Biology Reference
In-Depth Information
2010
). This is despite the absence in this organism of the enzymes for haem
biosynthesis, raising interesting questions about haem acquisition. The
recombinant FHb, however, was purified from
E. coli
extracts with haem
and FAD, as expected for FHbs from numerous other species.
In vitro
, the
FHb was capable of metabolizing NO efficiently in the presence of oxygen,
as well as exogenous FAD to compensate for the cofactor lost on purification
(
Mastronicola et al., 2010
). The occupancy of haem in the purified protein
was not reported. Although it might be argued that the globin acquired
haem from the host
E. coli
and that the protein must be inactive in
Giardia
,
given the absence of haem, the authors also showed that, under anaerobic
conditions, nitrite induces a NO-consuming activity that was sensitive to
cyanide and CO, implicating the participation of a haem protein
(
Mastronicola et al., 2010
) in NO metabolism. Interestingly, NO produc-
tion and NOS activity by this parasite has been reported and the presence
of an NOS-like sequence in the
G. lamblia
genome has been identified
(
Harris et al., 2006
).
One of the earliest reports of a globin in any microbe was that of a
haemoglobin- or myoglobin-like protein in the ciliated protozoan
Parame-
cium caudatum
, initially by Sato and Tamiya. This was confirmed by
Keilin
and Ryley (1953)
, who also showed that another ciliate
Tetrahymena
pyriformis
possessed a similar protein, judging by whole cell spectroscopy.
The
T. pyriformis
and
T. thermophila
proteins have been isolated and charac-
terized (
Korenaga, Igarashi, Matsuoka, & Shikama, 2000
). Recently, O
2
association and dissociation rates and autoxidation rate constants were
determined for the
T. pyriformis
protein (
Igarashi, Kobayashi, &
Matsuoka, 2011
) and shown to be similar to those reported for the
M. tuberculosis
HbN, a bacterial globin implicated in NO tolerance. A Fe
(III)-H
2
O complex was formed following the reaction of NO with the
Fe(II)-O
2
complex in the crystal state. Although this was interpreted by
the authors as indicating a function for this globin in NO detoxification,
we are unaware of any studies that clearly show this
in vivo
or that measure
protein turnover in the presence of NO. In contrast, the O
2
kinetics of
the
P. caudatum
protein (
Irie & Usuki, 1980; Iwaasa et al., 1989; Smith,
George, & Preer, 1962; Steers, Barnett, & Lee, 1981; Usuki, Hino, &
Ochiai, 1989
), with which the
Tetrahymena
protein shares high amino
acid identity (
Yamauchi, Mukai, Ochiai, & Usuki, 1992; Yamauchi,
Ochiai, & Usuki, 1992; Yamauchi, Tada, Ochiai, & Usuki, 1993;
Yamauchi et al., 1995
), are similar to those of sperm whale myoglobin
