The oxidation-reduction and
spectroscopic
properties of various forms of nitrous oxide reductase from Pseudomonas
stutzeri were investigated. The high-activity form I of the enzyme
(purple, 8 Cu, Mr 140000) was reduced
by a
large variety of cationic, anionic and photochemically generated
agents.
The blue form III was the only product found in these experiments under
anaerobic conditions. Reductive (dithionite) and oxidative
(ferricyanide)
titrations showed that the conversion of the purple form I to the blue
species III was fully reversible in the absence of dioxygen. Two
kinetically
different phases of the reaction of form I with a stoichiometric amount
of dithionite (1e--equivalent/Cu) were detected: in the fast
phase (seconds), the purple chromophore with
lmax
at 540 nm disappeared almost completely, whereas in the slower phase
(minutes)
the blue species with lmax
around 650 nm was generated. Irrespective of the nature of the
reductant
the blue species did not react even at large excess of reductant. It
was
reoxidized by ferricyanide, hydrogen peroxide and nitric oxide.
A new, catalytically inactive
derivative
of nitrous oxide reductase (form V, 2 Cu, Mr
140000) was isolated from a transposon Tn5-induced mutant with
defective
chromophore biosynthesis. The pink color of the mutant protein faded
almost
completely after addition of 0.5e--equivalent/Cu. In this
case
no blue species was found, similar to earlier observations for the
regenerated,
catalytically inactive protein.
Varying with the sample and the
pH, 50 - 80% of the total copper of form I was in an electron-
paramagnetic-resonance-(EPR)-silent
state as compared to 47% in the mutant protein. The broad, featureless
EPR signal recorded at 9.32 GHz for the blue, reduced form III of
nitrous
oxide reductase represented approximately 20% of the total copper.
For the blue species no resolution
enhancement was achieved at 34 GHz. At this frequency both forms I and
V showed similar EPR signals with apparent g-values at 2.16 and
1.99. At 9.32 GHz, form V had an EPR signal with g||
at 2.18, A||
= 3.55 mT (4 or 5 lines, in contrast to form I) and g |
at 2.03. Above 100 K the splitting of the g||
region into seven equidistant lines in the EPR signal of the
high-activity
form I and the hyperfine structure of the perpendicular transition
disappeared.
Carbon monoxide and nitric oxide,
but not nitrous oxide, had marked effects on the spectroscopic
properties
of the purple form I. Marked effects were also obtained for the
exogenous
ligands nitrite, azide, cyanate and thiocyanate. The purple chromophore
disappeared in the presence of these agents and the gII region
of
the corresponding EPR spectra at 9.32 GHz broadened. No superhyperfine
structure originating from the interaction between the Cu(II) centers
of
nitrous oxide reductase and these ligands was detected. Nitric oxide
also
reacted with the reduced form III of the enzyme, giving a species with
the spectroscopic properties of the pink form II. A considerable amount
of nitrite was generated in the reaction of nitric oxide and the purple
form I, depending on the partial pressure and the reaction time.
When form I was mixed with hydrogen
peroxide or potassium superoxide at 0°C, a blue intermediate with a
broad shoulder around 640 nm was observed. The EPR spectrum of the
reaction
product showed the presence of type 2 Cu(II) centers with g||
= 2.26, A||
= 18.5 mT and g | = 2.06.
The present results indicate that
the coordination sphere of the purple Cu centers in nitrous oxide
reductase
are rather labile towards subtle changes in the environment such as pH
and exogenous ligands. The spectroscopic properties of the blue species
and its persistence in the presence of strong reductants point towards
a catalytic site with Cu in a ¢reduced'
state, stabilized by thiol or disulfide sulfur with substantial spin
density
delocalized onto sulfur.