Deletion mutants of tyrosine hydroxylase identify a region critical for heparin binding.

Phenylalanine hydroxylase, tyrosine hydroxylase, and tryptophan hydroxylase constitute a family of tetrahydropterin-dependent aromatic amino acid hydroxylases. It has been proposed that each hydroxylase is composed of a conserved C-terminal catalytic domain and an unrelated N-terminal regulatory domain. Of the three, only tyrosine hydroxylase is activated by heparin and binds to heparin-Sepharose. A series of N-terminal deletion mutants of tyrosine hydroxylase has been expressed in Escherichia coli to identify the heparin-binding site. The mutants lacking the first 32 or 68 amino acids bind to heparin-Sepharose. The mutant lacking 76 amino acids binds somewhat to heparin-Sepharose and the proteins lacking 88 or 128 do not bind at all. Therefore, an important segment of the heparin-binding site must be composed of the region from residues 76 to 90. All of the deletion mutants are active, and the Michaelis constants for pterins and tyrosine are similar among all the mutant and wild-type enzymes.