Pulsed EPR Study of Amino Acid and Tetrahydropterin
Binding in a Tyrosine Hydroxylase Nitric Oxide Complex: Evidence for
Substrate Rearrangements in the Formation of the Oxygen-Reactive Complex
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Abstract
Tyrosine hydroxylase is a nonheme
iron enzyme found in the nervous
system that catalyzes the hydroxylation of tyrosine to form l-3,4-dihydroxyphenylalanine, the rate-limiting step in the biosynthesis
of the catecholamine neurotransmitters. Catalysis requires the binding
of three substrates: tyrosine, tetrahydrobiopterin, and molecular
oxygen. We have used nitric oxide as an O<sub>2</sub> surrogate to
poise Fe(II) at the catalytic site in an <i>S</i> = <sup>3</sup>/<sub>2</sub>, {FeNO}<sup>7</sup> form amenable to EPR spectroscopy. <sup>2</sup>H-electron spin echo envelope modulation was then used to
measure the distance and orientation of specifically deuterated substrate
tyrosine and cofactor 6-methyltetrahydropterin with respect to the
magnetic axes of the {FeNO}<sup>7</sup> paramagnetic center. Our results
show that the addition of tyrosine triggers a conformational change
in the enzyme that reduces the distance from the {FeNO}<sup>7</sup> center to the closest deuteron on 6,7-<sup>2</sup>H-6-methyltetrahydropterin
from >5.9 Å to 4.4 ± 0.2 Å. Conversely, the addition
of 6-methyltetrahydropterin to enzyme samples treated with 3,5-<sup>2</sup>H-tyrosine resulted in reorientation of the magnetic axes
of the <i>S</i> = <sup>3</sup>/<sub>2</sub>, {FeNO}<sup>7</sup> center with respect to the deuterated substrate. Taken together,
these results show that the coordination of both substrate and cofactor
direct the coordination of NO to Fe(II) at the active site. Parallel
studies of a quaternary complex of an uncoupled tyrosine hydroxylase
variant, E332A, show no change in the hyperfine coupling to substrate
tyrosine and cofactor 6-methyltetrahydropterin. Our results are discussed
in the context of previous spectroscopic and X-ray crystallographic
studies done on tyrosine hydroxylase and phenylalanine hydroxylase