Zhl. Abraham et al., PH-DEPENDENCE FOR BINDING A SINGLE NITRITE ION TO EACH TYPE-2 COPPER CENTER IN THE COPPER-CONTAINING NITRITE REDUCTASE OF ALCALIGENES XYLOSOXIDANS, Biochemical journal, 324, 1997, pp. 511-516
The first quantitative characterization of the interaction of NO2- wit
h the Cu-containing dissimilatory nitrite reductase (NiR) of Alcaligen
es xylosoxidans using steady-state kinetics, equilibrium gel filtratio
n and EPR spectroscopy is described. Each molecule of this protein con
sists of three equivalent subunits, each containing a type-1 Cu atom a
nd also a type-2 Cu atom at each subunit interface. Enzyme activity in
creased in a biphasic manner with decreasing pH, having an optimum at
pH 5.2 and a plateau between pH 6.1 and 5.8. Equilibrium gel filtratio
n showed that binding of NO2- to the oxidized NiR was also pH-dependen
t. At pH 7.5, no binding was detectable, but binding was detectable at
lower pH values. At pH 5.2, the concentration-dependence for binding
of NO2- to the enzyme showed that approx. 4.1 NO2- ions bound per trim
eric NiR molecule. Unexpectedly, NiR deficient in type-2 Cu centres bo
und 1.3 NO2- ions per trimer. When corrected for this binding, a Value
of 3 NO2- ions bound per trimer of NiR, equivalent to the type-2 Cu c
ontent. The NO2--induced changes in the EPR parameters of the type-2 C
u centre of the oxidized enzyme showed a similar pH-dependence to that
of the activity. Binding constants for NO2- at a single type of site,
after allowing for the non-specifically bound NO2-, were 350 +/- 35 m
u M (mean +/- S.E.M.) at pH 7.5 and < 30 mu M at pH 5.2. The apparent
K-m for NO2- with saturating concentrations of dithionite as reductant
was 35 mu M at pH 7.5, which is 10-fold tighter than for the oxidized
enzyme, and is compatible with an ordered mechanism in which the enzy
me is reduced before NO2- binds.