Mutation; Phenylalanine Hydroxylase/chemistry; Phenylalanine Hydroxylase/genetics; Protein Folding; Amino Acid Substitution; Biocatalysis; Heat-Shock Proteins/metabolism; Humans; Hydrogen-Ion Concentration; Isoquinolines/pharmacology; Maltose-Binding Proteins/chemistry; Maltose-Binding Proteins/genetics; Maltose-Binding Proteins/metabolism; Microscopy, Electron; Models, Molecular; Mutagenesis, Site-Directed; Mutant Proteins/chemistry; Mutant Proteins/metabolism; Mutant Proteins/ultrastructure; Osmolar Concentration; Phenylalanine Hydroxylase/metabolism; Phenylketonurias/enzymology; Phenylketonurias/genetics; Phosphorylation; Protein Conformation; Protein Multimerization/drug effects; Protein Structure, Secondary; Protein Structure, Tertiary; Recombinant Fusion Proteins/chemistry; Recombinant Fusion Proteins/genetics; Recombinant Fusion Proteins/metabolism; Serine/genetics; Serine/metabolism; Substrate Specificity; Temperature