Kinetic study of monophenol and o-diphenol binding to oxytyrosinase
The complex reaction mechanism of tyrosinase involves three enzymatic forms, two overlapping catalytic cycles and a dead-end complex. The deoxytyrosinase form binds oxygen with a high degree of affinity, K s O 2 = 46.6 ± 2.4 μM. The mettyrosinase and oxytyrosinase forms bind monophenols and o-diphen...
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Published in | Journal of molecular catalysis. B, Enzymatic Vol. 32; no. 5; pp. 185 - 192 |
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Main Authors | , , , , , , |
Format | Journal Article |
Language | English |
Published |
Amsterdam
Elsevier B.V
01.03.2005
Elsevier Science |
Subjects | |
Online Access | Get full text |
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Summary: | The complex reaction mechanism of tyrosinase involves three enzymatic forms, two overlapping catalytic cycles and a dead-end complex. The
deoxytyrosinase form binds oxygen with a high degree of affinity,
K
s
O
2
=
46.6
±
2.4
μM. The
mettyrosinase and
oxytyrosinase forms bind monophenols and
o-diphenols, although the former is inactive on monophenols. Analytical expressions for the catalytic and Michaelis constants of tyrosinase towards phenols and
o-diphenols have been derived. Thus, the Michaelis constant of tyrosinase towards monophenols
(
K
m
M
)
and
o-diphenols
(
K
m
D
)
are related with the catalytic constants for monophenols
(
k
cat
M
)
and
o-diphenols
(
k
cat
D
)
, respectively, and with the binding rate constants of the
oxytyrosinase form with these substrates (
k
+4 and
k
+6, respectively), by means of the expressions
K
m
M
=
k
cat
M
/
k
+
4
and
K
m
D
=
k
cat
D
/
k
+
6
. From these expressions, we calculate the values of the binding rate constant of
oxytyrosinase to the substrates (monophenols and
o-diphenols) for tyrosinases from different biological sources, and reveal that the
o-diphenols bind more rapidly to
oxytyrosinase than the monophenols. In addition, a new kinetic constant
K
m
D
(
M
)
=
k
cat
M
/
2
k
6
(the Michaelis constant for
o-diphenol in the monophenolase activity), is derived and determined. Thus, it has been shown that tyrosinase has apparently higher affinity towards
o-diphenols in its monophenolase than in its diphenolase activity. |
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ISSN: | 1381-1177 1873-3158 |
DOI: | 10.1016/j.molcatb.2004.12.005 |