Bovine adrenal tyrosine hydroxylase (TH) is isolated in a partially inhibited state with the feed-back inhibitors adrenaline and noradrenaline tightly coordinated to high-spin (S = 80230-G/asset/equation/feb2001457939080230g-math-si1.gif?v=1&s=592b95fdcd82a51a60163531a44a32288d46a2a0)
) Fe(III) at the active site. In addition to the charge-transfer interaction with iron, an additional charged group in the polypeptide chain, with an apparent pK a of about 5.3 at 4°C, is involved in the binding of catecholamines. Protonation of this group increases the pseudo-first order rate constant for the dissociation of the TH-[3H]noradrenaline complex more than 100-fold at 4°C. At pH 7.0 and 30°C, phosphorylation of Ser-40 causes a 6-fold increase in the rate constant for this dissociation.
