Determination of the secondary structures of proteins by circular dichroism and optical rotatory dispersion

Physicochemical studies of dinitrophenylated bovine serum albumin

The structural changes of bovine serum albumin (BSA) as a function of dinitrophenylation have been studied by disc gel electrophoresis, sedimentation velocity analyses, and circular dichroism. These experiments were designed to understand the molecular bases for the change in immunogenicity and antigenicity of BSA upon dinitrophenylation. Dinitrophenylated BSA tends to aggregate to dimers and higher aggregates. A concomitant large change in electrophoretic mobility was also observed. Circular dichroism studies reveal a large decrease in the alpha-helical structure of the BSA molecule.

Studies on the disulfide region of alpha 1-protease inhibitor

The single disulfide bond of purified human alpha 1-protease inhibitor was reduced with dithiothreitol in the absence of denaturant and the resultant sulfhydryl groups were alkylated with iodoacetamide-1-C14. The product was found to be fully functional as an inhibitor of trypsin and elastase in esterolytic and proteolytic assays. The modified protein was also found to be nearly identical to native alpha 1-protease inhibitor when analyzed by immunological, electrophoretic, and spectral methods. The performic acid oxidized inhibitor, on the other hand, was devoid of any enzyme inhibitory activity. Analysis of the derivatized protein by amino acid analysis and by radioactive counting revealed only a single cysteine-containing peptide. The alkylated inhibitor was digested with cyanogen bromide and then trypsin, and subjected to two-dimensional peptide mapping. A single cysteine-containing peptide was recovered and shown to have the sequence Phe-Asn-Ile-Gln-His-Cys-Lys. A variety of experiments involving gel filtration or dialysis of reduced or oxidized alpha 1-protease inhibitor indicate that this Cys-peptide is covalently bound to either free cysteine or to glutathione via a disulfide bridge.

Estimation of the secondary structure of protein A from S. aureus by CD-spectroscopy

The secondary structure of protein A(SpA) was estimated to 31% alpha-helix, 13% beta-structure and 56% random coil by CD-spectroscopy, which is close to the sum of the secondary structures of the SpA fragments. This supports that the SpA molecule is composed N-terminally of four Fc-binding units each consisting of two antiparallel alpha-helices interconnected by random coil segments and C-terminally of an region mainly in random coil.

Structural features of human leukocyte interferon A as determined by circular dichroism spectroscopy

The solution structure of human leukocyte (alpha) interferon-A (LeIF-A) purified from E. coli extracts was investigated by circular dichroism spectroscopy. At pH 8.2, the native molecule exhibits 40-70% alpha-helix and no clearly detectable beta-structure. The tertiary structure includes a closely-packed interior containing at least one tryptophan side-chain. Titration to pH 1.5 produces a partial loss of structural features which are rapidly regained on return to neutral pH.

Conformational study of purified epoxide hydrolase from rat liver

Hepatic epoxide hydrolase (EC 3.3.2.3) was purified from phenobarbital-treated rats by ion-exchange chromatography followed by hydrophobic chromatography. The enzyme had a specific activity of 300--400 nmol min-1 mg-1 protein with benzo[a]pyrene-4,5-oxide as the substrate. Circular dichroism (CD) spectra of the purified enzyme gave two negative bands, centered at 210 nm and 222 nm, respectively. The mean residue ellipticity at 222 nm was 12,9000 deg X cm(2) X dmol(-1), which indicated the presence of about 35% alpha-helical structures. Sodium dodecyl sulfate (SDS) greatly affected the shape of the CD spectra, which were gradually shifted to the blue. This suggested a decrease in the aggregation state of the protein. Electrostatic interactions were important in the organization of the enzyme structure since the conformation was stable between pH 7.4 and pH 10. At pH-values 5.0, 6.0 and 12.0, the CD bands underwent considerable changes in both amplitude and shape. Moreover there was a good correlation between the optimal pH range of the epoxide hydrolase activity and the organization state of the protein. After membrane reconstitution with liposomes, the conformation of the enzyme was not significantly modified by the presence of dimyristoyl L-alpha-phosphatidylcholine or other phospholipids. This constancy was obtained over a wide range of molar ratios of phospholipids to protein (0--500). However, phospholipids did increase the thermal stability of the enzyme. Fluorescence measurements of diphenylhexatriene (DPH) bound to dimyristoyl L-alpha-phosphatidylcholine indicated that addition of epoxide hydrolase modified the thermal transition of the lipid phase. On the other hand, electron paramagnetic resonance (EPR) signals of the nitroxide-labelled fatty acid, 2-(14-carboxy-tetradecyl)-2-ethyl-4,4-dimethyl-3,3,-oxazolidiny-oxyl, bound to the phospholipid, indicated that the presence of the protein decreased by about 53% the correlation time of the label, suggesting that its motion had increased. In conclusion, phospholipid-epoxide hydrolase interactions enhanced the fluidity of dimyristoyl L-alpha-phosphatidylcholine liposomes without changing the secondary structure of the enzyme. Electrostatic interactions also played an important role in the conformational stability of the protein.

Circular dichroic and fluoropolarimetric studies on tryptophyl residues in acid-induced isomerization of bovine plasma albumin

The acid-induced isomerization (the N-F transition) and expansion of bovine plasma albumin were studied by measuring circular dichroic spectra and fluorescence polarization of tryptophyl residues. Decreases in the magnitude of ellipticities at 208, 222, 262 and 268 nm were observed in the N-F transition and acid-expansion. However, increases in the magnitude of ellipticities at 295-300 nm observed in the initial part of the N-F transition exactly correlated with the increase of rotational relaxation time of tryptophyl side chains obtained by fluorescence polarization measurement.

Raman spectral evidence for a mu-oxo bridge in the binuclear iron center of ribonucleotide reductase

The Raman spectrum of the B2 subunit of Escherichia coli ribonucleotide reductase shows a peak at 496 cm-1 that appears to be in resonance with the 370-nm electronic transition of the binuclear iron center in both the native and radical-free forms of the protein. Exposure of the protein to H218O causes the peak to shift to 481 cm-1, indicating that the vibrational mode is due to an Fe-O moiety in which the oxygen can exchange with solvent. The rate of oxygen exchange (kobsd = 8.3 x 10-4 s-1) is consistent with a mu-oxo-bridged structure. Protonation of the oxygen is unlikely since the Fe-O vibration fails to shift to lower frequency in D2O. Instead, there is a gradual increase in the vibrational frequency with time to a maximum value of 502 cm-1 after 3 h in 70% with time to a maximum value of 602 cm-1 after 3 h in 70% D2O. Apparently, the deuteration of successive protein functional groups causes a slight alteration in the structure of the binuclear iron center. The resonance Raman characteristics of the Fe-O-Fe group in protein B2 are similar to those previously reported for the mu-oxo-bridged binuclear iron center in hemerythrin. A further similarity between the two proteins is the high degree of alpha-helical content. Circular dichroism measurements place this value at approximately 60% for the B2 subunit of ribonucleotide reductase.

Subunit interactions in pig heart fumarase--II. Study of tetramer-dimer equilibrium in function of enzyme concentration and temperature

The dissociation of pig fumarase tetramers into two dimers was studied as a function of temperature in the absence of denaturating agents. 1. At high temperatures a kinetical and structural study of dissociation and reassociation was performed. 2. At low temperatures fumarase dissociation was induced by limiting the enzyme concentration.

Conformational transitions in serum high density apoproteins of hypercholesterolemic monkeys

Conformational transitions in serum high density apolipoproteins of normal and hypercholesterolemic monkeys have been studied by circular dichroism. This study has revealed that under hypercholesterolemic conditions the secondary structure of apolipoproteins suffers permanent changes which could be observed even after a lengthy procedure of isolation, purification and delipidation of high density lipoprotein.

Subunit interactions in pig heart fumarase--I. Study of tetramer-dimer equilibrium in dilute urea solutions

1. Information about the interaction forces at the contact regions between fumarase subunits is obtained from kinetical and structural studies of dissociation and reassociation. 2. In dilute urea solutions an equilibrium between tetramers and dimers is established. 3. No appreciable unfolding of the individual subunits is observed in these conditions.

Conformational differences between high clotting human alpha-thrombin and nonclotting gamma-thrombin

The conformations of human alpha-thrombin and gamma-thrombin have been compared by circular dichroism, solvent perturbation different spectroscopy, and chemical modification. Circular dichroism studies indicate that proteolytic conversion of alpha-thrombin to gamma-thrombin is accompanied by considerable conformational changes which include a decrease in alpha-helical content from 5-7% to 0-1%. Solvent perturbation at pH 6.0 obtained with 20% ethylene glycol, 20% glycerol, and 20% dimethyl sulfoxide indicates an apparent exposure of 3.5 +2- 0.2 tryptophan and 7.8 +/- 0.1 tyrosine residues in alpha-thrombin and 4.6 +/- 0.2 tryptophan and 9.2 +/0 0.3 tyrosine residues in gamma-thrombin. This increased exposure is substantiated by the greater reactivity of tryptophan residues in gamma-thrombin toward dimethyl (2-hydroxy-5-nitrobenzyl) sulfonium bromide. It suggests that gamma-thrombin is a less compact molecule than the parent alpha-thrombin. Solvent perturbation studies of alpha-thrombin and gamma=thrombin inhibited by phenyl-methanesulfonyl fluoride showed that 0.3 +/- tryptophan and 0.9 +/- 0.3 tyrosine residues in alpha-thrombin and 0.6 +/- 0.3 tryptophan and 1.3 +/- 0.4 tyrosine residues in gamma-thrombin were blocked by the inhibitor. These subtle differences in the extent of blocking of tyrosine and tryptophan suggest a tighter conformation in the catalytic site of gamma-thrombin compared to that of alpha-thrombin.

Study of conformational states and reversibility of histone complexes

The core histone complex (H3:H4:H2A:H2B)2 and products of dissociation, the H2A:H2B: dimer and the H3:H4 tetramer, were isolated from chicken erythrocyte chromatin by several literature methods as well as gel filtration on Bio-Gel A15m at various salt concentrations. The conformational and oligomeric characteristic of these histone complexes were compared to analogous histone complexes prepared by renaturation of individually acid-extracted histones by circular dichroism (CD) and analytical gel filtration chromatography. The salt-extracted core histone complex (independent of method of preparation), the purified dissociation products, the H2A:H2B dimer, and the H3:H4 tetramer in 2 M NaCl, 10 mM sodium phosphate, 0.25 mM EDTA and 0.1 mM DTT, pH 7.0, have conformation which are identical, by the criteria of similar CD spectra, with complexes prepared from acid-extracted histones, Likewise, the salt-extracted complexes may be cycled through solvents of low ionic strength (10 mM sodium phosphate, pH 7.0 or 50 mM NaOAc, pH 5.0) or 1 mM HCl and returned to 2.0 M NaCl, 10 mM sodium phosphate, 0.25 mM EDTA, and 0.1 mM DTT, pH 7.0, in a completely reversible manner. Thus it would appear that acid-denatured histones are capable of being fully renatured to yield native-like complexes.

Circular dichroism studies on the interaction of tryptophan synthase with pyridoxal 5'-phosphate

The interaction between the beta 2 subunit of tryptophan synthase and the coenzyme pyridoxal 5'-phosphate (PLP) is characterized by induced circular dichroism (CD) in the near-UV (260-285 nm) and in the visible region (320-480 nm, extrinsic Cotton effect). Because of its high mean residue ellipticity ([theta] = 56 deg cm2 dmol-1 for the isolated holo-beta 2 subunit and 102 deg cm2 dmol-1 for the alpha 2-holo-beta 2 complex, respectively) the latter has been used to define different conformational states of the beta 2 dimer via CD titrations. Fitting the obtained binding parameters to the known data from equilibrium dialysis leads to the result that the low-affinity state of the isolated beta 2 subunit shows a 3 times greater rotational strength than the holoenzyme in the high-affinity state. The generation of the final CD amplitude is characterized by a rate constant intermediate between the values for the formation of the internal aldimine and for the regain of enzymatic activity. Interaction of the alpha 2-apo-beta 2 bienzyme complex with the cofactor leads to a hyperbolic binding curve which is apparently free of contributions caused by unspecific PLP binding outside the active center. The determined dissociation constant of 9 x 10(-7) M is in good agreement with the value of 1 x 10(-6) M as obtained by equilibrium dialysis. Binding kinetics reveal a very slow process with a rate constant of 2.6 x 10(-4) s-1, significantly smaller than that for the regain of catalytic activity during reconstitution of the enzyme.

Interactions of human serum albumin with some alkylureas

The interactions of human albumin with urea, methyl-, N,N'-dimethyl-, ethyl-, N,N'-diethyl-, propyl-, and butylurea were studied by means of calorimetry and circular dichroism. It has been found that the enthalpies of interaction of the alkylureas with human serum albumin are distinctly different from those of urea. Thus, the transfer of the protein from water to aqueous urea solutions is accompanied by release of heat, i.e., the overall reaction is exothermic, whereas the transfer of the same protein of solutions of alkylureas is characterized by consumption of heat, i.e., the overall reaction is endothermic. By examining the far ultraviolet circular dichroism in behavior reflects the presence of the hydrophobic moiety in the urea molecule.

A circular dichroism study of undegraded human ceruloplasmin

The CD spectrum of human ceruloplasmin (Cp) has been studied between pH 6.90 and 12.00 in the far-ultraviolet, near-ultraviolet, and visible light regions. The spectrum in the far-ultraviolet region showed that undegraded holo and apo single-chain ceruloplasmin and a cleaved ceruloplasmin preparation have a low content of alpha helix but a high content of beta and unordered structure. A conformational transition accompanied by a decrease in beta and an increase in unordered structure occurred at pH 11.10 for intact ceruloplasmin. This transition probably involved the ionization of buried tyrosines, as shown by the increase of a near-ultraviolet band at 250 nm. The copper atoms may contribute to the stability of the native structure since the conformational transition occurred at a low pH value (10.50) in the case of apoceruloplasmin. The apo-Cp also presented a more intense CD band at 292 nm, suggesting the presence of tryptophan(s) near the environment of copper(s) in the molecule where no tyrosine residue seems to be involved. The spectrum between 320 and 700 nm of intact and cleaved Cp was resolved into six Gaussian bands which were assigned to type-1 copper atoms. Important changes in only two of these bands upon pH increase (bands III at 541 nm and VII at 322 nm) confirmed the nonequivalence of the two blue coppers in human ceruloplasmin.

Physico-chemical properties of pregnant mare serum gonadotropin

Pregnant mare serum gonadotropin exhibits a dissociation at acid pH as shown by the drop of s20,w values from 3.52 S at pH 8.1 to 2.52 S at pH 2.0. The dissociation is accompanied by an absorbance change with a maximum at 287 nm and a parallel loss of both follicle-stimulating hormone (FSH) and luteinizing hormone (LH) activities as followed by radioreceptor assays. The apparent pKa of the acid transition is 3.45 with an extremely slow and temperature-dependent rate at pH 2.0 (1.8 . 10(-4) s-1 at 37 degrees C). By gel filtration the molecular weight of the active hormone is estimated to be 45 000 (rather than the previously reported 53 000-64 000). The active conformation of the hormone includes beta sheet structure (34%) as for other gonadotropin hormones with a minor but significative amount of alpha-helix. Four tyrosine residues were titrated, two of pKa = 10.3 and two of pKa = 11 out of a total of seven tyrosines. The parallel changes in FSH and LH activities during the preparation and the acid transition suggest that the two biological activities are intrinsic properties of the same molecular entity.

Subunit structure and dissociation of Callinectes sapidus hemocyanin

The hemocyanin of the blue crab, Callinectes sapidus has two major components sedimenting with approximate sedimentation coefficients of 17 S and 25 S. Molecular weight data based on light scattering and sedimentation equilibrium measurements at pH 7.8 suggest that the two components have molecular weights of approximately 450 000 and 900 000 in the presence of stabilizing Ca2+. In the absence of Ca2+, the molecular weights are found to be about 5% lower, suggesting some dissociation of the hemocyanin components. Circular dichroism and optical rotatory dispersion measurements in the far-ultraviolet region gave nearly identical spectra for the two components. Based on the reference parameters of Chen et al. (Chen, Y.H., Yang, J.T. and Martinez, H.M. (1972) Biochemistry 11, 4120--4131 and Chen, Y.H., Yang, J.T. and Chan, K.H. (1974) Biochemistry 13, 3340--3359), estimates of 16--20% alpha-helix, 40--60% beta-structure, and 30--40% random organization were obtained for the two hemocyanin components. Exposure to 6 M Gdn HCl gave light scattering molecular weights of approx. 68 000 and 77 000, which is close to one-sixth of the molecular weight of the 17 S component. These results support the view that the two components of C. sapidus hemocyanin share the hexameric and dodecameric organization common to arthropod hemocyanins. The salts of the Hofmeister series and the ureas are found to dissociate the dodecameric component with the former exhibiting the usual order of effectiveness of NaCl, NaBr, NaI, and NaClO4 dissociation, while the ureas show an inverse order of decreasing effectiveness in going from urea to methyl-, ethyl- and propylurea. This suggests that polar and ionic interactions are relatively more important than hydrophobic interactions for the stabilization of the dodecameric form of C. sapidus hemocyanin. The dissociation behavior of the 17 S hexameric species by GdnHCl in the 0--1.5 M concentration region (where essentially no denaturation occurs), based on light scattering molecular-weight measurements, is satisfactorily accounted for by equations describing the dissociation of hexamers to monomers.

Muscle beta-actinin and serum albumin of the chicken are indistinguishable by physicochemical and immunological criteria

Chicken muscle beta-actinin is considered to be one of the "true" myofibrillar components due to its specific binding to isolated myofibrils. Surprisingly, the direct comparison of this muscle protein with serum albumin, both isolated from chicken, showed that they behaved identically under several electrophoretic conditions. Furthermore, immunoreplica gels and double-immunodiffusion tests with antibodies prepared against beta-actinin established the serological identity of both proteins. No significant differences were found by circular dichroic spectroscopy or in amino acid composition. In addition, the amino-terminal sequences of both proteins were identical (H2N-Asp-Ala-Glu-His-Lys-Ser-Glu-Ile-Ala-His-Arg-Tyr-Asn-Asp-Leu-). Combined, these results strongly indicate that muscle beta-actinin and serum albumin are similar, if not identical.

Conformation of pardaxin, the toxin of the flatfish Pardachirus marmoratus

Pardaxin is the toxic component isolated from the secretion of the Red Sea flatfish Pardachirus marmoratus. Pardaxin has attracted considerable attention recently because of its use as shark repellent. Conformational information regarding the peptide backbone, disulfide bonds, and tyrosine chromophores was obtained using Raman and circular dichroism (CD) spectroscopy. Analysis of the Raman spectra indicates that the toxin consists of 39% alpha-helix, 23% beta-structure, and 39% random coil. The occurrence of a band at 510--512 cm-1 indicates the conformational geometry of the disulfide C-C-S-S-C-C linkages to be gauche-gauche-gauche. From the lack of a peak in the 2500--2700 cm-1 region we concluded that all half-cystines are involved in disulfide linkages. The far-ultraviolet CD spectrum of pardaxin showed positive 196 and negative 208 and 222 nm bands. The best fit secondary structure based on the CD ellipticities was found to be: 23% alpha-helix, 21% beta-structure, and 56% random coil. Under highly acidic and alkaline conditions the far-ultraviolet pardaxin spectra showed extensive, reversible loss of its helical structure. Due to certain biological characteristics of pardaxin the possible influence of salt on pardaxin's conformation was examined. No evidence of conformation perturbation was detected in either the CD or Raman spectra of pardaxin.

The binding of oxidised coenzyme to bovine-liver glutamate dehydrogenase studied by circular-difference spectroscopy

1. The binding of NAD+ to glutamate dehydrogenase may be followed quantitatively by titration, using high-sensitivity circular dichroism (CD) difference spectroscopy. 2. The CD of the bound coenzyme in the binary complex E . NAD closely resembles that of bound ADP, although the affinity is much lower, being 350-fold less for NAD+ at 20 degrees C in 0.1 M phosphate, pH 7. 3. A family of CD spectra may be analysed by unconstrained linear regression assuming only three components: free enzyme, free coenzyme, and a single binary complex, E . NAD. 4. Taking the molar CD of bound ADP as representing the molar CD of the adenine chromophore of bound NAD+, the linear regression shows the formation of a simple 1 : 1 complex E . NAD with Kd = 0.72 mM in a simple binding process without positive or negative cooperativity. 5. NADP+ binding is more than 10-fold weaker than NAD+ binding. 6. From the similarity of the CD of bound ADP and bound NAD+ it is probable that NAD+, in forming a simple binary complex, binds preferentially at the regulatory (adenine nucleotide) binding site (site II). 7. Direct evidence has been obtained for the binding of a second molecule of NAD+ to the ternary complex E . NAD . glutarate. This process occurs with low affinity and is probably also located at the adenine regulatory site. 8. This second-site binding of NAD+ may contribute to the phenomena of non-Michaelis-Menten kinetics and apparent negative homotropic interactions in the binding of NAD+, previously attributed to subunit-subunit cooperative interactions.

Reconstitution of the hepatic asialoglycoprotein receptor with phospholipid vesicles

A solubilized detergent-free preparation of the hepatic binding protein specific for asialoglycoproteins associates spontaneously with small unilamellar lipid vesicles. This process is independent of the phase transition of the lipid and effectively restores the specific binding activity of the receptor protein. The insensitivity of the resulting lipid-protein complex to ionic strength provides evidence for a hydrophobic interaction. There is a perturbation of the lipid phase transition concomitant with addition of the protein. Circular dichroism studies indicate that the protein undergoes a conformational change on association with lipid. Binding of specific ligand produces further physical changes in the receptor as indicated by alterations in the tryptophan fluorescence quenching pattern.

Physical-chemical properties of ubiquitin

The secondary structure of ubiquitin, the environment of its single tyrosine residue and its potential for interacting noncovalently with histone 2A or DNA, have been probed by circular dichroism (CD), ultraviolet absorbance, fluorescence and ancillary techniques. The results indicate that ubiquitin has a stable secondary structure containing only a low percentage of alpha-helix or beta-sheet. The ubiquitin tyrosine has an elevated pKa arising from the influence of a spatially proximate carboxylate which also causes a marked quenching of the tyrosine fluorescence at neutral pH; the influence of this carboxylate is lost when the protein is unfolded in 7 M guanidine. No evidence has been obtained for the presence of allosteric noncovalent interactions between free ubiquitin and either histone 2A or purified unfractionated DNA. The results suggest that one function of ubiquitin (or of the ubiquitin segment of protein A24) may be to interact with a chromatin component other than histone 2A or DNA, and/or that ubiquitin functions within 2A as a steric blocking group of a region of the nucleosome.

Solution studies on heme proteins. Circular dichroism and optical rotation of Glycera dibranchiata hemoglobins

Circular dichroism (CD) and optical rotatory dispersion (ORD) spectra of several liganded derivatives of the monomer and polymer hemoglobin components of the marine annelid, Glycera dibranchiata were measured over the wavelength range 650--195 nm. The differences observed between the monomer and polymer components for the heme dichroic bands in the visible, Soret and ultraviolet wavelength regions seem to result from changes in the heme environment, geometry and coordination state of the central heme iron in these proteins. Within the Soret region, the liganded derivatives of the monomer hemoglobin exhibit predominantly negative circular dichroic bands. The heme band at 260 nm is also absent for the monomer hemoglobin. The ORD and CD spectra in the far-ultraviolet, peptide absorbing region suggest also differences in the alpha-helix content of the monomer and polymer hemoglobins. The values for the single-chain G. dibranchiata hemoglobin are in the expected range (about 70% alpha-helix) as predicted by the X-ray structure of this protein. The lower estimates of the alpha-helix content for the polymer hemoglobin (approx. 50%), may reflect the differences in amino acid composition, primary structure and polypeptide chain foldings. Changes in oxidation state and ligand binding appears to have no pronounced effect on the helicity of either the monomer or polymer hemoglobins. The removal of the heme moiety from the monomer hemoglobin did result in a major decrease in its helix content similar to the loss of heme from myoglobin.

Purification and physicochemical characterization of a new rat plasma proteinase inhibitor, alpha 1-inhibitor III

A three-stage procedure was used to isolate an additional proteinase inhibitor in rat plasma tentatively called alpha 1-inhibitor III. A 20% yield was obtained after two successive gel filtrations on Ultrogel Ac-A. 33-4 followed by an ion-exchange chromatography on DEAE-cellulose. This method was chosen since it permits further study of the enzyme binding properties of the isolated molecule. The purified material was first controlled to retain an inhibiting capacity towards serine proteinases using bovine chymotrypsin. The isolated molecule has an apparent molecular weight of 215 000, a pI of 4.65, an E1%1cm, 280 nm of 7.50 and a sedimentation coefficient of 8.6 S. It contains approx. 15% carbohydrates and is made up of a single peptidic chain. Study of the periodic structure by circular dichroism has demonstrated a low alpha-helix content (4--5%) whereas the beta-sheet conformation accounts for approx. 30% of the peptidic moiety. Tryptophan residues have been shown to be mainly responsible for the molecular fluorescence most of them being non-accessible to the solvent since only 25% of the tryptophanyl fluorescence was quenched in presence of I.

Further characterization of the lipid-depleted bovine rhodopsin obtained by cholate-ammonium sulfate fractionation

The rhodopsin preparation obtained by the method of ammonium sulfate fractionation contained 3-6 mol phospholipid and about 18 mol cholate per mol rhodopsin. The purified rhodopsin had 74% helical structure and showed a visible CD spectrum different from that of rhodopsin in the membrane. The rhodopsin was stable below but denatured gradually above 20 degrees C. The lifetime of metarhodopsin I was long in this preparation. Regeneration capacity was low and only 30% of the original rhodopsin was regenerable by addition of 11-cis-retinal after bleaching. 50 mol of phosphatidylcholine were maximally bound to 1 mol rhodopsin when the purified rhodopsin was mixed with phosphatidylcholine in 0.5% cholate. The rhodopsin recombined with lipid has properties similar to those of the original rhodopsin in the membrane. Exchange of cholate for other detergents was easily performed by dialysis. The rhodopsin preparation in which cholate was exchanged for digitonin gave almost the same CD, thermal stability and regenerability as those of native rhodopsin in the membrane but metarhodopsin I still retained its long lifetime.

Circular dichroism comparative studies of two bacterial collagenases and thermolysin

The recently isolated and purified collagenase produced by Achromobacter iophagus, the collagenase from Clostridium histolyticum, and thermolysin, three enzymes having common properties, were studied by circular dichroism. From the spectra of the aqueous solutions obtained in the peptide region, the fraction of alpha helix, beta sheet and aperiodic segments in the three proteins could be estimated. Good similarity was found between Achromobacter collagenase and thermolysin, which both contain a high fraction of alpha helix. Side-chain contributions were analyzed in the aromatic region of thespectra: effects of pH and of organic solvents were observed, showing the strong influence of surroundings on the stabilization of the proteins.