Fax: + 39 081 674090, Tel.: + 39 081 674279, E-mail: mazzarella@chemistry.unina.it Abbreviations: Hb, hemoglobin; AFHb, Antarctic fish Hb; HbTb, Trematomus bernacchii Hb; Hb1Tn, major Hb
Trang 1The oxidation process of Antarctic fish hemoglobins
Luigi Vitagliano1,*, Giovanna Bonomi2,*, Antonio Riccio3, Guido di Prisco3, Giulietta Smulevich4
and Lelio Mazzarella1,2
1 Istituto di Biostrutture e Bioimmagini, CNR, Napoli; 2 Dipartimento di Chimica, Universita` degli Studi di Napoli ‘Federico II’, Complesso Universitario di M.S Angelo, Napoli; 3 Istituto di Biochimica delle Proteine, CNR, Napoli, Italy; 4 Dipartimento di Chimica, Universita` degli Studi di Firenze, Polo Scientifico, Sesto Fiorentino, Italy
Analysis of the molecular properties of proteins extracted
from organisms living under extreme conditions often
highlights peculiar features We investigated by UV-visible
spectroscopy and X-ray crystallography the oxidation
pro-cess, promoted by air or ferricyanide, of five hemoglobins
extracted from Antarctic fishes (Notothenioidei)
Spectro-scopic analysis revealed that these hemoglobins share a
common oxidation pathway, which shows striking
differ-ences from the oxidation processes of hemoglobins from
other vertebrates Indeed, simple exposure of these
hemo-globins to air leads to the formation of a significant amount
of the low-spin hexacoordinated form, denoted
hemi-chrome This hemichrome form, which is detected under a
variety of experimental conditions, can be reversibly
trans-formed to either carbomonoxy or deoxygenated forms
with reducing agents Interestingly, the spectra of the fully
oxidized species, obtained by treating the protein with
ferricyanide, show the simultaneous presence of peaks
corresponding to different hexacoordinated states, the
aquomet and the hemichrome In order to assign the heme region state of the a and b chains, the air-oxidized and ferricyanide-oxidized forms of Trematomus bernacchii hemoglobin were crystallized Crystallographic analysis revealed that these forms correspond to an a(aquomet)-b(bishistidyl-hemichrome) state This demonstrates that the
a and b chains of Antarctic fish hemoglobins follow very different oxidation pathways As found for Trematomus newnesihemoglobin in a partial hemichrome state [Riccio, A., Vitagliano, L., di Prisco, G., Zagari, A & Mazzarella,
L (2002) Proc Natl Acad Sci USA 99, 9801–9806], the quaternary structures of these a(aquomet)-b(bishistidyl-hemichrome) forms are intermediate between the physiolo-gical R and T hemoglobin states Together, these structures provide information on the general features of this inter-mediate state
Keywords: Antarctic fish; hemichrome; hemoglobin; hexa-coordination; oxidation
Hemoglobins (Hbs) are members of the globin superfamily
devoted to the transport of oxygen to cells [1] Except for the
Antarctic fish belonging to the icefish family, these proteins
are present in all vertebrates In these organisms, Hbs are
typically tetrameric proteins consisting of two pairs of
identical a and b chains While sharing a common general
mechanism of action, Hbs extracted from different
verte-brates have acquired specific functional properties in
response to major evolutionary pressures In Antarctic fish
the evolutionary process of cold adaptation has produced
unique hematological characteristics [2–5] In fact, the blood
of Antarctic fish contains fewer erythrocytes and less Hb than fish of temperate water so far studied Furthermore,
as constancy characterizes the conditions of the Antarctic marine environment, the blood of these fish is endowed with
a markedly reduced Hb multiplicity However, the charac-terization of Antarctic fish Hbs (AFHbs) has shown that they retain most of the structural and functional properties typical of Hbs of fish living in temperate environments
As found in other fish Hbs, the activity of AFHbs may be differently modulated by external effectors Indeed, although most AFHbs display the Root effect [3], namely low oxygen affinity with loss of co-operativity at low physiological pH, the major Hb of Trematomus newnesi (Hb1Tn) does not show this effect [6] Interestingly, this
Hb exhibits very high sequence identity (95%) with Hb of Trematomus bernacchii(HbTb) which, conversely, exhibits a strong Root effect [7]
We have recently shown that, in contrast with human and other mammalian Hbs, Hb1Tn rapidly forms low-spin hexacoordinated oxidized species (hemichromes) when exposed to air [8] In addition, we have determined the crystal structure of one of the intermediates of the oxidation process of this Hb [9] This intermediate is characterized
by a different binding state of the a and b chains A CO molecule is bound to the a heme iron, whereas a bishistidyl complex is observed at the b heme This structure, the first
Correspondence to L Mazzarella, Dipartimento di Chimica,
Univer-sita` degli Studi di Napoli Federico II, Complesso Universitario di
M.S Angelo, via Cinthia, I-80126 Napoli, Italy.
Fax: + 39 081 674090, Tel.: + 39 081 674279,
E-mail: mazzarella@chemistry.unina.it
Abbreviations: Hb, hemoglobin; AFHb, Antarctic fish Hb; HbTb,
Trematomus bernacchii Hb; Hb1Tn, major Hb component of
Trematomus newnesi; Hb2Tn, minor Hb component of Trematomus
newnesi; HbCTn, cathodic Hb of Trematomus newnesi; HbGa,
Gymnodraco acuticeps Hb; HbTbCO, carbomonoxy form of HbTb;
aeHbTbOx and fcHbTbOx, structures of HbTb oxidized by air and
ferricyanide, respectively.
*Note: These authors contributed equally to this work.
(Received 23 October 2003, revised 22 January 2004,
accepted 24 February 2004)
Trang 2example of a tetrameric Hb in the hemichrome state, has
demonstrated that the iron coordination by distal His,
usually associated with denaturating states, may be
toler-ated in a native-like Hb structure [9] Furthermore, the
analyses [9] of the quaternary structure and the critical
interface a1b2 have revealed that this partial hemichrome
state has an intermediate structure between the relaxed (R)
and tense (T) Hb functional states [10,11]
We here report extensive spectroscopic and
crystallo-graphic analyses of the oxidation process of AFHbs In
particular, we show that oxidation through hemichrome
formation is a common mechanism of five AFHbs extracted
from three Antarctic fishes of the dominant, largely endemic
Notothenioidei suborder In this framework, we
demon-strate that these hexacoordinated states may be successfully
reduced to deoxy and carbomonoxy forms Interestingly,
the crystal structure of two oxidized forms of HbTb
provides novel information on the different oxidation
pathway of the a and b chains of AFHbs and on the
accessible quaternary structures of tetrameric Hbs
Experimental procedures
Spectroscopic analyses
AFHb oxidation pathways were followed by UV-visible
spectrophotometric analysis In particular, T bernacchii Hb
(HbTb), Gymnodraco acuticeps Hb (HbGa) and the three
Hb components (major, minor and cathodic, hereafter
denoted Hb1Tn, Hb2Tn and HbCTn, respectively) of
T newnesi were considered The proteins were purified
following the procedures developed by di Prisco and
coworkers [6,7,12] and oxidized by exposing their
carbo-monoxy forms to air in 60 mMTris/HCl (pH 7.6) or 60 mM
potassium phosphate (pH 6.0) buffers at 20C The
oxidation process was initiated by exposing a cuvette
containing 600 lL of protein to air The proteins were also
oxidized by using ferricyanide These met-Hb derivatives
were prepared by oxidation of the carbomonoxy forms
using excess potassium hexacyanoferrate (III) in 60 mM
Tris/HCl (pH 7.6) or 50 mM CAPS
(3-cyclohexylamino-1-propanesulfonic acid)/NaOH (pH 10.0) at 20C followed
by gel filtration on a Sephadex G-25 column previously
equilibrated and eluted with 60 mMTris/HCl (pH 7.6) or
CAPS/NaOH (pH 10.0) buffers to remove the oxidant
The oxy form of HbTb was obtained by exposing a
CO-bound Hb solution to strong light under an intense flux of O2
To check the reversibility of the oxidation process, the
met-Hb forms were chemically reduced by adding 2–3 lL
sodium dithionite (20 mgÆmL)1) to 50 lL Hb solution
For comparative purposes all the experiments were
repeated on human Hb and on sea bass hemolysate This
hemolysate contains five Hb components as a result of the
combination of four different globins [13]
Denaturation was identified by the overall decrease in
intensity of the electronic absorption spectrum and the
increase in the intensity ratio of the aromatic band versus
the Soret band It is worth mentioning that the increase in
the aromatic band in the Soret region was followed by a
significant precipitation of the protein However, the
presence of the precipitate did not prevent the crystallization
of the oxidized forms (see below)
UV-visible electronic absorption spectra were recorded with a Jasco 560 spectrophotometer (Jasco Corporation, Tokyo, Japan) at room temperature
Crystallographic studies The oxidized forms of HbTb used for the crystallographic experiments were prepared using two different procedures
In the first one, HbTbCO was exposed to air and subsequently crystallized The free interface diffusion tech-nique was used by pouring the protein (final concentration
5 mgÆmL)1) in 60 mM Tris/HCl (pH 7.6) on a solution containing 14% (w/v) MPEG 5000 (Fluka) into a capillary sealed in air at 20C Single crystals suitable for X-ray analyses were obtained after 1 week Diffraction data were recorded at 2.4 A˚ resolution on these crystals 35 days after their appearance
A Nonius DIP2030b imaging plate mounted on a Nonius FR591 rotating anode (Nonius BV, Delft, the Netherlands) was used for data collection Results and statistics of data processing, carried out using the programDENZO[14], are reported in Table 1 In contrast with Hb1Tn, for which the crystals of the carbomonoxy [15] and the air-exposed [8,9] forms are nearly isomorphous, the crystals of this air-exposed form of HbTb, hereafter referred to as aeHbTbOx, are not isomorphous to the crystals of HbTbCO [7] The crystals are monoclinic (space group C2), with cell dimen-sion, a¼ 108.52 A˚, b¼ 65.09 A˚, c¼ 55.75 A˚ and b¼ 113.48 An ab dimer constitutes the asymmetric unit
In addition, crystallization trials were also set up for the oxidized form of HbTb prepared by using ferricyanide (fcHbTbOx), as reported in the previous section Crystals suitable for X-ray analysis were obtained using conditions very similar to those adopted for aeHbTbOx The final protein and MPEG5000 concentrations were 6.0 mgÆmL)1 and 12% (w/v), respectively In these experiments the capillaries were sealed under CO Crystals appeared after
1 week and were used for data collection 3 months later Diffraction data were collected at 2.5 A˚ resolution by using a Nonius rotating anode/imaging plate system The crystals are triclinic (space group P1) with cell dimen-sions a¼ 55.99 A˚, b ¼ 62.98 A˚, c ¼ 63.50 A˚, a ¼ 77.1, b¼ 69.8 and c ¼ 84.2 An a2b2tetramer constitutes the
Table 1 Data collection statistics R-merge ¼ S hkl S i |I i – <I>|/|I i |.
aeHbTbOx fcHbTbOx Crystal data
Data processing Resolution range (A˚) 20.0–2.4 25–2.5 Number of observations 47657 38318 Number of unique reflections 14020 23515
Trang 3asymmetric unit Statistics of the data collection are
reported in Table 1
Both structures (aeHbTbOx and fcHbTbOx) were solved
by molecular replacement using the programAMORE [16]
and the structure of Hb1Tn(hemi) (1LA6 Protein Data
Bank code) [9] as a starting model Straightforward
solutions were obtained using the ab dimer as search
model The overall position of the molecule was initially
refined by a rigid body minimization Subsequently, the
individual chains were refined as distinct rigid units The
rigid body refinement cycles were followed by atomic
positional refinements and B factor optimizations by using
the programCNS[17] Each refinement run was followed by
manual intervention using the molecular graphic programO
[18] to correct minor errors in the position of the side chains
In both structures the electron density maps corresponding
to the heme regions showed that a water molecule was
bound to the heme iron of the a chains, whereas in the
b chain there was a clear indication of the formation of a
bishistidyl complex The bond distance between the heme
iron and the Ne2atom the Fe of the bishistidyl complex was
restrained and refined to 2.0 A˚ In the final steps of the
refinement, water molecules were identified and included in
the refining models A detailed description of the refinement
statistics of the two structures is reported in Table 2 The
atomic coordinates of aeHbTbOx and fcHbTbOx have
been deposited in the Protein Data Bank, with entry codes
1S5Xand 1S5Y, respectively
Comparative analyses of AFHb structures
To analyze the structural variations associated with
hemi-chrome formation, the structures of aeHbTbOx and
fcHbTbOx were compared with those of the HbTbCO
(PDB code 1PBX) [7] and deoxy HbTb (HbTb-deoxy)
(PDB code 1HBH) [19], which were used as reference
structures to evaluate the position of these two structures
along the R fi T transition pathway Furthermore, to
measure the structural variability of Hbs in partial
hemi-chrome states, aeHbTbOx and fcHbTbOx were also
compared with Hb1Tn(hemi) [9] Specifically, this task
was achieved by evaluating root mean square deviations
and by generating difference distance matrices The
differ-ence distance matrix is indeed a sensitive probe for
investigating structural differences between two models [20] In this procedure, distances between pairs of Ca atoms are measured in each model The differences in the corresponding Ca distances between the two models are then evaluated and used as elements of the matrix
Results
Spectroscopic studies of the oxidation process of AFHbs Oxidation of AFHbs exposed to air.The oxidation process
of five AFHbs (Hb1Tn, Hb2Tn, HbCTn, HbTb, and HbGa) was initially followed by exposing the carbomonoxy forms to air As an example, the entire oxidation process of HbTb at pH 7.6 is reported in Fig 1 The spectrum of the oxy form is reported for comparison The spectrum of the
CO complex is characterized by the presence of Soret (418 nm) and Q bands (538 and 567 nm) which are very similar to those of human HbA-CO [21] On exposure to air, the spectrum starts to change After 18 h, the Soret band broadens and blue-shifts to 414 nm, and the a band red-shifts to 575 nm These spectral changes are consistent with the formation of the oxy form (Fig 1, top spectrum) [A rapid formation of the oxy form is also observed when Hb1Tn is exposed to air This observation suggests that the structure of Hb1Tn exposed to air, previously reported as
an a(CO)/b(hemichrome) [9], probably corresponds to
an a(O2)/b(hemichrome) state.] Concomitantly, a weak shoulder at 630 nm becomes evident This band, assigned to
Table 2 Refinement statistics R-factor ¼ S hkl (||F hkl obs| ) k|F hkl calc||)/
S hkl |F hkl obs| R-free ¼ S h (||F obs | ) k|F calc ||)/S h |F obs | where h is a
sta-tistical subset (5%) of data.
aeHbTbOx fcHbTbOx Resolution range (A˚) 20.0–2.4 25.0–2.5
Number of protein atoms 2153 4306
Number of water molecules 27 82
Root mean square deviations from ideal values
Bond lengths (A˚) 0.011 0.010
Dihedral angles () 17.2 17.4
Improper angles () 0.93 0.94
Fig 1 Electronic absorption spectra of air-exposed T bernacchii Hb The spectra were recorded at 20 C in 60 m M Tris/HCl (pH 7.6) with a protein concentration of 0.4 mgÆmL)1 The first five spectra from the bottom to the top were recorded after exposing HbTbCO to air for 0,
18, 49, 71, and 140 h The top spectrum was recorded on the oxy form
of HbTb The spectrum of HbTbCO exposed to air for 140 h was recorded on a sample containing HbTb at a concentration of 1.2 mgÆmL)1 The Soret band of the latter spectrum was not recorded because the protein was too concentrated The first four spectra from the bottom to the top of the Soret region correspond to HbTbCO exposed to air for 71, 49, 18, 0 h The region between 450 and 700 nm was expanded eightfold.
Trang 4the CT1 band of a hexacoordinated (6c) high-spin (HS)
form, is typical of a species with a water molecule
coordinated to the heme [21,22] After 49 h the Soret band
further downshifts to 408 nm, and the Q bands broaden
After 71 h, just before denaturation, the formation of a
new species characterized by a Soret band at 407 nm and
Q bands at 530 and 565 nm is observed In a parallel
experiment, carried out on a protein three times more
concentrated, the formation of this state was also observed
after 71 h of exposure of the protein to air, and became
clearly evident in the spectrum recorded after 140 h, when
the oxy form had almost disappeared These maxima are
typical of a 6c low-spin (LS) heme with an endogenous
ligand coordinated to the sixth position of the heme iron
(hemichrome) Interestingly, all AFHbs form low-spin
hexacoordinated hemichrome states, characterized by the
occurrence of peaks in the visible region at 530 and 565 nm
(Fig 2) The slight red-shift observed for the Hb2Tn,
Hb1Tn, and HbTb is due to the presence of the oxy form
(Fig 1) Notably, all spectra are characterized by the
simultaneous presence of an aquomet 6cHS species (as
judged by the presence of the weak CT1 band at 630 nm)
However, the time evolution of the different species depends
on the concentration of the protein (data not shown) and it
slightly varies among the five AFHbs It is worth noting, for
example, that the appearance of hemichrome is faster in
Hb1Tn than in HbTb
Hemichrome formation has also been detected for
AFHbs in media containing high concentrations of MPEG
5000 (12% w/v) and salt (0.5Mammonium sulfate) (data
not shown) The oxidation pathway exhibited by AFHbs
is very different from that reported for other vertebrate
Hbs, including those extracted from fish living in temperate
waters so far studied [23,24]
For comparative purposes the oxidation processes of
human Hb and sea bass hemolysate were analyzed under
the same experimental conditions No evidence of
hemichrome formation was detected on exposure of their
CO complexes to air Even after 185 h, before denaturation, human Hb only shows the formation of the oxy form In the same time period the sea bass Hb spectrum is characterized
by the coexistence of the oxy form with a very low amount
of aquo 6c HS species (weak CT1 peak at 630 nm, data not shown)
Finally, the influence of the Hb quaternary structure on hemichrome formation was analyzed by exposing HbTb to air at pH 6.0 As this Hb is endowed with a strong Root effect [7], the HbTb R/T equilibrium is shifted toward the T state at acidic pH Indeed, the T state form of HbTb was crystallized by Fermi and coworkers by simply lowering the
pH of the carbomonoxy form of the protein to 6.0 [19] In the early stages of the oxidation process (2–9 h, Fig 3) the exposure of HbTbCO to air leads to the formation of the deoxy form, as suggested by the appearance of shoulders at
434 and 556 nm This species evolves towards the formation
of the hemichrome state (24–30 h) The overall oxidation process of HbTbCO at pH 6.0 is faster than at pH 7.6 However, the relative intensity of the bands in the visible region suggests that the amount of the 6c HS grows at
pH 6.0 at the expense of the hemichrome Therefore, it appears that the latter form is favored at higher pH, as found for the 6c LS hydroxo in human Hb [21,22]
Chemical oxidation of AFHbs The oxidation process of AFHbs in air requires many hours and the proteins denature before reaching complete oxidation Therefore,
to obtain the final completely oxidized form, chemical oxidation of AFHbs by potassium ferricyanide was also studied Figure 4 compares the met forms of the various AFHbs at pH 7.6 with those of sea bass and human Hbs
in the visible region For the latter protein, the spectrum obtained at pH 10.0 is also reported The electronic absorption spectra of human and sea bass Hbs at pH 7.6 are characterized by bands at 498, 541, 576, and 630 nm, and a shoulder at 600 nm, indicative of a dominant 6c HS aquomet state in equilibrium with a hydroxy 6c coordina-tion (HS and LS states) [22,25] At alkaline pH, the spectrum of human Hb shows the presence of only the hydroxy forms (bands at 541, 576, and 600 nm) [22]
Fig 2 Electronic absorption spectra of air-exposed derivatives of the
five AFHbs The spectra were recorded after exposure of the
carbo-monoxy forms to air at pH 7.6 The spectra were recorded after 69 h
for Hb2Tn and HbC, 75 h for Hb1Tn, 92 h for HbGa, and 140 h
for HbTb The concentration of HbTb, Hb1Tn and HbC was
1.2 mgÆmL)1, whereas the concentration of Hb2Tn and HbGa was
0.7 mgÆmL)1.
Fig 3 Electronic absorption spectra of air-exposed T bernacchii Hb at
pH 6.0 The spectra were recorded after exposure of the carbomonoxy forms of the Hbs to air at pH 6.0, as indicated The concentration of HbTb was 0.45 mgÆmL)1 The region between 450 and 700 nm was expanded eightfold.
Trang 5Although at pH 7.6 the aquomet form is present in all
Hbs under investigation, the spectra of human and sea bass
Hbs do not show the presence of the hemichrome state
(characterized by the bands at 530 and 565 nm), which is the
dominant species in the spectra of all AFHbs It should be
mentioned, however, that minor differences are observed
among AFHb spectra Indeed, in addition to the
hemi-chrome 6c LS form and the 6c HS aquomet species present
in the spectra of HbCTn, HbGa, and HbTb, the peaks at
541 and 576 nm in the spectrum of Hb2Tn also indicate the
formation of a 6c LS hydroxymet form The possibility that
these bands may be due to the presence of the oxy form of
Hb2Tn may be ruled out by considering that the protein
had been treated with excess ferricyanide
The effect of temperature on the oxidation process of
Hb1Tn and HbTb was analyzed by oxidizing these Hbs
with ferricyanide at 4C The spectra obtained in these
experiments are virtually identical with those obtained at
20C (data not shown)
Chemical reduction, using sodium dithionite, of oxidized
HbTb leads to the formation of the deoxygenated form
which, subsequently in CO, evolves toward the formation of
HbTbCO (Fig 5) The low intensity ratio of the aromatic
versus the Soret band indicates that this HbTbCO form
holds a folded structure These observations suggest that
this hemichrome form, although intermediate along the
unfolding pathway of these proteins, retains a well-defined
structure This result is corroborated by the crystallographic
analyses reported below
Crystallographic studies on the oxidized forms of HbTb
Overall quality of the structures.The structure of
aeHbT-bOx was refined to an R-factor of 19.0% (R-free 23.3%)
using diffraction data in the resolution range 20.0–2.4 A˚
The final model includes 27 water molecules The structure
of fcHbTbOx was refined to an R-factor of 19.9% (R-free 24.7%) using diffraction data in the resolution range 20.0– 2.6 A˚ Eighty-two water molecules were included in the final model In both structures, the electron density is well defined for both the main chain and the side chain of most of the residues As frequently reported in R state Hbs, the regions corresponding to the CD loop (residues 45–52) and the C-terminus (residues 145–146) of the b subunit are com-pletely disordered The stereochemical parameters of the refined structure (Table 2) are in close agreement with those obtained for well-refined protein structures at the same resolution
Although aeHbTbOx and fcHbTbOx reveal significant differences at the quaternary-structure level, the tertiary structures and iron-binding states are virtually identical in these two oxidized forms of HbTb (see below)
Structure of the heme regions In both aeHbTbOx and fcHbTbOx, differences in the heme structures of the a and b subunits were evident from inspection of the first electron density maps In particular, analysis of the heme region of the a subunits shows that a water molecule is bound to the heme iron (Fig 6A) Given the pH of the crystallization medium (pH 7.6), the electron density of the ligand of the a iron could also correspond to a hydroxide ion This possibility can be, however, ruled out by taking into account the UV spectrum of air-exposed HbTb (Fig 1), which does not indicate the formation of a detectable amount of the hydroxymet species
A completely different picture emerges from analysis of the electron density maps corresponding to the b heme (Fig 6B) The iron atom coordinates both the proximal
Fig 4 Electronic absorption spectra of the met-Hb derivatives obtained
by treating the five AFHbs with excess potassium ferricyanide at pH 7.6.
For comparative purposes the spectra of human Hb (HbA at pH 7.6
and 10.0) and sea bass hemolysate (EMSP, pH 7.6) are also included.
The concentration of the Hbs, measured before oxidation, was
5.0 mgÆmL)1.
Fig 5 Chemical reduction of the met form of T bernacchii Hb Upper trace: HbTbCO treated with excess potassium ferricyanide Lower trace: carbomonoxy form obtained by reduction of the met form with excess sodium dithionite The spectra of the Soret region were recorded
at a protein concentration of 1 mgÆmL)1, whereas the concentration used to collect the spectra in the region 450–650 nm was 8 mgÆmL)1.
Trang 6(92b) and distal (63b) histidine residues Combining these
observations with the above spectroscopic results, the
present structure can be confidently assigned to an
a(aqu-omet)b(hemichrome) form In both structures, analysis of
the heme coordination geometry of the bishistidyl form shows that the Ne2–Fe–Ne2angle deviates significantly from linearity This finding is in agreement with the geometrical features of the bishistidyl complex in Hb1Tn(hemi) [9] The nonlinearity of the Ne2–Fe–Ne2 angle in these structures may be ascribed to the strain imposed by the protein matrix
Tertiary and quaternary structure of aeHbTbOx and fcHbTbOx Despite the different functional properties of Hb1Tn and HbTb and the different binding state of the a-heme iron of Hb1Tn(hemi) [9] and aeHbTbOx/fcHbT-bOx, the formation of the bishistidyl complex produces similar structural modifications in these two AFHbs In fact, the coordination of distal His by the b-heme iron in both aeHbTbOx and fcHbTbOx is associated with a scissors-like motion of helices E and F As found in Hb1Tn(hemi), the distance between the Caatoms of distal and proximal His
is 12.5 A˚ The value of this distance is usually larger than 14.0 A˚ in both ligand-bound and deoxygenated tetrameric Hbs [9] The formation of the bishistidyl complex also requires a significant shift of the heme group Indeed, as shown in Fig 7, in the oxidized forms of HbTb the heme group moves toward the exterior of the protein by 1 A˚
Fig 6 Electron-density Fo-Fc omit maps of the heme regions of
aeHbTbOx (A) a heme; (B) b heme The maps were contoured at
3.3 r A portion of the helices E and F are also shown to illustrate their
orientation.
Fig 7 Heme shift on hemichrome formation at the b chains of HbTb The EF regions of aeHbTbOx (black) and HbTbCO (gray) are shown after superimposition of the structurally conserved core composed of helices B, G and H.
Trang 7The variations detected at the level of the tertiary
structure propagate to the quaternary structure through
the a1b2 interface The displacements of helix F and FG
corner of the b subunit, which are necessary for
hemichrome formation, are not compatible with the R
state of HbTb Therefore, the protein acquires a novel
quaternary structure which is intermediate between the
canonical T and R states Indeed, the root mean square
deviations resulting from the superimposition of the
aeHbTbOx tetramer on the structures of HbTbCO and
deoxy-HbTb are 1.50 and 1.65 A˚, respectively Similar
deviations are found for fcHbTbOx Even more intriguing
is the analysis of the difference distance matrices of these
T bernacchiiHb structures (Fig 8) The similarity of the
matrix relative to the structures deoxy-HbTb and
HbTbCO (Fig 8A) and the matrix computed from the
structures of aeHbTb and HbTbCO (Fig 8C) provides
convincing evidence that the structural alterations that
occur on hemichrome formation coincide with the
mod-ifications associated with the structural transition from the
R to the T functional states [10,11]
Discussion
In this study, the oxidation process of five Hbs isolated from
three Antarctic fish species was investigated by combining
spectroscopic and crystallographic techniques In particular,
Hbs extracted from T newnesi (Hb1Tn, Hb2Tn and HbC),
T bernacchii(HbTb) and G acuticeps (HbGa) were
con-sidered
These three notothenioid species occupy well-separated
places in the phylogenetic tree [26] In fact, two of these
species (T newnesi and T bernacchii) belong to the family
Nototheniidae (subfamily Trematominae), whereas G
acu-ticepsbelongs to the family Bathydraconidae A
compar-ative analysis of the sequences of these AFHbs reveals the
occurrence of substitutions in important regions of the
protein, e.g the heme pocket and the a1b2interface The five
Hbs analyzed in this study also show different functional
properties Indeed, whereas the oxygen affinity of Hb1Tn
[6], Hb2Tn [6], and HbGa [12] is only slightly affected by
pH, the other two Hbs [6,7] exhibit a strong Root effect
Despite these differences, here we demonstrate that these
Hbs share a common oxidation pathway, which is
remark-ably different from that exhibited by other tetrameric Hbs,
including those extracted from the investigated species living
in temperate climates [23,24] In addition to the commonly
observed aquomet and hydroxymet forms, oxidation of
AFHbs leads to the formation of a significant amount of a
reversible hemichrome form This finding, which is in line
with a preliminary analysis of the oxidation process of
Hb1Tn [8], is particularly surprising as hemichrome
forma-tion is often associated with denatured states of tetrameric
Hbs [27]
The strong tendency of AFHbs to form hemichromes is
strenghthened by the observation that bishistidyl complexes
were invariably detected despite changing the oxidizing
agent (air or ferricyanide), the ionic strength of the medium,
and the temperature (4 and 20C) By analyzing the
oxidation process of a Root-effect Hb (HbTb) at pH 6.0, we
have demonstrated that hemichrome formation also occurs
when the protein is constrained to the T state
Fig 8 Difference distance matrices (A) deoxy-HbTb vs HbTbCO; (B) deoxy-HbTb vs aeHbTbO; (C) aeHbTbOx vs HbTbCO In each map, blue regions represent residues that move closer in the second structure, whereas the converse happens in the red regions The picture was generated using the program ESCET [20].
Trang 8The crystal structures of HbTb oxidized either by air or
ferricyanide reveal that a and b chains follow different
oxidation processes In fact, the formation of a bishistidyl
complex occurs only in the heme iron of the b subunits On
the other hand, the electron density indicates a water
molecule bound to the heme iron of the a chains This
finding suggests that the a and b chains possess a
significantly different degree of freedom in the tetrameric
structure of AFHbs In the absence of information on the
oxidation products of isolated Hb chains, it is difficult to
decide whether the two chains are intrinsically endowed
with a different flexibility or their mobility is differently
constrained in the tetramer The study of isolated chains of
human Hb has demonstrated that a chains are more ready
to form hemichrome than b chains [28], indirectly
support-ing the latter possibility
The different ligation state of the a and b chains may
account for the anomalous behaviour of this partial
hemichrome form when it is treated with reducing agents
Unlike hemichromes of other hemoproteins, which can be
reduced to hemochrome, the oxidized form of HbTb is
reduced by dithionite to deoxy HbTb This may be
explained by considering that the a chains are necessarily
reduced to the deoxy state This process drives the
allosterically regulated protein toward the deoxy state
As reported for Hb1Tn(hemi) [9], the quaternary
struc-tures of the fully oxidized forms of HbTb, aeHbTbOx and
fcHbTbOx, are intermediate between the R and T states
Comparative analyses on these three structures, derived from
three different crystalline forms, provide information on the
invariant features as well as on the overall flexibility of this
intermediate R/T state In all three structures, the
scissoring-like motion of the b-heme pocket produces a rearrangement
of the b FG corner His b97 takes a position that is
intermediate between those taken by this residue in R and T
structures These alterations are transferred through the a1b2
interface to helix F of the a chain The position of the helix is
locked by displacement of the Tyr a141 side chain, which
takes a conformation similar to that reported in the T state
Despite these conserved structural elements, the three
structures display significant differences with regard to the
overall structure This can be inferred from the overall root
mean square deviations between the structures that lie in the
range 0.40–0.70 A˚ This finding suggests that this R/T state is
endowed with a certain degree of flexibility despite the
structural constraint of the bishistidyl complex at the b heme
A molecular-graphics analysis carried out to identify the
specific structural basis responsible for the unusual
oxida-tion of AFHbs did not provide a conclusive answer
However, some amino-acid substitutions occurring in the
CD region (residues 42–52) and the heme pocket (residues
60–95) of the b subunit have been identified as potential
candidates that may facilitate hemichrome formation in
AFHbs Although rather flexible in all mammalian Hb
structures, the CD region of the b subunit is completely
disordered in AFHbs structures in both the R [7,15] and
R/T hemichrome [9] states As reported for the
nonsymbi-otic rice Hb [29], a high flexibility of the CD region may be
essential for the direct coordination of distal His to the heme
iron in AFHbs The greater mobility of this region may be
ascribed to the presence of an extra glycine residue (Gly43 in
HbCTn and Gly44 in Hb1Tn, Hb2Tn, HbTb, HbGa) in
the AFHbs sequences compared with human Hb and to replacement of Pro51 (human sequence) with Ala In this context, it is noteworthy that the formation of bishistidyl complexes in the a chain of crystalline horse hemoglobin [30] and in neuroglobin [31] is associated with large displacements of the CD corner The formation of the bishistidyl complex may also be facilitated by replacement
of Ala70 of the human Hb sequence with Gly residue in AFHbs Indeed, in human Hb [32], the methyl group of Ala70 is placed between two substituents of the heme group, and probably prevents the shift of the heme required for the formation of the bishistidyl complex It can be also surmised that flexibility of the CD region may be essential for the direct coordination of distal His to the heme iron as well as for heme dissociation
It cannot be excluded, however, that hemichrome forma-tion may be favored by a greater overall flexibility of AFHbs Indeed, by analogy with proteins from psychrophilic organ-isms [33], AFHbs may have acquired an enhanced plasticity, which allows the distortions required for hemichrome formation to be fully active at very low temperatures Finally, in the last few years it has been shown that bishistidyl complexes are functional states of several important monomeric and dimeric globins, such as neuroglobins [34], truncated Hbs [35,36] and nonsymbiotic Hbs [29,37] Although, on the basis of the available data, similar roles cannot be postulated for tetrameric Hbs, the present data show that bishistidyl complexes are, however, accessible states of a subclass of tetrameric Hbs Further-more, it has been shown that a significant amount of oxidized Hb is present in mammalian [38] as well as in fish [39] erythrocytes If this trend were to be extended to Antarctic fish, at least those under investigation, our data would imply that, under physiological conditions, a significant amount of Hb is present in a partial hemi-chrome state in these organisms
Acknowledgements This paper is dedicated to the memory of Eraldo Antonini, eminent biochemist, prematurely deceased on 19 March 1983 We thank Giosue¢ Sorrentino and Maurizio Amendola for their skilful technical assistance and Luca De Luca for help with the photograph layout. References
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