

Low phenylalanine concentration:

If the enzyme is inactive, PAH will look like its inactive form, with phenylalanine located in the allosteric site / inactive site so that PAH is a different shape. This will not allow as much phenylalanine to be converted into tyrosine, as we wait for Phe levels to increase.

High phenylalanine concentration:

If the enzyme is active, PAH will look like its regular form, with an open active site that's the size of phenylalanine so that reactions can occur. (Like a hand fitting into a glove, or a lock into a key.) Phenylalanine will be the substrate, the thing acted upon by the enzyme. This will eventually raise the amount of tyrosine that's created and lower the concentration of phenylalanine.

Think about it like this. Ever heard of homeostasis? That's how the body likes to be the same, keep everything regulated, all the time. So if there are high levels of something, the body is going to try to keep that something down, and if there are low levels, it's going to try to increase the concentration of that thing.