http://www.loc.gov/mods/v3 http://www.loc.gov/standards/mods/v3/mods-3-2.xsd
Studies of the chemical mechanism of tyrosine hydroxylase
xii, 122 leaves : illustrations ; 28 cm.
The iron-containing enzyme tyrosine hydroxylase (TYH)
catalyzes the hydroxylation of tyrosine to
dihydroxyphenylalanine, the initial step in the biosynthesis
of the catecholamine neurotransmitters dopamine,
norepinephrine, and epinephrine. A series of 4-X-substituted
(X=H, F, Br, Cl, CH3, CH30) phenylalanines has been
characterized as substrates to gain insight into the
mechanism of hydroxylation. Multiple hydroxylated products
were formed in most cases. As the size of the substituent at
the 4-position increased, the site of hydroxylation switched
from the 4- to the 3-position of the aromatic ring. The
total amount of product formed with each amino acid showed a
good correlation with the [] parameter of the substituent,
with [] values of -4.30︢.7 or -5.60︢.8 when
tetrahydrobiopterin or 6-methyltetrahydropterin,
respectively, was used as cosubstrate. These values are
consistent with a highly electron deficient transition state
for hydroxylation. Oxygen addition at the 4-position
resulted in either elimination of the substituent to form
tyrosine or an NIH shift to form the respective 3-X-tyrosine.
The relative amount of the product due to an NIH shift
decreased in the order Br>CH3>Cl>>F-CH30-0. A labile 4a
hydroxytetrahydropterin product was formed with all amino
acid substrates tested. For poorly coupled substrates the
amount of pterin product formed was greater than the amount
of amino acid product formed. A chemical mechanism for
hydroxylation by tyrosine hydroxylase is presented to account
for amino acid and tetrahydropterin product formation.
Formation of the NIH shift product with 4-methylphenylalanine
and 4-chlorophenylalanine showed isotope effects of 0.820︢.03
and 0.79+ 0.32, respectively. This is consistent with a
sequential mechanism where hydroxylation of the aromatic ring
occurs before the NIH shift. Studies using 3,4
cylohexenylalanie with tyrosine, phenylalanine, and
tryptophan hydroxylase showed formation of two products,
consistent with discrete attack at either the 3- or 4-carbon
and not formation of an epoxide intermediate. Removal of the
N-terminal domain of recombinant human phenylalanine
hydroxylase relieved allosteric control by phenylalanine.
The enzyme's metal content was 0.27 mole Fe per mole enzyme
subunit, and it was activated by added iron.
Issued also on microfiche from University Microfilms Inc.
Includes bibliographical references: pages 115-119.