(Received for publication, December 2, 1996)
From the Department of Life Sciences, National Tsing Hua University, Hsinchu 30043, Taiwan
Heparin and heparan sulfate have recently
been shown to bind to snake cardiotoxin (CTX) and to potentiate its
penetration into phospholipid monolayer under physiological ionic
conditions. Herein we analyze the heparin-binding domain of CTX using
10 CTXs from Taiwan and African cobra venom. We also performed computer modeling to obtain more information of the binding at molecular level.
The results provide a molecular model for interaction of CTX-heparin
complex where the cationic belt of the conserved residues on the
concave surface of three finger -sheet polypeptides initiates ionic
interaction with heparin-like molecules followed by specific binding of
Lys residues near the tip of loop 2 of CTX. The dissociation constants
of CTXs differ by as much as 4 orders of magnitude, ranging from ~140
µM for toxin
to ~20 nM for CTX
M3, depending on the presence of Lys residues near the tip of loop 2. High affinity heparin binding becomes possible due to the presence of
Arg-28, Lys-33, or the so-called consensus heparin binding sequence of XKKXXXKRX near the tip of the loop.
The well defined three-finger loop structure of CTX provides an
interesting template for the design of high affinity heparin-binding
polypeptides with
-sheet structure. The finding that several cobra
CTXs and phospholipase A2 bind to heparin with different affinity may
provide information on the synergistic action of the two venom
proteins.
Heparin and heparan sulfate (HS)1 belong to the glucosaminoglycan subclass of glycosaminoglycans (GAGs) and attract special attention because they exhibit greatest structural diversity (1, 2). Many physiological functions, including wound healing, cell attachment and spreading, recruitment of inflammatory cells (chemokines), and regulation of cell proliferation and differentiation (cytokines) are ascribed to them (for reviews, see Refs. 3 and 4). Heparan sulfate proteoglycans are ubiquitous and abound at cell surfaces, wherefrom they control the entry or availability of approaching entities (3, 5). Numerous proteins, such as growth factors (for review, see Ref. 6), antithrombin III (7), phospholipase A2 (PLA2) (8, 9), among others, bind to heparin with dissociation constants ranging from µM to nM. We recently found that CTX A3, a major component of Taiwan cobra venom, with selective cytolytic activity, also binds to heparin and HS under physiological conditions (10).
In view of the growing importance of functional role of heparin binding protein being identified, significant effort has been devoted, but with contradictory results, to elucidate the so-called heparin binding consensus sequence (11). An attempt to identify a heparin binding motif by linearly aligning a broad collection of alleged heparin binding sequences has been made and a consensus, XBBXBX and XBBBXXBX derived (12). Nevertheless, some of the proteins most affected by heparin do not contain this sequence in their heparin binding site (Ref. 13 and references therein). In addition, recent analysis of heparin binding sites of human antithrombin III (14) points to the requirement of Lys residues outside the previously proposed pentasaccharide binding region. This implies that complicated structural features other than simple cluster of basic residues must operate, and a well defined binding motif remains to be elucidated. The recently solved heparin-basic fibroblast growth factor (FGF) co-crystal structure (15) will promote our understanding further.
Cobra venom CTXs are a family of highly homologous, small (~60-62
amino acids), basic, water-soluble, but membrane- active, polypeptides
with well defined three-finger loop -sheet structures (16-20). Also
known as cytotoxins, they target a variety of cell types, in particular
cardiac myocytes (21, 22). Although their binding to phospholipids is
demonstrated (23, 24), lipids alone cannot account for their high
specificity. We have recently suggested that sulfated oligosaccharide
may be a target of CTX action and that this binding potentiates its
penetration into phospholipid membrane (10). The molecular mechanism of
this interaction and the amino acid residues involved remain to be determined.
We undertook a study of interaction of 10 CTXs isolated from venom of
Naja atra,2 Naja
mossambica and Naja nigricollis with heparin for the
following reasons. Cardiotoxins are highly homologous and their
three-dimensional structures are superimposable (for review, see Ref.
20). Differing in some cases by as little as one residue, these
naturally occurring variants present themselves as a series of readily
available basic proteins that might allow establishment of trends
underlying the interaction of basic -sheet proteins with anionic
carbohydrates of the cell surface. This approach, though less specific
than site-directed mutagenesis, can be directly applicable in
understanding the toxicity of cobra venom. For instance, the results of
the present study shows that all the toxins, and some PLA2 from the same venom, bind to heparin, albeit with varying avidity. Furthermore, we analyzed heparin-induced conformational change of CTXs by circular dichroism (CD) spectroscopic and computer molecular dynamic (MD) modeling methods to address the questions of binding motif and stoichiometry of CTX-heparin complex. The proposed model reveals a new
type of
-sheet heparin-binding motif with structural feature to
explain the high-affinity of three-finger toxins with receptors.
Crude venom from Naja naja atra, Naja
mossambica mossambica and Naja nigricollis nigricollis
(according to previous nomenclature) was purchased from Sigma. Heparin
(porcine intestinal mucosal) of average Mr 15,000 (HMW heparin) and homolytically depolymerized heparin of average
Mr 3,000 (LMW heparin) used in this study were also
purchased from Sigma. All other chemicals were of reagent grade. Egg
sphingomyelin and phosphatidylcholine were purchased from Avanti Polar
Inc. CTXs from N. atra and N. mossambica venom were purified by SP-Sephadex C-25 ion exchange chromatography and
reverse phase HPLC and assayed for hemolytic and fusion activities as
described (23, 24). Toxin (T
) was purified from venom of
N. nigricollis according to Fryklund and Eaker (26). Purity of CTXs, analyzed by SDS-polyacrylamide gel electrophoresis and analytical reverse phase HPLC, was found to be higher than 99%. Protein concentration was determined by the Lowry method.
Heparin-induced structural change in CTXs was analyzed by CD spectroscopy. Spectra were recorded on AVIV 62A DS spectropolarimeter (Lakewood, NJ); details of calibration of the spectrometer are reported (19). All experiments were performed in 20 mM sodium phosphate buffer, pH 7.4, due to technical difficulty in monitoring ellipticity at 195 nm using high ionic strength buffer. Typically, 20 µM CTX was titrated against HMW or LMW heparin, and at each point, the solution was incubated for 3 min at room temperature before scanning. Titrations in the reverse order were performed similarly by titrating 20 µM of LMW heparin against CTX. Spectra, average of four repeats, were obtained by scanning from 260 to 190 nm. A 1-mm cell, bandwidth of 1 nm, and time constant of 1 s were used to collect data, reported as ellipticity in millidegrees. Temperature was maintained at 25 °C. Heparin-induced structural perturbation was analyzed according to change in ellipticity around 192-195 nm.
Fluorescence MeasurementsIntrinsic fluorescence intensity of Tyr or Trp of CTXs was monitored to determine dissociation constant of the toxins with HMW heparin as reported (10). Briefly, the excitation and emission wavelengths were set at 285 nm and 318 nm for Tyr and at 291 nm and 345 nm for Trp, respectively, by using SLM 4800 fluorescence spectrometer. The dissociation constant, Kd, was determined by non-linear least squares fitting of data by assuming that n independent, but equivalent, binding sites are present in heparin using the following equation,
![]() |
(Eq. 1) |
![]() |
(Eq. 2) |
![]() |
(Eq. 3) |
![]() |
(Eq. 4) |
Change in turbidity of samples was monitored using Beckmann DU-70 spectrophotometer. Typically, CTXs in 20 mM sodium phosphate buffer, pH 7.4, were titrated against LMW heparin and the A320 was monitored at each point. In some cases, a reverse titration with constant LMW heparin (20 µM) against increasing CTX concentrations was also studied to reveal the binding mode of CTXs with LMW heparin. The procedure and result are similar to our previous experiment with HMW heparin, but the extent of aggregation as monitored by optical density is in general higher than that detected for HMW heparin.
Heparin Affinity ChromatographyTo determine the relative affinities of CTXs, crude venom (3.0-3.8 mg) from N. atra, N. mossambica, and N. nigricollis was loaded onto Heparin-Sepharose column (HiTrap, 1 ml, Pharmacia), fitted to Bio-Rad FPLC system. The column was washed extensively with 10 mM Tris buffer, pH 7.4, and bound fractions were eluted with a linear gradient of 0-1 M NaCl in 10 mM Tris buffer, pH 7.4. The eluted fractions were analyzed for protein concentration by measuring A230 using Beckmann DU-70 spectrophotometer. The assignment of respective toxin peaks was done by comparing the HPLC retention times of the fractions with that of purified toxins.
CTX and PLA2 AssaysCTX activity of each fraction was assayed by adding 100 µl aliquots of the fraction to a solution containing 20 mM sphingomyelin vesicles in a total volume of 1 ml 10 mM Tris buffer containing 100 mM NaCl and 1 mM EDTA, pH 7.4, and monitoring the fusion/aggregation activity of the vesicles at A450 as described (23). PLA2 activity of eluates was assayed, by adding 100 µl aliquots of each fraction to 4 ml of assay buffer containing 2 mM dimyristoylphosphatidylcholine, 20 mM CaCl2, 10 mM Triton X-100 ml, pH 8.0. The amount of NaOH required to titrate fatty acid released from dimyristoylphosphatidylcholine vesicles, to maintain pH at 8.0, was monitored using Autotitrator (Radiometer, Copenhagen).
Molecular ModelingThe co-ordinates of x-ray structures of CTX A5 were taken from Brookhaven Protein Data Bank and used as the initial structures for molecular modeling. Structure of heparin was obtained from the recently determined structure of hep-bFGF complex (15). Model building, docking and energy minimization were performed on IBM RS6000 375 and Silicon Graphics Indigo II workstation using QUANTA/CHARMM Version 4.0 software. Polar hydrogen atoms were added to the x-ray structures by standard procedure in QUANTA. The initial structures of toxin and heparin were relaxed by energy-minimization to remove initial nonbonded contacts.
Docking of the molecules was performed by using the Modeling Module of
QUANTA/CHARMM. Heparin was docked on the concave side of the CTX A5
molecules as indicated by the aggregation property of CTXs (see
results). Comparison of the binding strengths of CTXs determined at
various ionic strengths suggests that two basic residues, Arg/Lys-28
and Lys-33, are involved in binding. Further optimization of
electrostatic interaction allows CTX to be docked in an orientation
perpendicular to the three antiparallel -sheet strand. The docked
complex structures were energy-minimized and then subjected to
molecular dynamics (MD) simulation. MD simulation was performed with
distance-dependent dielectric constant. A time step of 1 fs
was used. The nonbonded cutoff value was set at 15 Å, and the
nonbonded pair list was updated every 20 time steps. Typical simulation
time for each complex was ~40 ps. The final structures generated by
MD simulations were energy-minimized and used for analysis. We also
carried out preliminary reference MD simulations with explicit all-atom
solvent molecules to ensure that our simulation was consistent with
that performed in solution.
CTXs exhibit a sharp
positive band near 195 nm and weak negative minimum near 215 nm,
characteristic of -sheet, in their CD spectrum. We have shown that
titration of CTX A3 with heparin, monitored by change in ellipticity at
195 nm, and ~215 nm, yields a two-phase curve, initial decrease in
ellipticity ~195 nm followed by recovery of the same ellipticity at
higher GAG concentrations (10). We have proposed that there exist two
non-equivalent binding states and that the second binding state may be
a manifestation of the aggregation of CTX molecules that accompanies
saturated binding. To understand the details of binding, we performed
similar titration of 10 CTXs.
All CTXs studied were found to bind to heparin; similar two-phase
profile was obtained. Fig. 1,
A-C, show, respectively, the representative
spectra (top panels) and the plot of ellipticity versus heparin concentration (bottom panels) of
CTXs A5, M4, and M1 (see Ref. 24 for details of classification). It can
be seen that up to ~2 µM heparin, ellipticity ~195 nm
decreases steadily as a function of heparin, reaches minimum, and
increases again for higher heparin concentrations. Ellipticity ~215
nm exhibited similar change but with opposite sign (data not shown).
Interestingly, in the presence of excess heparin, ellipticity of CTX A5
rises slightly higher than that for free form, whereas ellipticity of CTX M1 remains considerably lower than for free protein and is intermediate for CTX M4. Also, the effective concentration (amount of
heparin required to produce maximum perturbation) varies from 2 to 2.8 µM for different toxins. The results of titration of 10 CTXs studied against HMW heparin are summarized in Fig.
2, A and B, and Table
I.
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For the sake of convenience of description we define ellipticity of
free toxin as , that at maximum perturbation as
2,
and that in the presence of excess heparin as
1 (Table
I). In the change profile at ~195 nm, then, the point corresponding
to maximum perturbation is
2. The difference between
1 and
, referred to here as
1,
denotes contribution to the magnitude of change of the first binding
state, because this stronger binding state should predominate for
excess heparin. The difference,
2
1, represents contribution of second binding state,
assuming that the two sites are mutually independent.
As can be seen from Table I, CTX M1 and T undergo maximum
conformational change upon first binding (high
1)
while CTXs M2, M4, A3, and A5 undergo change upon second binding (high
2 -
1). Effective concentration of
heparin required to produce maximum change in CTXs
(
2) also varies for different CTXs. That for CTX M4,
A3, A2, A1, and M3 falls in the range of 1.8-2.0 µM, and
that for CTXs M1, T
, M2, A5, and A4 varies from 2.4 to 3.0 µM. We have suggested that significant conformational
change occurs due mainly to the aggregation of CTX-heparin complex
around saturation; this indicates that the latter group prefers to
remain in the aggregated state even when more heparin is added. It is
interesting to point out that the presence of basic residue appears to
correlate with the suggested aggregation state of CTX-heparin complex.
With the exception of CTX A4, the only CTX containing Arg at the
N-terminal, all CTXs containing either Arg-6 or Lys-6 dissociate at
lower effective heparin concentration of 1.8-2.0 µM
(Tables 1 and 2). Additional evidence to support this interpretation
can be obtained from turbidity data presented later.
HMW Heparin contains ~10 pentasaccharides and thus many CTX molecules may bind to heparin at maximum perturbation. This forced indulgence or molecular crowding may introduce constraints in the protein. To delineate the contribution of molecular crowding, we performed binding study using depolymerized heparin (average Mr 3000, LMW heparin) which would be free of crowding. The composition of this heparin is heterogeneous (27); it contains saccharides of varying lengths while some sequences present in HMW heparin may be altogether absent in this lower homologue. This property of LMW heparin, however, allows identification of several trends that remain hidden with the higher homologue.
Shown in Fig. 3A are representative CD
spectra of CTX A3 alone and in presence of LMW heparin. The profile of
change in ellipticity versus concentration of LMW heparin is
plotted in Fig. 3B. Like for HMW heparin, ellipticity ~195
nm decreased to minimum at 20 µM heparin and resumed that
of free protein at 80 µM heparin. Similar titration with
trisulfated heparin disaccharide and hyaluronan, the GAG lacking
sulfate, revealed that no change occurred in CTX A3 (Fig.
3B; see also Ref. 10). This indicates that a sulfated stretch longer than disaccharide is required for binding.
The reversibility of binding and regaining of ellipticity of CTX A3 at higher concentrations of LMW heparin are similar to that for HMW heparin. The effective concentration of LMW derivative, 20 µM, however, is ~10-fold higher than for HMW heparin. But the difference in their molecular weights is ~5-fold. The higher effective concentration for LMW heparin can be explained by the absence in this sulfated oligosaccharide of some sequences required for binding. We therefore performed similar titration in the reverse order to gain insight into binding.
LMW heparin (20 µM, constant, effective concentration) was titrated against CTX A3. Fig. 3C shows profile of ellipticity of CTX A3 in absence (open symbols) and presence (closed symbols) of the heparin versus concentration of CTX A3. It can be seen that upon addition of CTX A3 to heparin, the ellipticity rises steadily, and the rise is comparable to that for free protein (compare with control), which indicates that this binding causes no perturbation. Since heparin is present in excess here, this binding can be considered to be strong. After addition of 12 µM of CTX A3; however, further addition of toxin does not cause an increase in ellipticity, which instead remains constant until 20 µM. This indicates that maximum perturbation occurs around here since the added CTXs do not contribute to CD signal as expected. Optical density measurement suggests that the turbidity increases abruptly from 0.02 to 1.0 from 12 to 20 µM protein (data not shown). Beyond 20 µM, the ellipticity again rises sharply and the rise is comparable to that of free CTX A3 (compare with control), but the turbidity remains at around 1.0. This indicates that binding is saturated after 20 µM and that most conformational change occurs due to aggregation of CTX-heparin complex. In conclusion, CTX A3 binds similarly to LMW and HMW heparin. The effective binding sites available in LMW heparin are, however, less than those of HMW heparin even after normalizing with their respective molecular weights.
We then performed titration of 10 CTXs with LMW heparin and monitored
change in ellipticity. The results are summarized in Fig. 2,
C and D to allow comparison with similar
experiments done with HMW heparin (Fig. 2, A and
B). For convenience of our discussion, the results obtained
for HMW heparin are classified according to the difference in their
effective charge, whereas those obtained for LMW heparin are classified
according to the differences in their degree of conformational change.
It can be seen that some CTXs undergo small structural perturbation
(Fig. 2C), while others undergo large change in ellipticity
(Fig. 2D) upon binding to LMW heparin. A similar result can
also be observed for HMW heparin, but the interpretation is complicated
because of the combined effect of molecular crowding and aggregation on
the detected conformational change. The magnitude of change,
i.e. the values of 2, are listed in Table
I. All S-type CTXs (CTX containing Ser-29) exhibit smaller conformational change, and CTX M2 and all P-type CTXs (CTX containing Pro-31) exhibit larger conformational change (higher
2
1 or
1).
Comparison of their amino acid sequences indicates that Arg-28 and/or
Lys-33 may enhance heparin-induced conformational change of the
CTXs.
Comparison of 2 values of CTXs with LMW and HMW
heparin reveals that LMW heparin, on the same CTX, exerts smaller
effect. This implies that molecular crowding arising out of the
superstructure of the higher homologue contributes to structural
perturbation of CTXs. Alternatively, the smaller effect of LMW heparin
may be a manifestation of the absence in this polysaccharide of
numerous sequences present in the HMW homologue. In summary, both
molecular crowding and specific side chain interaction in the
CTX-heparin complex play a role in heparin-induced CTX conformational
changes.
Interestingly, binding of CTXs with LMW heparin was accompanied by
change in turbidity of the solution. As shown in Fig. 4, the profile of turbidity change can be classified into two groups: one
with slow decline (panel A) and the other with rapid decline (panel B) of turbidity, with increasing heparin
concentration. This result strengthens our previous suggestion that
CTXs containing Arg-6 or Lys-6 tend to dissociate easily when heparin
is in excess. As reported previously (16), CTXs can be considered as
slightly concave molecules with most of the basic residues lying on the concave side. Arg-6 or Lys-6, however, are located on the convex side
of the surface and thus the absence of any ionic interaction helps
dissociate the CTX/heparin complex at saturation. In other words, as
this basic residue is exposed to the backside of CTX/heparin complex it
is not involved in binding.
Binding Strength of CTXs with Heparin
Change in conformation
and aggregation state of CTX-heparin complex help understand the mode
of interaction, but do not reflect the binding strength. We therefore
performed heparin-affinity chromatography and fluorescence spectroscopy
binding measurements on the interaction of various CTXs with heparin to
determine the relative binding strengths of the CTXs. The CTXs from
N. atra bind to heparin with similar strength (Fig.
5A), whereas significant variation in the
binding strength is found for CTXs from N. mossambica and
N. nigricollis (Fig. 5, B and C).
Interestingly, several PLA2 from N. mossambica and N. nigricollis bind to heparin stronger than most CTXs as indicated
by the salt elution profiles.
To determine the dissociation constant of CTX-heparin complex,
intrinsic fluorescence intensity of Tyr or Trp residues of CTX was
measured as a function of heparin-CTX ratio. Fig.
6A shows representative fluorescence (F/F)
profiles for CTX M4, M3, A2 and T
to indicate the variation in
binding strength. Fitting of the data by non-linear least squares
method based on Equation 1, presented in Material and Method section,
allows determination of Kd and n. These values are
listed in Table II, along with amino acid sequences of
CTXs. Also shown is the salt concentration required to elute CTX from
heparin affinity matrix, and the net positive charge content of
CTXs.
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The determined Kd values (Table II) of the toxins, and hence their binding strengths, vary by 4 orders of magnitude. The presence of basic residues near the tip of loop 2 appears to be most crucial in enhancing the binding strength. This observation confirms our previous conclusion based on the conformational change of CTXs that Lys-33 and/or Arg-28 are indeed involved in CTX-heparin interaction. It should be pointed out that although the relative binding strength determined by salt elution is in general consistent with that determined by spectroscopic method, distinct differences are clearly present, as for CTX M3 and M4. Conclusions of the binding strength of heparin binding proteins thus should not rely solely on the concentration of NaCl required to elute them from affinity matrices.
To demonstrate clearly that the different binding strengths reflect the
involvement of basic residues, salt-dependence measurements of
fluorescence intensity were performed on several selected CTXs. As can
be seen in Figs. 6B and 7, binding of CTX with heparin exhibits significant salt-dependence. Surprisingly not all binding behavior of CTX can be understood in terms of
macromolecule-polyelectrolyte theory, which holds good for most heparin
binding proteins as suggested in earlier studies (28, 29).
Specifically, a nonlinear dependence of log Kd with
log [Na+] can be seen for T, a CTX that exhibits weak
binding under physiological conditions (Fig.
7A).
A reasonable, linear salt-dependence of the binding is, however,
detectable for CTX M2 (Fig. 7, A and B). T and
CTX M2 differ by only three amino acid residues at positions 29, 31 and
33 (Table II). Substitution of Ala-29, Pro-31, and Met-33 in T
by
Gly-29, Ser-31, and Lys-33 in CTX M2 increases the binding strength by three orders of magnitude in physiological buffer, although their binding strengths are similar in low salt buffer. The exact implication of this observation is not clear at present, but the results indicate clearly that the presence of Lys-33 is crucial for the specific binding
of CTX with heparin.
As indicated in Table II, the binding strength of CTX is enhanced by one order of magnitude if another charged residue is present at position 28 (compare CTX M4 with A3). Salt dependence study further substantiates the suggestion that Arg-28 (or Lys-28, for CTX A5) is indeed involved in binding since the number of purely ionic interactions between the two species, Z, is higher for CTX M4. A similar conclusion can also be derived by comparing the salt-dependence curves of CTX A3 and M2 (Fig. 7B). It should be pointed out that salt dependence curves of dissociation constant determined for CTX M2 and M4 are not perfectly linear (Fig. 7B). Therefore, quantitative comparison based on macromolecule-polyelectrolyte theory may lead to substantially large values of Z than those predicted based on the charge difference (29). However, Arg-28 must also be involved in the binding of Lys-33 containing CTXs.
CTX M3 binds to heparin with exceptionally high strength. There is, however, no Lys-33 in the molecule. Instead, two Lys residues are located consecutively, at positions 31 and 32. Interestingly, the amino acid sequence of CTX M3 from 29 to 38 positions follows the so-called consensus sequence, xBBxxxBBx, of high affinity binding region of aFGF from amino acid position 126 to 135 (13, 30). We therefore conclude that the charge distribution near the tip of loop 2 region is important and is responsible for specific interaction between CTX and heparin.
We found that CTX M3 also binds to heparan sulfate (HS), the ubiquitous GAG on cell surface, with a dissociation constant of 6 µM (Fig. 6C). We have shown that HS binds to CTX A3 with Kd value of 16 µM in physiological buffer (10). The binding of CTX M3 to HS is thus about 3 times stronger than that of CTX A3, the difference between their heparin binding strength, on the other hand, is two orders of magnitude. Therefore, the strong association of CTX M3 with heparin can only be related to higher sulfation of heparin relative to HS.
Computer Modeling of the Molecular Interaction Between CTXs and HeparinCTXs contain 9 discontinuous, basic residues (highlighted in Table II) capable of serving as binding sites for heparin. The three-dimensional structure of CTXs reveals that the protein exhibits significant polarization with the conserved basic residues, at positions 2, 24, 38, 46, 52 and 60, residing on the concave face of the three-finger disk-like structure, and the acidic residues opposite (16, 17). Since our experimental results suggest the involvement of Lys/Arg-28 and Lys-33 in the binding and they are known to be located on the same concave surface of the conserved basic residues, we performed computer modeling by docking the CTX molecules on the concave side of the molecule and carried out MD simulation to obtain detailed information of the binding at molecular level. The same conclusion as that based on the aggregation property of CTX/heparin complex was reached.
Shown in Fig. 8A are the stereo plots of the
final energy-minimized structures of CTX A5-heparin complex generated
by MD simulation. In addition to the two designated Lys at positions 28 and 33, heparin can also interact with CTX A5 via the side chains of
Lys-24, Arg-38, Lys-46, Lys-52, and Lys-60 which are distributed as a positively charged band on the concave side of the molecules. Interestingly, these positively charged amino acid residues are conserved in all CTXs sequence available and are located at the same
position as revealed by their three-dimensional structures. Electrostatic interaction, in the binding of CTX-heparin may be reinforced by this positively charged band structure. Possible electrostatic interactions between CTXs and heparin are found to
involve OSO3
group of the second, third, fourth,
and sixth saccharides,
NSO3
group of the second and
sixth saccharides, and the
CO2
group of the third
and fifth saccharide. That only five saccharides participate in binding
suggests that sulfated pentasaccharide may constitute an average
binding unit as proposed by us recently (10).
We have herein studied binding of 10 CTXs with LMW and HMW heparin
by spectroscopic and affinity chromatographic techniques. Correlation
of the binding data with amino acid sequence of CTXs points to the
importance of the distribution of charged residues at the tip of loop 2 of CTX. Despite the high homology these naturally occurring variants,
basic toxins, display in their primary and three-dimensional
structures, significant differences in their affinities for acidic
heparin are found. Mere presence of conserved basic residues does not
warrant high affinity heparin binding under physiological condition,
although strong binding occurs under low salt condition, as observed
for T. High affinity heparin binding is only observed for CTXs
containing specific basic residues near the tip of loop 2; however, the
conserved, discontinuous, basic residues Lys-24, Arg-38, Lys-46,
Lys-52, and Lys-60 are also involved in the interaction between CTXs
and heparin due to electrostatic attraction.
The toxin contains two distinct cationic clusters: one primary
recognition binding region comprising the conserved residues, which
allows heparin to subsequently interact with another cationic cluster
located at the tip of loop 2. This conclusion is consistent with recent
study on the binding of heparin with other proteins. For instance,
primary recognition site for heparin was concluded to lie at positions
122-137 of aFGF, although most other basic residues from other
clusters are banded like an equator around the spherical FGF (30).
Recently, the requirement of Lys and Arg residues outside the proposed
pentasaccharide binding region for high affinity heparin binding is
demonstrated for human antithrombin III (14, 31). High affinity binding
structural motif of heparin can thus be considered to consist of two
regions as illustrated in the schematic diagram, shown in Fig.
8B, based on the structure of -sheet CTX.
Interestingly, of the total 9 conserved basic residues in CTXs, Lys-2
has previously been shown to stabilize the -sheet structure (19),
while Lys-12, Lys-18, and Lys-37 bind to phospholipids (17). Thus, all
the 9 conserved basic residues in CTXs appear to have structural or
functional roles, other than to simply increase solubility of this
amphiphilic polypeptide. In this respect, CTX appears to be efficiently
designed for interaction with membrane components. The small dimension
of CTX molecules (35 × 24 × 15 Å) (16) also indicates that it is
probably one of the smallest polypeptides with well defined
three-dimensional structure that interacts with sulfated
oligosaccharide of heparin.
The conserved basic residues of CTXs implicated in heparin binding
follow the order -B-X2n-1-B-, where
X is any residue and B is basic residue. Thus, discontinuous
basic residues separated by odd number of any residue appears to
constitute a suitable heparin binding motif in the -sheet proteins
studied. Although such discontinuous distribution is in contrast to
clustering of basic residues in other heparin-binding protein with
-helix (13), it is not surprising in view of the alternating
distribution of amino acid side chain in
-sheet polypeptides.
Therefore, suitable structural binding motif of heparin can be
identifiable, as illustrated in the presently proposed model of
CTX-heparin interaction, for heparin binding protein with known
three-dimensional structure.
A similar spatial distribution of conserved basic residues is also found in the structurally related three finger venom neurotoxins (NTXs) that bind avidly to acetylcholine receptors (Table III). Sequence alignment of 10 NTXs (also from cobra venom) reveals that all the five basic residues are localized on the same face of NTX molecule and may thus be accessible if guided by the specific recognition site of NTX, -W-X-D-H- (32), which are known to be near the tip of loop 2 (shown in Table III). This distribution of basic residues in NTXs suggests that they may use the binding mode similar to CTX to bind to acetylcholine receptor. It has recently been demonstrated that modification of Arg residues of loops 2 and 3 and C-terminal of NTX from King cobra caused a 90-92% decrease in lethality and abolished acetylcholine receptor binding. These results indicate that in addition to the established recognition site of loop 2, other cationic residues of different regions are also important in receptor binding and lethality (33, 34). Amino acid sequences in NTX show distinct distribution of charged residues from position 50 to 60 (Table III). Three different classes of NTXs, K-type, E-type and G-type, containing Lys-54, Glu-54 and Gly-54, respectively, emerge upon sequence alignment. It is suggested that substitution of acidic or neutral residues at this position may further modulate the specific interaction of NTX with its receptor.
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The fact that CTXs and several PLA2 exhibit significantly different binding strength to heparin (Fig. 5) contains important biological implications in that such binding to heparin-like oligosaccharides may serve to concentrate proteins with different properties at the site of action. It is known that PLA2 and CTXs act synergistically on many cell systems (21, 22). Binding of human PLA2 type II to proteoglycans causes differential effect on its enzymatic activity as demonstrated recently (8), an indication that heparin modulates the toxicity of these polypeptides. Since both toxins have different targets and since all the known targets of these toxins lie at the membrane, future study of the interplay between CTX and PLA2 near the membrane surface may provide information on GAG-guided protein action on membranes.
W. Wu thanks Professors C. Ho and J. Prestegard for initial discussion on interaction of CTXs with carbohydrates.