The binding of tyrosine hydroxylase to negatively charged lipid bilayers involves the N-terminal region of the enzyme

Tyrosine hydroxylase (TH) is the rate-limiting enzyme in the synthesis of catecholamines. We have studied the association of recombinant human TH with model membranes by using either liposomes or silica gel beads coated with single phospholipid bilayers (TRANSIL). The use of TRANSIL beads has allowed the determination of apparent dissociation constants (Kd) for the binding of the enzyme to negatively charged bilayers (Kd=230-380 microM, at pH 6.0-7.0). Binding to the bilayers is accompanied by a decrease in enzyme activity. Proteolysed forms of the enzyme show decreased binding affinity and two putative amphipathic N-terminal alpha-helices are proposed to be involved in membrane binding. As seen by circular dichroism, binding to the bilayer does not seem to induce significant changes on the secondary structure content of the enzyme, but alpha-helical structures appear to be stabilized against thermal denaturation in the membrane-bound state. Thus, amphitropism, a mechanism that regulates the function of peripheral proteins by weak binding to membrane lipids, may add to the factors that regulate both the activity and the stability of TH.