From the Department of Biotechnology, Albanova
University Center, KTH, SE-106 91 Stockholm, Sweden, the
§ Department of Cell and Molecular Biology, Structural
Biology, Uppsala University, SE-751 24 Uppsala, Sweden, the
¶ Department of Fiber and Polymer Technology, KTH, SE-100 44 Stockholm, Sweden, the
Department of Biochemistry, Uppsala
University, SE-751 23 Uppsala, Sweden, and the
** Laboratorium für Organische Chemie, ETH
Hönggerberg, CH-8093 Zürich, Switzerland
Received for publication, October 28, 2002, and in revised form, December 18, 2002
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ABSTRACT |
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The extracellular flavocytochrome cellobiose
dehydrogenase (CDH; EC 1.1.99.18) participates in lignocellulose
degradation by white-rot fungi with a proposed role in the early events
of wood degradation. The complete hemoflavoenzyme consists of a
catalytically active dehydrogenase fragment (DHcdh)
connected to a b-type cytochrome domain via a linker
peptide. In the reductive half-reaction, DHcdh catalyzes
the oxidation of cellobiose to yield cellobiono-1,5-lactone. The active
site of DHcdh is structurally similar to that of glucose oxidase and cholesterol oxidase, with a conserved histidine residue positioned at the re face of the flavin ring close to the
N5 atom. The mechanisms of oxidation in glucose oxidase and cholesterol oxidase are still poorly understood, partly because of lack of experimental structure data or difficulties in interpreting existing data for enzyme-ligand complexes. Here we report the crystal structure of the Phanerochaete chrysosporium DHcdh
with a bound inhibitor, cellobiono-1,5-lactam, at 1.8-Å
resolution. The distance between the lactam C1 and the flavin N5 is
only 2.9 Å, implying that in an approximately planar transition state,
the maximum distance for the axial 1-hydrogen to travel for covalent
addition to N5 is 0.8-0.9 Å. The lactam O1 interacts intimately with
the side chains of His-689 and Asn-732. Our data lend substantial
structural support to a reaction mechanism where His-689 acts as a
general base by abstracting the O1 hydroxyl proton in concert with
transfer of the C1 hydrogen as hydride to the re face of
the flavin N5.
Cellobiose dehydrogenases
(CDHs1; EC 1.1.99.18) are
extracellular fungal flavocytochromes that are believed to participate in lignocellulose degradation by fungi. They are oxidoreductases carrying protoheme and FAD cofactors bound to separate domains. In vitro, CDH from the white-rot Basidiomycete
Phanerochaete chrysosporium depolymerizes cellulose,
hemicelluloses, and lignin (Refs. 1-3; for review, see Ref. 4) as well
as other polymers (5). The exact biological function of CDH has been a
subject of lively debate, but recent results suggest that the enzyme is
important for invasion and colonization of wood (6).
The catalytic site is located in the flavoprotein domain, where the
reductive half-reaction proceeds by oxidation of
INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
CONCLUSIONS
REFERENCES
-cellobiose (apparent kcat 15.7 s
1
and Km 0.11 mM, see Ref. 7) to yield
cellobiono-1,5-lactone (Fig. 1) and the
concomitant two-electron reduction of FAD. In dilute aqueous solution,
cellobionolactone hydrolyzes to cellobionic acid. Results from
1H NMR spectroscopy show that the product from cellobiose
oxidation by CDH is unequivocally cellobionolactone, and thus,
cellobionic acid is not formed on the enzyme (8). During the ensuing
oxidative half-reaction, the flavin is re-oxidized by an electron
acceptor, either directly or via the cytochrome domain (9). At present, the most favored mechanism for in vivo degradation of
biopolymers by CDH is the reduction of ferric compounds present in wood
in the presence of hydrogen peroxide to form hydroxyl radicals through a Fenton-type reaction (2, 10). The cytochrome domain has been
implicated in this reaction (10) since the generation of hydroxyl
radicals proceeds by one-electron reduction; however, the flavin domain
is also able to generate hydroxyl radicals (1). Besides cellobiose,
soluble cellodextrins, mannobiose and lactose, are good or acceptable
substrates for CDH, whereas monosaccharides are poor substrates (7).
CDH can use a large number of electron acceptors; however, reduction of
oxygen is slow (11).
View larger version (22K):
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Fig. 1.
Molecules discussed in the text.
a, cellobiose; b, cellobionolactone;
c, cellobionolactam.
We recently reported the crystal structure of the dehydrogenase
fragment of CDH without ligand (DHcdh) showing that it is closely related to that of members of the glucose-methanol-choline (GMC) family of oxidoreductases (12). The GMC oxidoreductases glucose
oxidase (GOx (13, 14)) and cholesterol oxidase (ChOx (15, 16)) are
similar in overall structure and active-site architecture to
DHcdh (12). GOx catalyzes the oxidation of
-D-glucose to D-glucono-1,5-lactone. Thus,
GOx and CDH perform similar chemistry in the reductive half-reaction.
The bifunctional enzyme ChOx, however, catalyzes the oxidation and
isomerization of cholesterol to 4-cholesten-3-one. Although CDH and GOx
catalyze similar chemical reactions, the structural details of the
catalytic site in the immediate vicinity of the flavin ring in CDH is
more similar to those in ChOx; that is, two conserved residues (His and
Asn) at the re face of the flavin ring in a similar
conformation and with near identical geometry relative to the flavin N5
(12). In GOx, however, the asparagine is replaced by a histidine
residue. Based on modeling of cellobiose in the DHcdh
active site, we suggested two glucosyl-binding sites at the
re face of the isoalloxazine ring with the reducing end of
cellobiose bound to the innermost site C close to the flavin
ring and the non-reducing end of cellobiose residing in the distant
site B (12).
Over the years, the most widely accepted reaction mechanisms for flavin-assisted dehydrogenation include the carbanion mechanism (17, 18), the radical mechanism (19, 20), and the hydride-transfer mechanism (21-23). In general, crystallographic data are not sufficient per se to conclusively discriminate between these mechanisms. However, with the increasing number of available ligand complexes for redox-active enzymes, the hydride-transfer mechanism is gaining in popularity as a general mechanism for dehydrogenation, although the radical mechanism has not been disproved. The only structure available for a GMC enzyme-ligand complex is that of Brevibacterium sterolicum ChOx with bound dehydroisoandrosterone (16). The authors propose a radical mechanism, although the structural details of ligand binding did not exclude any of the possible mechanisms. The situation is, at least partly, complicated by the dual activity of ChOx.
We have used the inhibitor 5-amino-5-deoxy-cellobiono-1,5-lactam (Cblm;
Ki ~0.25 mM at 35 °C, data not
shown), which has a geometry similar to that of the product and of a
possible transition state (TST), to study the active-site interactions and delineate a possible reaction mechanism for CDH. This is the first
crystal structure of a CDH with a ligand bound in the active site. The
reaction mechanism for enzymatic oxidation of cellobiose by CDH is
discussed in the light of the present structure of a complex between
the dehydrogenase fragment of CDH and Cblm, determined at 1.8-Å resolution.
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EXPERIMENTAL PROCEDURES |
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Crystallization, Data Collection, and Refinement-- DHcdh was purified and crystallized as described elsewhere (7, 12). Data were collected at 100 K from crystals soaked overnight in reservoir solution containing 1 mM Cblm. Data collection and model refinement statistics are summarized in Table I. Data reduction and scaling were carried out using MOSFLM (24) and SCALA (25), respectively. Our reported structure of DHcdh (Protein Data Bank code 1KDG (12)) was used as starting model for crystallographic refinement against DHcdh·Cblm amplitudes. Initial refinement was done with CNS (26), and manual re-building was done with the program O (27). Starting coordinates for Cblm were generated using CORINA (28) followed by manual fitting of the model to the electron density. Final refinement was done with REFMAC5 (29) at 1.8-Å resolution usg anisotropic scaling, hydrogens in their riding positions, and atomic displacement parameter refinement using the translation, libration, screw-rotation model. The flavin cofactor, inhibitor, glycosylation adducts, and the substrate and flavin binding domains of DHcdh (12) were defined as rigid bodies during translation, libration, screw-rotation refinement. The model contains 2 protein molecules (residues 215-755), two 6-hydroxylated FAD molecules; 5 N-acetylglucosamine residues (3 in molecule A and 2 in B), 2 Cblm molecules, and 1007 water molecules (533 for A, 473 for B, and 1 located on a non-crystallographic symmetry 2-fold axis). The bending angle of the isoalloxazine ring was calculated as described previously (12).
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Modeling of Cellobiose--
Cellobiose was modeled manually in
the active site of DHcdh guided by the observed binding
pattern for Cblm. To relieve geometric strain and impose favorable van
der Waals contacts, the model was subjected to energy minimization with
CNS (26) without the x-ray pseudo-energy term. During the energy
minimization, only atoms within a sphere of 5.5 Å from the linking
oxygen in the glycosidic bond were allowed to move. Atoms within a
cushion of 3.5 Å around the sphere were refined with harmonic restraints.
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RESULTS |
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Overall Structure--
The 1.5-Å crystal structure of
DHcdh (Protein Data Bank code 1KDG) has been reported
elsewhere (12). In brief, DHcdh consists of an FAD binding
subdomain and a substrate binding subdomain (Fig.
2a). The entrance to the
active site is located at the subdomain interface, where a 12-Å-long
tunnel leads down to the flavin ring. The structures of
DHcdh and DHcdh·Cblm are nearly identical
with root mean square deviation values (all atoms) of 0.35 and 0.34 Å for pair-wise least-squares comparisons of A and B molecules, respectively. The electron density for the protein as well as for the
inhibitor is of excellent quality (Fig. 2b). The average residual B factor for the ligand (all atoms) is 8.2 and 8.4 Å2 for the A and B molecule, respectively.
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The Flavin Cofactor-- As observed for DHcdh, the non-covalently bound flavin cofactor in DHcdh·Cblm is present as 6-hydroxylated FAD. The butterfly bending angle of the flavin ring is less pronounced in DHcdh·Cblm (molecule A, 7°; molecule B, 11°) compared with DHcdh (22°). The flattening of the flavin ring appears to result from an induced fit where the N5 moves into the isoalloxazine plane to accommodate the incoming C1 atom of the lactam ring. The angle defined by the flavin N5 and N10 with the backbone nitrogen atom of Gly-310 is 136° and the N5-Gly-310 N distance is 3.2 Å.
Binding of Cellobionolactam--
Inhibitor interactions are
outlined in Fig. 3, a and
b. The glucosyl-binding sites and the substrate-binding
residues are located at the re face of the isoalloxazine
ring. The lactam moiety of Cblm, corresponding to the reducing end of
cellobiose, is bound in site C with four protein residues
and three solvent molecules within hydrogen-bonding distance of its
exocyclic carbonyl and hydroxyl groups: O1-His-689 N2,
O1-Asn-732 N
2, O2-Ser-687 O, O2-His-689
N
2, and O3-Asn-688 N
2. The C3 and C6
hydroxyl groups can form one and two water-mediated hydrogen (H) bonds,
respectively. Thus, a total of eight H-bonds are possible in site
C. The endocyclic lactam nitrogen is positioned near the
flavin N5 (3.2 Å) and O4 (2.9 Å). The C1 atom of the lactam moiety,
which corresponds to the site of oxidative attack in cellobiose, binds
in a position 2.9 Å in front of and below the N5-C4a locus of the
isoalloxazine ring, defining an angle of 108° (molecule A) and 110°
(molecule B) with the N5-N10 atoms of the flavin ring. These values are
in agreement with those typically observed in flavoenzymes (30). It
should be noted that the lactam C1 and O1 are almost perfectly aligned
with the flavin N5 and C4a, respectively (C1-N5, 2.9 Å; O1-C4a, 2.9 Å). The short distance (molecule A, 2.5 Å; molecule B, 2.6 Å) of the
H-bond formed between the lactam O1 and His-689 N
2
suggests that this interaction is strong and that His-689 is suitably
positioned to deprotonate the substrate hydroxyl group.
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The glucosyl moiety of the ligand resides in site B where it
forms a total of five ligand-protein H-bonds: O2-Glu-279
O2, O2-Arg-586 N
, O3-Glu-279 O
1,
O3-Arg-586 N
2, and O6-Asn-688 N
2. The
aromatic ring of Phe-282 has rotated (molecule A, 7°; molecule B,
11°) compared with the DHcdh structure to stack more
planar with the B-site pyranose ring. Five solvent-mediated
H-bonds are observed, one each for O2, O3, and O6 and two for O4. A
total of 10 H-bonds and one planar hydrophobic stacking interaction contribute to binding in site B. Judged solely from the
number of possible interactions, site B is likely to make
substantial contribution to the binding of cellobiose, which is in line
with reported kinetic constants for di- and monosaccharides (7).
Coupled Tyrosine-Serine Flipping--
Only two residues undergo
conformational change in response to inhibitor binding (Fig.
3b). In the non-liganded DHcdh structure, Tyr-609 is kept out of the C-site by H-bonds formed by its
hydroxyl group with backbone atoms of Asn-732 and the Gln-582 side
chain. The active-site tunnel is filled with solvent molecules, of
which one water molecule binds in front of the flavin ring within
H-bonding distance from His-689 N2.
In the DHcdh·Cblm complex, however, the tyrosine
side chain has discarded its H-bonding partners and moved into site
C (Fig. 3b). In the new position, Tyr-609 resides
below the lactam ring where O coordinates two ordered water
molecules not present in the non-liganded structure. One water is
located below the lactam O6, where it is involved in a H-bond network
with Tyr-609 O
, Thr-581 O
, Asn-732 N
2, and the C6
hydroxyl group. The second water molecule is below the lactam C3
hydroxyl group and forms H-bonds to Tyr-609 O
, the lactam O3, and
another water molecule. The tyrosine and its two coordinated water
molecules (colored yellow in Fig. 3, b and c) elevates the "floor" of site C.
The flipping of the tyrosine side chain imposes two additional changes
in the active site as follows. (i) To accommodate the Tyr-609 side
chain in its new position, the side chain of Ser-519 also flips and
thereby exchanges its two H-bonding partners (Ser-519 O-Ser-687
O
, and Ser-519-water) for two new ones (Ser-519 O
-Met-685 O, and
Ser-519 O
-new water). The concomitant flips of Tyr-609 and Ser-519
give rise to a local backbone-C
displacement of 0.6 Å and 0.8 Å at
residue 609 and 519, respectively; (ii) the re-positioning of the
Tyr-609 side chain together with the presence of the ligand effectively
traps a water molecule (Wat1366) in a position 3 Å from the
C
1-N
1 edge of the His-689 imidazole ring
(Fig. 3, a-c), i.e. the imidazole side opposite
to that interacting with the C1 hydroxyl in cellobiose. This water
molecule is not present in the non-liganded structure.
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DISCUSSION |
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Comparison of Cellobionolactam with Modeled Cellobiose-- Structurally, cellobionolactam resembles the product cellobionolactone, with the only difference that the endocyclic O5 oxygen is replaced by an N-H function (Fig. 1). In both compounds C1 is coplanar with O1, C5, and O5 (or lactam nitrogen). We thus assume that the tetrahedral configuration of the anomeric center of cellobiose is changing toward a planarized intermediate during its transition to a sp2-hybridized carbon in cellobionolactone.
Based on the Cblm binding pattern, we have produced a docking model of
cellobiose in DHcdh. Minor steric restraints near the flavin N5 and His-689 N2 enable the planar lactam C1==O
to penetrate somewhat farther (0.3-0.4 Å) into site C
compared with the equatorial O1 group of cellobiose (Fig.
3c). The C1-N5 distance is 2.9 Å in the cellobiose model, and the shortest distance that the C1 hydrogen (1-H) would need to
traverse to bind covalently to N5 is, thus, roughly 0.9-1.0 Å. In the
Cblm complex, the corresponding transfer distance (imagining a 1-H as
in the TST) would be slightly less, 0.8-0.9 Å. Although these
distances do not differ significantly, the precise geometry of the 1-H
relative to N5 is slightly different in cellobiose as compared with a
TST in that 1-H in a TST would be closer to being aligned so as to
interact with the lowest unoccupied molecular orbital of the flavin
(Fig. 3c). The assumption that the observed inhibitor
binding is also valid for a TST implies that upon approaching the TST
the substrate slides slightly deeper into the active site. This results
in a better alignment of the 1-H with the flavin lowest unoccupied
molecular orbital. Similar to what has been suggested for several other
flavoproteins (30), the resulting negatively charged flavin
hydroquinone may be stabilized at the N1-C2==O locus of the
isoalloxazine ring by the positive dipole of the C-terminal
helix.
These small but distinct differences in binding may be interpreted as
the oxidative site favoring a partially planar TST mimicked by the
lactam ring in our structure.
Mechanistic Implications--
Similar specific relative geometry
of the substrate and cofactor-reactive groups has been observed in
crystal structures of ligand complexes for
nicotinamide-dependent (31), flavin-dependent (32), and quinone-dependent (33) oxidoreductases, all of
which have been assigned a hydride-transfer mechanism. From a purely structural viewpoint, the close proximity between C1 and N5 together with the relative geometry of the atoms appears to favor a general base-catalyzed hydride-transfer mechanism (Scheme
1, panel a). General
base-assisted deprotonation of the C1 hydroxyl group by His-689 in
concert with the expulsion of 1-H as hydride via a planar or nearly
planar TST would be entirely consistent with the experimentally
observed binding of Cblm and modeled cellobiose.
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The carbanion mechanism in its classical implementation requires that His-689 abstracts 1-H as a proton, resulting in a substrate carbanion. The carbanion then performs a nucleophilic attack at N5 to form a covalent C1-N5 adduct. The subsequent elimination reaction proceeds by the concomitant formation of a double bound between C1-O1 and uptake of the O1 hydroxyl proton by the flavin O4 and ultimately by N5. To accommodate the carbanion reaction, the C-site glucosyl residue would need to tilt forward toward His-689 by at least 45-90° to position the 1-H for proton abstraction. Such a conformational change in the spatially restricted active site is unlikely, structurally and energetically. The non-reducing end of the substrate is anchored in site B (Fig. 3b), and the conformational change would introduce unreasonable strain in the glycosidic bond between sites B and C. The intimate interaction between His-689 and O1 makes it difficult to find any reasonable incentive for this residue to abstract the more distant 1-H. The carbanion mechanism is also highly unlikely from a purely chemical point of view in that the generated substrate carbanion would be conjugatively destabilized, as has been well established by work of Eliel and co-workers (34). Thus, we suggest that the carbanion mechanism is incompatible with cellobiose oxidation by CDH.
Apart from a hydride-transfer mechanism, a radical mechanism is compatible with the present structure (Scheme 1, panel b). In this reaction, one electron may be transferred from the substrate O1 to the flavin C4a or N5 concomitantly with the abstraction of the O1-H as a proton by His-689, resulting in a flavin radical and a substrate radical. The subsequent step involves a transfer of the 1-H as a hydrogen radical to N5. Although the structure is compatible with the radical mechanism, the two radical species implied have not been demonstrated. The failure to detect the radical species spectroscopically does not, however, provide conclusive evidence against the radical mechanism. The formation of a cellobiosyl radical may be slow and its subsequent decomposition rapid, making detection of an electron spin resonance signal difficult. We may conclude that a physically meaningful difference between the hydride and the electron-transfer mechanisms hinges upon the temporal sequence, or concertedness, of events.
Studies on Structurally Unrelated Enzymes with Similar Substrate
Specificity--
The structure of soluble glucose dehydrogenase from
Acinetobacter calcoaceticus in complex with its substrate,
-D-glucose, has been reported by Oubrie et
al. (33). This enzyme catalyzes the oxidation of
-D-glucose to gluconolactone but uses pyrroloquinoline quinone as cofactor. Similar to what is discussed here for CDH, a
hydride-transfer mechanism was assigned to soluble glucose
dehydrogenase (33) based on the specific orientation of 1-H relative to
the C5 in pyrroloquinoline quinone (corresponding to the flavin N5 in
CDH). In soluble glucose dehydrogenase, the distance between the
substrate C1 and pyrroloquinoline quinone C5 is 3.2 Å, implicating a
transfer distance of 1.2 Å for 1-H. The overall structure of CDH and
soluble glucose dehydrogenase (Protein Data Bank code 1CQ1) and their
active sites display no obvious similarity, but nevertheless,
interesting details emerge when superimposing the active sites. A
superposition with reference to C1 in the two enzyme complexes aligns
the C5 of the pyrroloquinoline quinone cofactor in soluble glucose
dehydrogenase within 0.7 Å of the flavin N5 in CDH. The
N
2 atoms of the proposed active base in soluble glucose
dehydrogenase (His-144) and CDH (His-689) are only 0.4 Å apart, and
the distance between Asn-732 N
2 in CDH and Arg-228
N
2 in soluble glucose dehydrogenase is 1.7 Å. Thus, the
precise geometry of the tetrad defined by the hydrogen acceptor of the cofactor, the C1, the proton acceptor of the general base catalyst, and
the assisting residue coincide remarkably well despite different structure and cofactor dependence.
Studies on Structurally Related GMC Oxidoreductases-- Although no results are available from site-directed mutagenesis studies on CDH, the residues proposed to participate in catalysis (12) have been mutated in the related enzymes GOx and ChOx. In B. sterolicum ChOx (35), Streptomyces ChOx (36), and Penicillium amagasakiense GOx (37), replacement of the proposed catalytic base (His-689 in CDH) resulted in enzyme variants with drastically reduced or abolished catalytic performance as measured by kcat, whereas Km values were practically unaffected, thus supporting the assignment of His-689 in CDH as a general base catalyst. On the other hand, mutation of the Asn-732 counterpart in Streptomyces ChOx (N480A, N480Q) and P. amagasakiense GOx (H563A, H563V) resulted in inactive enzymes. In the light of the present structure and the mutant data for GOx and ChOx, we propose a dual role for Asn-732 where it (i) helps to position the substrate with respect to the flavin, and (ii) by offering a H-bond to O1, also facilitates proton abstraction by His-689.
The only crystal structure available for a GMC oxidoreductase-ligand
complex is that of B. sterolicum ChOx with bound
dehydroisoandrosterone (16). In ChOx, His-447 (His-689 in CDH) has been
proposed to activate a water molecule (Wat541) for nucleophilic attack
on the substrate. This water molecule occupies the position of the substrate C1-O1 group in CDH. Thus, CDH and ChOx share the same reaction geometry, although the reaction in ChOx is suggested to be
relayed through a water molecule. For GOx, no experimentally determined
complex with substrate or substrate analogue is available, but
-D-glucose has been modeled in the active site (14, 37), resulting in a position of the substrate relatively similar to that of
the C-site glucosyl moiety of Cblm in CDH; the substrate and
the catalytic residues are positioned at the re face of the flavin ring, and the C1 hydroxyl group is equidistantly positioned between His-689 and Asn-732 (His-520 and His-563 P. amagasakiense GOx). Molecular dynamics calculations of a
glucose-GOx complex with a water-mediated interaction between the
substrate O1 and the active histidine similar to that observed for ChOx
resulted in expulsion of the water molecule, suggesting that a
water-relayed mechanism is unlikely in GOx (14). Thus, direct
interaction between the proposed catalytic base and the substrate is in
agreement with our observed mode of Cblm binding to CDH.
Induced Fit and Water Trapping--
Before another reductive
half-reaction can occur, a total of two electrons acquired by the
flavin needs to be transferred to an electron acceptor during the
ensuing oxidative half-reaction, and the hydrogens transferred to N5
and His-689 N2 have to be suitably disposed of. For the
N5 hydrogen, the most probable destination of a proton is to bulk water
concomitantly with two single-electron or a two-electron transfer upon
flavin re-oxidation (depending on the electron acceptor used). In the case of the proton withdrawn from the substrate 1-hydroxyl group by
His-689 N
2, the structure provides some hypothetical,
but interesting scenarios.
As described above, the conformational change in Tyr-609, assisted by a
concomitant flip of the Ser-519 side chain, orchestrates the formation
of a highly ordered network of H-bonds below the ligand in site
C. This imposes an effective restriction of this site to
perfectly accommodate the lactam ring and thereby induce an optimal fit
of the TST-like ligand to the protein. The movement of the Tyr-609 side
chain into site C appears to occur in response to Cblm
binding and results in the entrapment of a water molecule (Wat1366)
close to His-689 C1 (Fig. 3,
a-c). In the non-liganded structure, the
position of Wat1366 is occupied by the CZ-OH group of Tyr-609. The
water 1366 is particularly interesting in that it is the only water
molecule within H-bonding distance to the imidazole group of the
proposed catalytic base in the ligand structure, and it may, thus,
serve as a secondary proton acceptor. The substrate proton acquired by
His-689 can be transferred to Wat1366 by a 180° rotation about
2 of the imidazole. In DHcdh·Cblm, Wat1366
interacts at the center of the aromatic ring of Tyr-609, which is
likely to increase its affinity for the extra proton at His-689. A
H3O+-
interaction (38) with the tyrosyl ring
may, thus, promote proton transfer to Wat1366 and stabilize the
resulting oxonium ion. The next step may be (i) that the oxonium ion
triggers the Tyr-609 side chain to swing out from site C and
resume its original position and thereby displace the product from the
active site, (ii) that product departure itself triggers the tyrosine
side chain to leave the active site and release the oxonium ion to exchange a proton with bulk water, or (iii) that protonation of the
histidine and the subsequent flip of the imidazole ring forces the
product to leave due to an emerging unfavorable contact between the
product carbonyl oxygen and the
-1 carbon of His-689 as a consequence of C
1 assuming the position of
N
2.
Nevertheless, the tyrosine flip clearly helps to induce an optimal fit
of the catalytic site for the inhibitor, and it generates a cavity for
a solvent molecule that may accept a proton from the catalytic base.
Wat1366 makes no direct contact with the site of oxidative attack, and
hence, the C1-N5 pathway of a presumed hydride ion is completely
shielded from water. It should be stressed that the position of Wat1366
in DHcdh·Cblm is completely different from that of the
water molecule bound in front of His-689 in the non-liganded structure.
This stresses the inherent difficulty in assigning catalytic roles to
active-site water molecules in the absence of ligand or when
non-authentic binding of ligand occurs. Thus, the water molecule
implicated in catalysis by ChOx (16) should be carefully evaluated,
although the dual function of ChOx may actually justify the presence of
a catalytic water positioned between the substrate and the catalytic histidine.
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CONCLUSIONS |
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The crystal structure of the CDH flavoprotein with bound inhibitor
provides support for a hydride-transfer mechanism for dehydrogenation with His-689 acting as a general base catalyst, deprotonating the
equatorial 1-hydroxyl group. The axial 1-H is aligned for a concerted
hydride transfer from C1 to N5 via a transition state characterized by partial planarization of C1. The binding of the inhibitor is in agreement with hydrogen transfer at the re
side of the flavin ring. We also suggest that Asn-732 is bifunctional in that it both participates in the formation of a productive enzyme-substrate complex and that it supports deprotonation by His-689
by serving as a H-bond donor to the 1-hydroxyl group. From a structural
and biochemical viewpoint, the reaction mechanism is simple and
requires only minor structural changes in the substrate and protein in
order for the 1-H to be expelled. The 1-hydrogen would then need to
traverse a distance of less than 1 Å for covalent attachment to the
flavin N5 atom. Although the perfect setup of the active site for
hydride transfer is evident, we cannot rule out the possibility of a
radical mechanism using structural data alone. The results provide a
structural platform for the use of conventional biochemical and
biophysical techniques as well as quantum mechanical and classical
molecular mechanics approaches to further investigate the molecular
mechanism of cellobiose oxidation by CDH.
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ACKNOWLEDGEMENT |
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We thank the beamline staff at European Synchrotron Radiation Facility ID14-EH4 (Grenoble, France) for assistance during data collection.
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FOOTNOTES |
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* This work was funded by grants from the Swedish Research Council for Environment, Agricultural Sciences, and Spatial Planning and the Swedish Research Council (to C. D.).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
The atomic coordinates and the structure factors (code 1NAA) have been deposited in the Protein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers University, New Brunswick, NJ (http://www.rcsb.org/).
To whom correspondence should be addressed. Tel.:
46-8-5537-8296; Fax: 46-8-5537-8468; E-mail:
divne@biotech.kth.se.
Published, JBC Papers in Press, December 19, 2002, DOI 10.1074/jbc.M210961200
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ABBREVIATIONS |
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The abbreviations used are: CDH, cellobiose dehydrogenase; Cblm, 5- amino-5-deoxy-cellobiono-1,5-lactam; ChOx, cholesterol oxidase; DH, dehydrogenase fragment; DHcdh, CDH flavoprotein without ligand; GMC, glucose-methanol-choline; GOx, glucose oxidase; NCS, non-crystallographic symmetry; TST, transition state; Wat, water.
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