cd1
nitrite
reductase from Pseudomonas stutzeri is a di-haem-containing
enzyme,
comprising a c-type haem and a d-type haem. Studies
with
the highly related cd1 nitrite reductase
of
Pseudomonas
aeruginosa have established that this enzyme undergoes fast
(microsecond)
and global structural relaxation upon CO photodissociation from the
reduced
enzyme. A key difference between the Ps. aeruginosa and Ps.
stutzeri
enzyme
is the absence of a flexible N-terminal extension in the Ps.
stutzeri
enzyme. In
Ps. aeruginosa cd1 nitrite reductase
the N-terminal extension wraps around the second subunit of the
homodimer
and with Tyr10 stabilizing a water
molecule
co-ordinated to the d1-haem. Given the
intimate
association of the N-terminal extension with the d1-haem,
we hypothesized that the presence of the N-terminal extension likely
contributes
to the fast structural reorganization seen during photodissociation of
CO from the reduced enzyme. In the present study we have investigated
the
kinetics of CO association and CO photodissociation of Ps. stutzeri
cd1 nitrite reductase (which lacks the
N-terminal
arm seen in the Ps. aeruginosa enzyme) to probe the role and
influence
of the N-terminal arm in the fast global structural reorganization seen
with Ps. aeruginosa. Surprisingly, we find that Ps.
stutzeri
cd1 nitrite reductase also undergoes fast
structural
reorganization during CO photodissociation. We also show, in
stopped-flow
experiments, that the kinetics of CO binding and dissociation with
reduced
Ps.
stutzeri cd1 nitrite reductase are similar
to
those observed with Ps. aeruginosa enzyme, thus ruling out a
major
role for the N-terminal flexible arm found in Ps. aeruginosa in
the kinetics of these processes. Our data indicate that global
structural
reorganization following CO photodissociation is an intrinsic property
of the haem domains in cd1 nitrite
reductases.
The absence of an N-terminal extension, as in the Ps. stutzeri cd1
nitrite reductase, does not lead to loss of global structural
reorganization
following CO photodissociation.