Optical activity and conformation of cobra neurotoxin

Correcting the circular dichroism spectra of peptides for contributions of absorbing side chains

The aromatic and sulfur-containing side chains Trp, Tyr, Phe, Cys, and Met contribute to the CD spectra of peptides and proteins in the amide region, interfering with the analysis for secondary structure. We propose a method to correct the CD spectra of peptides undergoing the helix-coil transition for contributions due to absorbing side chains using singular value decomposition. The method uses the common basis vectors obtained from an analysis of the CD spectra of related peptides without the aromatic and sulfur-containing amino acids. The common basis vectors are fitted to a portion of the CD spectrum of the peptide being corrected, in the range that is unaffected by its sidechain contributions. Then the resulting coefficients from the fitting are used along with the common basis vectors to regenerate the entire corrected spectrum. The method is illustrated for the CD spectra of the peptide sequence acetyl-Y-VAXAK-VAXAK-VAXAK-amide, where X is substituted with the 20 naturally occurring amino acids. This peptide model adopts a random-coil conformation in 2 mm sodium phosphate buffer, pH 5.5, and becomes an alpha helix in methanol/buffer solutions. The difference between the original and corrected spectra shows the contribution from the aromatic and sulfur-containing side chains.

Amino acid residues within the sequence region alpha 55-74 of Torpedo nicotinic acetylcholine receptor interacting with antibodies to the main immunogenic region and with snake alpha-neurotoxins

The sequence region 55-74 of the alpha-subunit of the acetylcholine receptor (AChR) from Torpedo californica electroplax comprises the amino-terminal end of a sequence segment--residues alpha 67-76--forming the main immunogenic region (MIR), which is most frequently recognized by anti-AChR autoantibodies in myasthenia gravis. The synthetic sequence alpha 55-74 of Torpedo AChR binds alpha-bungarotoxin (alpha BTX), suggesting that amino acid residues within this sequence region may contribute to formation of an alpha BTX binding site. Using single-residue substituted synthetic analogues of the sequence alpha 55-74 of Torpedo AChR, in which each residue was sequentially substituted by either glycine or alanine, we sought identification of the amino acids involved in interaction with alpha-neurotoxins and with three different anti-MIR monoclonal antibodies (mAbs 6, 22, and 198). Substitution of Arg55, Arg57, Trp60, Arg64, Leu65, Arg66, Trp67, or Asn68 strongly inhibited alpha-toxin binding, whereas substitutions of Ile61, Val63, Pro69, Ala70, Asp71, or Tyr72 had marginal effects. Substitutions within the region alpha 68-72 significantly diminished binding of anti-MIR mAbs, although residue preferences differed among mAbs. Further, substituting Trp60 substantially reduced binding of mAb 198, and moderately affected binding of mAb 6, and substitution of Asp62 slightly but consistently affected binding of mAbs 6 and 22.

The role of an invariant tryptophan residue in alpha-bungarotoxin and cobrotoxin. Investigation of active derivatives with the invariant tryptophan replaced by kynurenine

Ozone oxidation converted the single, invariant, tryptophan residue to N2-formylkynurenine in alpha-bungarotoxin and cobrotoxin. Upon this modification, the lethal toxicity was significantly reduced in cobrotoxin but mostly retained in alpha-bungarotoxin. Each neurotoxin containing kynurenine instead of tryptophan retained the same antigenicity as the native toxin. Fluorescence and CD spectroscopy revealed that, although the environment and state of the kynurenine residue were similar, [Kyn29]cobrotoxin was much more sensitive to pH change than alpha-[Kyn28]bungarotoxin. In terms of lethal toxicity and conformational stability, the invariant tryptophan residue appears to play a more important role in cobrotoxin, imparting a higher lethal toxicity than that in alpha-bungarotoxin, which has a disulfide bond at Cys29-Cys33.

Unfolding processes of small globular proteins: the two-state vs multi-state model

1. The alcohol-induced unfolding of two homologous proteins, neurotoxin and cardiotoxin from Taiwan cobra (Naja naja atra) venom has been analysed. 2. It is postulated that the unfolding process for both proteins is a multi-state conformational transition. 3. It has been hypothesized that between the compact native state of the protein and its fully unfolded state there exists a quasi-continuous spectrum of conformational metastates of protein species. 4. The population distribution of these metastates is partially dependent on the nature of unfolding factors as well as the amino acid composition and sequence. 5. The sum of all transient conformational states and the protein species being in the folded and unfolded states respectively, can be detected by means of circular dichroism spectroscopy since the absorption of circularly polarized light is rapid relative to the rate of fluctuations of the protein structure.

Conformational properties of the neurotoxins and cytotoxins isolated from Elapid snake venoms

The review will critically assess the information available on the conformation of homologous neurotoxins and cytotoxins isolated from Elapid snakes. Particular attention will be given to the dynamics of the molecules in solution because there is the possibility that defined intramolecular rearrangements are involved at the sites of action. Such properties will be then reconciled with the known X-ray crystallographic and sequence data in order to derive likely structure-activity relationships.

Role of aromatic residues in the structure-function relationship of alpha-bungarotoxin

The conformation of alpha-bungarotoxin and its cyanogen bromide cleaved and nitrated derivatives was studied by circular dichroism (CD). Native toxin contains no helices but some beta forms and possibly beta turns. Its ordered conformation is little affected when the peptide bond between Met-27 and Trp-28 is cleaved; however, the CD due to Trp-28 is abolished. The CNBr-cleaved derivative retains its immunoaffinity toward anti-toxin sera but loses its neurotoxicity toward the acetylcholine receptor. On the basis of both CD and fluorescence spectra, Trp-28 is probably stabilized by a short-range interaction with the carboxylate group of Asp-30. The ordered conformation of the toxin is also unaltered when one of the two tyrosine residues, identified as Tyr-54, is nitrated with tetranitromethane. This Tyr(NO2)-54 derivative possesses both immunoaffinity and neurotoxicity. However, the toxin is denatured and loses its activities when the other tyrosine residue, Tyr-24, is also nitrated in 6 M guanidine hydrochloride, even after the denaturant is removed. Spectrophotometric titration of the toxin indicates that Tyr-54 has a normal apparent dissociation constant (pKa = 9.7) and Tyr-24 ionizes at pH above 11.2. Both tyrosine residues are in a polar environment, but Tyr-24 is not readily accessible to reagents and is stabilized by long-range interactions, probably involving Glu-41.

Reversed unfolding-refolding process of cobra neurotoxin

Circular dichroism and nuclear magnetic resonance spectroscopies have been used to study the unfolding process of cobrotoxin upon addition of fluoro alcohols/or sodium dodecyl sulfate to its aqueous solution. In each final unfolded state, the protein had its disulfide bonds intact. The unfolding process has been found to be reversible in the case of fluoro alcohol/water mixtures, while no such reversibility was found in the case of sodium dodecyl sulfate. However, when hexafluoro-2-propanol is added to the sodium dodecyl sulfate unfolded protein, refolding is induced. The mechanism of unfolding is discussed in terms of the different interactions which govern the protein conformation in solution.

Fatty acid synthetase complex from the insect Ceratitis capitata. Structural studies

The involvement of lipids in the structure and the activity of the fatty acid synthetase from the insect Ceratitis capitata has been previously established. Lipid-protein interactions were examined by circular dichroism. A thermal transition for both the structure and the activity of the enzyme complex takes place at about 50 degrees C; as the temperature is raised alpha-helix content decreases considerably and, concomitantly, the enzyme undergoes a marked inactivation. After 180 min at 37 degrees C, the secondary structure of the enzyme complex is 20% alpha-helix, 33% beta structure and 47% of not ordered structure against 43%, 26% and 31% as respective percentages for the native form of the complex. Lipolytic digestion of the complex was carried out with either lipase or phospholipase A2 or a mixture of both enzymes. Any of the lipolytic treatments induces a decrease of [theta]220 and the simultaneous digestion with lipase plus phospholipase during 90 min account for a limit structure with 8% of alpha-helix. The secondary structure of the complex after treatment with proteolytic enzymes, trypsin or chymotrypsin, had 15% alpha-helix, 20% beta structure and 57% of not ordered structure. The preservation of the alpha-helix content indicates that lipids protect certain of the bonds cleavable in the absence of lipids. The structural organization of the complex was studied through sequences of lipolytic and proteolytic treatments; final organization was dependent on the initial lipolytic digestion in agreement with the peptide bond shielding by the lipid component. Nitration of the complex with tetranitromethane modified almost completely all tyrosine residues of the polypeptide chains.

Refolding of reduced short neurotoxins: circular dichroism analysis

The four disulfide bonds of nine homologous short curare-like polypeptides are cleaved by reduced dithiothreitol. Air oxidation renaturations of the reduced compounds are followed by far-ultraviolet circular dichroism analysis, and the kinetics of refolding thus determined are compared. They indicate that three toxins refold 4--10 times more slowly than the six others. It is shown that a significant difference between the refolding kinetics still subsists when renaturations are made in the presence of various concentrations of thiol-disulfide exchange reagents or at various pH values. From an examination of the toxin sequences, it is proposed that a single additional amino acid insertion is responsible for the difference in the observed kinetics. This proposal is supported by temperature studies of renaturation kinetics.

Circular dichroism spectra of cleavage fragments of soybean trypsin-chymotrypsin inhibitor

Circular dichroism spectra of biologically active fragments of Bowman-Birk soybean inhibitor have been determined in acidic, neutral, and alkaline conditions. Neither fragment showed evidence of alpha-helix or beta-structure. Negative dichroism above 260nm has been assigned in both fragments primarily to disulfide bonds, with a minor contribution from tyrosine in a hydrophilic environment. The individual spectra of these fragments, and their sum between 230 and 340 nm have been compared with the spectra of the intact inhibitor and several structurally related proteins. Possible interactions which may give rise to CD bands in this region are discussed.

Conformation of colipase. Prediction of the secondary structure, circular dichroism and 360 MHz proton NMR studies of porcine colipase A

The secondary structure of porcine colipase (93 residues) was established according to the predictive method of Chou and Fasman (Chou, P.Y. and Fasman, G.D. (1974) Biochemistry 13, 211--222 and 222--245). The relative composition of the conformational regions was as follows: 5% alpha-helix (region 39--44), 25% beta-sheet (three regions, 7--11, 49--57 and 77--85) and eight beta-turns corresponding to 32% of the polypeptide. Colipase contains a large proportion (about 35%) of unordered structure. Estimated values for the alpha-helix and beta-sheet contents from the circular dichroism spectrum were in good accordance with the predicted model. A less satisfactory value was found for the beta-turns. A characteristic feature of the far ultraviolet dichroic spectrum is the presence of an unusual positive band at 225 nm that might be indicative of a particular spatial arrangement of the chromophores in the molecule. Two tyrosines (Tyr56 and Tyr57) and one histidine (His86) are at close vicinity in the three dimensional structure of the protein as shown by proton NMR studies. These residues are located at the end of two beta-sheet hydrophobic regions(49--57 and 77--85) which might play a role in the association of colipase with the lipid-water interface as indicated by results of the NMR studies of the taurodeoxycholate-colipase complex.

Reverse turns in peptides and proteins

The evidence that reverse turns frequently occur as structural components of proteins, as well as of linear and cyclic peptides, is overwhelming. This review summarizes and examines critically the experimental evidence derived from physical methods such as 1H and 13C nuclear magnetic resonance spectroscopy, spin-lattice relaxation time, circular dichroism, IR spectroscopy, and X-ray crystallography. Secondly, theoretical evidence obtained from energy calculations, which rely on empirical energy functions, and correlative methods, which rely on algorithms based on the frequency of occurrence of amino acids, is evaluated. In particular, those theoretical studies for which complementary physical studies have been completed are emphasized. Finally, examples of structure-function relationships involving reverse turns and their biological recognition are demonstrated.

Secondary structure prediction of anterior pituitary hormones. Lack of correlation between predicted values and circular dichroism data

The secondary structures of human somatotropin, human choriomammotropin, ovine and porcine prolactin, human, ovine and porcine beta-lipotropin, human and ovine lutropin, human thyrotropin, human corticotropin, alpha-melanotropin and human beta-melanotropin have been predicted by the method of Chou & Fasman. Predicted contents of alpha-helix and beta-sheet do not correspond well with values estimated from circular dichroism spectra.

Conformational studies of neurotoxin II from Naja naja oxiana. Selective N-acylation, circular dichroism and nuclear-magnetic-resonance study of acylation products

After treatment of neurotoxin II, a component part of the venom of the Middle Asian cobra Naja naja oxiana, with acetoxysuccinimide all five possible epsilon-acetylated-lysyl derivatives were obtained and the position of the label was established. Trifluoroacetylation of both the derivatives and the parent toxin yielded, respectively, the five acetyl-penta(trifluoroacetyl)-neurotoxins II and the hexa(trifluoroacetyl)-neurotoxin II, which were studied by circular dichroism (CD), 1H and 19F nuclear magnetic resonance (NMR) spectroscopy. The availability of this series of compounds made possible assignment of all six fluorine signals (from the N-terminal and the five epsilon-amino groups) in the hexa(trifluoroacetyl)-neurotoxin II NMR spectra and disclosure of the proximity of the Lys-26 and Lys-46 trifluoroacetyl groups. The pH dependence of the 19F NMR signals was determined and the pK values of the groups affecting the signal chemical shifts were calculated by a computer iterative program. In order to ascertain the relative accessibility of the lysyl side chains, the change in halfwidths of the hexatrifluoroacetylated neurotoxin II 19F signals, with addition of varying amounts of an iminoxyl spin probe, was determined. The data obtained are compared with the X-ray data on sea snake neurotoxins and the significance of the side chain interactions observed in solution is discussed.

Laser Raman studies on cobrotoxin

Laser Raman spectra of cobrotoxin under various conditions have been obtained. Comparison of the spectra of native cobrotoxin in lyophilized form and in aqueous solution indicates that the secondary structures of cobrotoxin are not significantly affected by the removal of the aqueous solvent. On going from the native to the partially reduced and the completely reduced, carboxy-methylated forms, characteristic peaks of the C-S-S-C and tyrosine ring in the region of 500--900 cm-1 showed definite changes in structure. The partially reduced form gave two peaks at 502 and 524 cm-1, suggesting difference in the conformation of the remaining disulfide bonds. As indicated by the present work, the conformation of the main chain of cobrotoxin in the native unperturbed state, in the partially reduced and in the completely reduced forms are the coexistence of beta-pleated sheet with random-coil structure, predominantly random coil, and predominantly random coil with the existence of an alpha-helix type structure, respectively. The effect of pH on the conformation of cobrotoxin in solution appeared to give rise to the change of the local structure of two aromatic residues common to all snake neurotoxins.

The 1H nuclear-magnetic-resonance spectra of Neurotoxin I and cardiotoxin Vii4 from Naja mossambica mossambica

Two toxins from the venom of Naja mossambica mossambica, neurotoxin I and cardiotoxin VII4, were investigated in aqueous solution by high-resolution 1H nuclear magnetic resonance (NMR) techniques at 360 MHz. The spectral characterization of the proteins included determination of the number of slowly exchanging amide protons which can be observed in 2H2O solution, measurement of the amide proton chemical shifts and exchange rates, characterization of the aromatic spin systems and the internal mobilities of aromatic rings, and studies of the pH dependence of the NMR spectra. For numerous resonances of labile and non-labile protons quite outstanding pH titration shifts were observed. It is suggested that these NMR parameters provide a useful basis for comparative structural studies of different proteins in the large group of homologous snake toxins. As a first application the NMR data presently available in the literature on neurotoxin II from Naja naja oxiana, toxin alpha from Naja nigricollis and erabutoxin a and b from Laticauda semifasciata have been used to compare these three proteins with neurotoxin I from Naja mossambica mossambica. This preliminary comparative study provides evidence that the same type of spatial structure prevails for these four homologous neurotoxins and that the folding of the backbone corresponds quite closely to that observed in the crystal structure of erabutoxin b. A second application is the comparison of cardiotoxin VII4 from Naja mossambica mossambica with the neurotoxins. The experimental data indicate that the folding of the polypeptide backbone is closely similar, but that the cardiotoxin molecule is markedly more flexible than the neurotoxins.

Circular dichroic analysis of protein conformation: inclusion of the beta-turns

The mean residue ellipticity, [theta], at any wavelength, lambda, of a protein in aqueous solution is expressed as [theta]lambda = fH[theta]H infinity(1-k/n) + f beta[theta]beta + ft[theta]t + fR[theta]R with two constraints: 1 > or = fj > or = 0 and sigma fj = 1. The subscripts H, beta, t, and R refer to the helix, beta-form, beta-turn, and unordered form. The fractions, fj's, of 15 proteins are based on X-ray crystallography, ft refers to the net beta-turn after cancelling those residues having dihedral angles of opposite sign. The [theta]H infinity of an infinite helix and its chain-length dependence factor, k, were computed from the myoglobin data (Chen et al., 1974, Biochemistry 13, 3350). The average number of residues per helical segment, n, for 15 proteins was about 10, which can be used for proteins of unknown structure. The reference spectra of other three structural elements are computed by a least-squares method. Once the reference spectra are chosen, the same equation above can be used to estimate the fractions of the secondary structure of a protein from its CD data points between 190 and 240 nm at 1-nm intervals. The computed helical content is usually good to excellent (concanavalin A is a notable exception). Inclusion of the beta-turn in the analysis improves the correlation for the estimates of the beta-form, but the computed beta t values are not significantly correlated with the X-ray results. Matrix formulation proves the equivalence of the least-squares method and the integral curve-fitting.

Circular dichroism spectra of isolated soybean and chickpea trypsin-chymotrypsin inhibitors

Circular dichroism spectra of trypsin-chymotrypsin inhibitors from soybeans and chickpeas have been determined in acidic, neutral and highly alkaline solutions. Neither protein contains alpha-helix although a small amount of beta-structure cannot be excluded. Negative dichroism above 250 nm has been assigned largely to disulfide bonds in both molecules with neither showing evidence for tyrosine residues buried in hydrophobic regions. The spectra of these inhibitors between 230 and 250 nm have been compared with the spectra of a number of structurally related proteins suggesting that previous interpretations of this region may have been incomplete or incorrect.

Conformation of snake toxic polypeptides studied by a method of prediction and circular dichroism

Short and long neurotoxins as well as cardiotoxins belong to three distinct families of homologous toxic polypeptides extracted from cobra venoms. A study of their conformation was undertaken by using the method of Chou and Fasman for prediction of secondary structures of proteins. To improve the reliability of this method, an averaging scheme was developed. The data obtained showed that all toxins have a predominant trend for beta-sheet nucleation. Moreover, predicted beta-sheet strands fitted well those actually observed from X-ray data. Thus, it seems that all toxins share similarities in their secondary structure. This proposition was supported by a comparative study of the CD spectra of a set of toxins. Nevertheless, the present data suggest also that each type of toxins possesses localized structural individualities which might be responsible for the biological and/or immunological specificities.

Conformation of erabutoxins a and b in aqueous solution as studied by nuclear magnetic resonance and circular dichroism

The 270-MHZ nuclear magnetic resonance (NMR) spectra of erabutoxins a and b have been observed in 2H2O solution. By the use of convolution difference and double resonance techniques, proton signals in the aromatic and methyl regions have been assigned. From the pH dependence of NMR chemical shifts, the pKa value of His-26 of erabutoxin b is found to be 5.8, whereas His-7 of erabutoxins a and b is not protonated at pH above 3. The imidazole ring of His-7 is protonated upon the denaturation at pH 2.85. The acid denaturation process has been followed by the His-26 and methyl proton signals and is found to be reversible but is slow as compared with NMR chemical shift time scale. The circular dichroism (CD) of erabutoxin b has also been observed. The denaturation is found to involve a major change from the beta-rich conformation to a disordered one. The NMR and CD changes upon acid denaturation are satisfactorily explained by the two-state process. The deuterium exchange rates of the C-2 protons of His-26 and His-7 of erabutoxin b indicate that His-26 is exposed to the solvent whereas His-7 is tightly buried in the interior of the protein globule. The pKa value of Tyr-25 is as high as about 12.0, possibly due to the hydrogen bond formation between the hydroxyl group of Tyr-25 and a carboxylate group. The hydroxyl group of Tyr-25 is reversibly titrated so that this group is not buried tightly in the interior of the protein globule. The line width of the aromatic proton signals of Tyr-25 is significantly broad at room temperature, suggesting a restricted rotation of the aromatic ring. The aromatic proton signals of Trp-29 are fairly sharp; this aromatic ring is exposed and mobile. Except for His-7, the micro-environments of Tyr-25, His-26, and Trp-29 residues and methyl proton signals of valine and isoleucine are consistent with the locations of alpha carbon atoms as elucidated by X-ray crystal analyses.

The status of tyrosyl residues in a Formosan cobra cardiotoxin

Spectrophotometric titration of Formosan cobra cardiotoxin showed that two of the three tyrosyl residues were titrated freely with a normal apparent pKa of 9.6 whereas the remaining one ionized at pH above 11.0. Nitration of cardiotoxin in Tris . HCl buffer with tetranitromethane resulted in the selective nitration of tyrosine 11 and tyrosine 22. It also revealed that tyrosine 51 was the abnormal one in the spectrophotometric titration. Complete nitration occurred in the presence of 6.0 M guanidine hydrochloride. Compared with the conformation of native cardiotoxin, the peptide conformation of the partially nitrated cardiotoxin did not change significantly but the conformation of the completely nitrated cardiotoxin changed remarkably. The biological activity of cardiotoxin was indeed affected by nitration, but the immunological activity was nearly intact even when all the tyrosine residues were nitrated.

The oxidation of methionine and its effect of the properties of cardiotoxin VII1 from Naja melanoleuca venom

Methionine residues 24 and 26 of cardiotoxin VII1 from Naja melanoleuca were oxidised to sulphoxides using N-chlorosuccinimide at pH 8.5. The number of equivalents of oxidant required for complete oxidation suggested that the methionine side-chains existed in a relatively "exposed" conformational state in cardiotoxin. The oxidised cardiotoxin was devoid of lethality. It was also non-haemolytic, both on its own and in the presence of phospholipase A2. However, it was still able to precipitate with anti-cardiotoxin antibody. CD studies indicated that the polypeptide backbone conformation was intact in the oxidised cardiotoxin but some perturbation of tyrosine residues was evident. The possibility of a direct or indirect involvement of the methionine residues in the biological activity of the cardiotoxin is discussed.

Properties of radiolabeled alpha-bungarotoxin derivatives and their interaction with nicotinic acetylcholine receptors

Column-purified monoiodinated, diiodinated, and tritiated derivatives of alpha-bungarotoxin (alpha-Bgt) are distinguished on the basis of their ultraviolet absorption and circular dichroism (CD) spectra. The pattern of changes in CD spectra on incorporation of iodine into a single tyrosine residue of alpha-Bgt and the widespread wavelength distribution of these effects are interpreted as reflecting primary chemical modification of the tyrosine chromophore as well as vicinal and global secondary structural changes. Native and tritiated alpha-Bgt are shown to be more effective than iodinated alpha-Bgt derivatives in competing for specific toxin binding sites on putative nicotinic acetylcholine receptors (nAChR) derived from rat brain reflecting functional perturbation of the modified toxin. In contrast, both membrane-bound and solubilized nAChR from Torpedo californica electroplax display little or no specific binding preference for native toxin, nor are there significant differences in lethal potency of alpha-Bgt derivatives toward mice. These results suggest that peripheral and putative central nAChR may differ in their alpha-Bgt binding properties and suggest the usefulness of modified toxin in detecting those subtle differences.

Location of disulfide bridges in cardiotoxin from the venom of Formosan cobra (NAJA NAJA ATRA)

The location of the four disulfide bridges in cardiotoxin from the venom of the Formosan cobra, Naja naja atra, was determined by isolating cystine-containing peptides from the enzymic and partial acid hydrolyzates of the intact toxin. The results indicate that the disulfide bridges are formed by half cystine residues 3--21, 14--38, 42--53, and 54--59. The amino acid sequence at the position 48--49 was revised to -Leu-Val- instead of Val-Leu- as reported previously.

Conformational stability of a snake cardiotoxin

A snake cardiotoxin from the venom of the Formosan cobra, Naja naja atra, is a basic polypeptide. The protein can be denatured in 6.0 M guanidine hydrochloride or at elevated temperatures. Its conformation remains virtually the same in solvents of lower polarity than water such as 1, 2-ethanediol or 1-propanol and 1, 2-ethanediol (1:1 v/v). The circular dichroism spectrum is atypical in water in that the peptide chromophores show a small negative CD band at 214 nm and a large positive one at 195 nm. To some extent the CD pattern resembles that of the beta-form but differs in specific positions and magnitudes. Considering that the theoretical CD of the reverse beta-bend and the characteristics of model polypeptides in beta-form manifest a similar pattern, we suggest that cobra cardiotoxin is rich in beta structure including beta pleated-sheets and beta reverse-turns.