[87] Fluorescence measurements

Mapping human calreticulin regions important for structural stability

Calreticulin (CALR) is a highly conserved multifunctional chaperone protein primarily present in the endoplasmic reticulum, where it regulates Ca²⁺ homeostasis. Recently, CALR has gained special interest for its diverse functions outside the endoplasmic reticulum, including the cell surface and extracellular space. Although high-resolution structures of CALR exist, it has not yet been established how different regions and individual amino acid residues contribute to structural stability of the protein. In the present study, we have identified key residues determining the structural stability of CALR. We used a Saccharomyces cerevisiae expression system to express and purify 50 human CALR mutants, which were analysed for several parameters including secretion titer, melting temperature (T_m), stability and oligomeric state. Our results revealed the importance of a previously identified small patch of conserved surface residues, amino acids 166-187 ("cluster 2") for structural stability of the human CALR protein. Two residues, Tyr172 and Asp187, were critical for maintaining the native structure of the protein. Mutant D187A revealed a severe drop in secretion titer, it was thermally unstable, prone to degradation, and oligomer formation. Tyr172 was critical for thermal stability of CALR and interacted with the third free Cys163 residue. This illustrates an unusual thermal stability of CALR dominated by Asp187, Tyr172 and Cys163, which may interact as part of a conserved structural unit. Besides structural clusters, we found a correlation of some measured parameter values in groups of CALR mutants that cause myeloproliferative neoplasms (MPN) and in mutants that may be associated with sudden unexpected death (SUD).

On the origin and correction for inner filter effects in fluorescence. Part II: secondary inner filter effect -the proper use of front-face configuration for highly absorbing and scattering samples

Fluorescence is an established technology for studying molecular processes and molecular interactions. More recently fluorescence became a leading method for detection, sensing, medical diagnostics, biotechnology, imaging, DNA analysis, and gene expression. Consequently, precise and accurate measurements in various conditions have become more critical for proper result interpretations. Previously, in Part 1, we discussed inner filter effect type I, which is a consequence of the instrumental geometrical sensitivity factor and absorption of the excitation. In this part, we analyze inner filter effect type II and discuss the practical consequences for fluorescence measurements in samples of high optical density (absorbance/scattering). We consider both the standard square and front-face experimental configurations, discuss experimental approaches to limit/mitigate the effect and discuss methods for correcting and interpreting experimental results.

On the origin and correction for inner filter effects in fluorescence Part I: primary inner filter effect-the proper approach for sample absorbance correction

Fluorescence technologies have been the preferred method for detection, analytical sensing, medical diagnostics, biotechnology, imaging, and gene expression for many years. Fluorescence becomes essential for studying molecular processes with high specificity and sensitivity through a variety of biological processes. A significant problem for practical fluorescence applications is the apparent non-linearity of the fluorescence intensity resulting from inner-filter effects, sample scattering, and absorption of intrinsic components of biological samples. Sample absorption can lead to the primary inner filter effect (Type I inner filter effect) and is the first factor that should be considered. This is a relatively simple factor to be controlled in any fluorescence experiment. However, many previous approaches have given only approximate experimental methods for correcting the deviation from expected results. In this part we are discussing the origin of the primary inner filter effect and presenting a universal approach for correcting the fluorescence intensity signal in the full absorption range. Importantly, we present direct experimental results of how the correction works. One considers problems emerging from varying absorption across its absorption spectrum for all fluorophores. We use Rhodamine 800 and demonstrate how to properly correct the excitation spectra in a broad wavelength range. Second is the effect of an inert absorber that attenuates the intensity of the excitation beam as it travels through the cuvette, which leads to a significant deviation of observed results. As an example, we are presenting fluorescence quenching of a tryptophan analog, NATA, by acrylamide and we show how properly corrected results compare to the initial erroneous results. The procedure is generic and applies to many other applications like quantum yield determination, tissue/blood absorption, or acceptor absorption in FRET experiments.

Tryptophan Fluorescence Changes Related to Ca(2+)-ATPase Function

Fluorescence measurements as monitored with either extrinsic or intrinsic probes constitute important ways with which to study the molecular properties of macromolecules. With high-quality spectrofluorimeters, it is, e.g., possible kinetically to follow local conformational changes, induced by ligands and inhibitors, with a sensitivity that is unsurpassed by any other physicochemical technique. We demonstrate here with Ca(2+) and two specific inhibitors of SERCA how this can be done by measurements of the intrinsic fluorescence of the tryptophan residues of SERCA.

Effect of curcumin on the amyloid fibrillogenesis of hen egg-white lysozyme

At least twenty human proteins can fold abnormally to form pathological deposits that are associated with several degenerative diseases. Despite extensive investigation on amyloid fibrillogenesis, its detailed molecular mechanisms remain unknown. This study is aimed at exploring the inhibitory activity of curcumin against the fibrillation of hen lysozyme. We found that the formation of amyloid fibrils at pH 2.0 in vitro was inhibited by curcumin in a dose-dependent manner. Moreover, quenching analysis confirmed the existence of an interaction between curcumin and lysozyme, and Van't Hoff analysis indicated that the curcumin-lysozyme interaction is predominantly governed by Van Der Waals force or hydrogen bonding. Curcumin was also found to acquire disaggregating ability on preformed lysozyme fibrils. Finally, we observed that curcumin pre-incubated at 25 degrees C for at least 7 days inhibited lysozyme fibrillogenesis better than untreated curcumin and the enhanced inhibition against HEWL fibrillation might be attributed to the presence of dimeric species.

Purification and characterization of a wound-inducible thaumatin-like protein from the latex of Carica papaya

A 22.137 kDa protein constituent of fresh latex was isolated both from the latex of regularly damaged papaya trees and from a commercially available papain preparation. The protein was purified up to apparent homogeneity and was shown to be absent in the latex of papaya trees that had never been previously mechanically injured. This suggests that the protein belongs to pathogenesis-related protein family, as expected for several other protein constituents of papaya latex. The protein was identified as a thaumatin-like protein (class 5 of the pathogenesis-related proteins) on the basis of its partial amino acid sequence. By sequence analysis of the Carica genome, three different forms of thaumatin-like protein were identified, where the latex constituent belongs to a well-known form, allowing the molecular modeling of its spatial structure. The papaya latex thaumatin-like protein was further characterized. The protein appears to be stable in the pH interval from 2 to 10 and resistant to chemical denaturation by guanidium chloride, with a DeltaG(water)(0) of 15.2 kcal/mol and to proteolysis by the four papaya cysteine proteinases. The physiological role of this protein is discussed.

Fluorescent labelling of 6-phosphogluconate dehydrogenase from Bacillus stearothermophilus

The thermophilic 6-phosphogluconate dehydrogenase from Bacillus stearothermophilus was inhibited upon specific modification of the -SH group of cysteine residues by 7-chloro-4-nitrobenzo-2-oxa-1, 3-diazole (NBD-Cl) at pH 7.0. By using 20-100-fold molar excess of NBD-CL the reaction occurs slowly at pH 7.0 as a first order process. Partial protection from inactivation was observed when the substrate 6-phosphogluconate or the coenzyme NADP was added to the reaction mixture. Complete inactivation was achieved upon modification of 1.9 of the six cysteine residues per mole of enzyme, which corresponds to nearly one residue per enzyme subunit. Circular dichroism measurements suggest that the gross structure of the protein molecule is practically unchanged upon reaction of the enzyme with NBD-Cl. Melting profile experiments revealed a single transition occurring at about 65 degrees C. Analogously, the profile of intensity of the fluorescence emission at 520 nm of the enzyme-bound S-NBD groups versus temperature indicated a midpoint of transition near 65 degrees C. Since this melting temperature corresponds closely to that observed with the native enzyme, these results would indicate that the molecular organizations of the native and modified enzyme are similar and stabilized by similar interactions within the polypeptide chain.

Structural and kinetic properties of a beta-hydroxyacid dehydrogenase involved in nicotinate fermentation

2-(Hydroxymethyl)glutarate dehydrogenase, the fourth enzyme of the anaerobic nicotinate fermentation pathway of Eubacterium barkeri, catalyzes the NADH-dependent conversion between (S)-2-formylglutarate and (S)-2-(hydroxymethyl)glutarate. As shown by its 2.3-A crystal structure, this enzyme is a novel member of the beta-hydroxyacid dehydrogenase family and adopts a tetrameric architecture with monomers interacting via their C-terminal catalytic domains. The NAD-binding domains protrude heterogeneously from the central, tetrameric core with domain rotation angles differing up to 12 degrees. Kinetic properties of the enzyme, including NADH inhibition constants, were determined. A strong NADH binding in contrast to weaker NAD(+) binding of the protein was inferred from fluorometrically determined binding constants for the dinucleotide cofactor. The data support either an Iso Ordered Bi Bi mechanism or a more common Ordered Bi Bi mechanism as found in other dehydrogenases.

Metal-enhanced e-type fluorescence

In this letter, we report metal-enhanced e-type fluorescence. Eosin in close proximity to silver island films (SiFs) shows enhanced e-type fluorescence with an approximately two-fold higher intensity observed from SiFs, as compared to a control sample. Our findings suggest two complementary mechanisms for the enhancement: surface plasmons can radiate e-type delayed fluorescence efficiently and enhanced absorption also facilitates enhanced emission from both S(1) and T(1) states. This observation is helpful in our understanding not only for studying the interactions between plasmons and fluorophores but also for our laboratories continued efforts to develop a unified plasmon-lumophore description.

The correction of reaction rates in continuous fluorometric assays of enzymes

The kinetic data obtained from the action of a cathepsin D-like enzyme from Biomphalaria glabrata hepatopancreas (digestive gland) on MOCAc-Gly-Lys-Pro-Ile-Leu-Phe-Phe-Arg-Leu-Lys(DNp)-D-Arg-NH(2), was studied as a data prototype, generated by means of a fluorogenic substrate. An initial fluorescence, due to incomplete energy transfer, of about 8% of the values attained after complete substrate hydrolysis; a non-linear standard curve even at microM concentrations and an exponential decay of the steady state fluorescence of reaction product of the order of 10(-4) x s(-1) were the main analytical problems encountered. The standard curves for fluorescence of the substrate reaction product after 48 h of hydrolysis, and the reference compound MOCAc-Pro-Leu-Gly-NH(2), were fitted by polynomial approximation and the point derivates used as calibration factors. Time dependence of the calibration factor for the reaction product was -2.96 x 10(-4) a.u microM(-1) x s(-1) that is, in the same order of observed enzymic reaction rates. A mathematical treatment was devised for obtaining rates corrected for errors derived from the three analytical problems indicated. The method is of general application in continuous fluorometric assays, irrespective of the particular enzyme used, but of special value for substrates that present significant initial fluorescence. The reaction rates were 11% higher; as calculated by means of the calibration factor [substrate]/(final-initial fluorescence intensities), which is the prevalent procedure in the literature; leading to underestimation of K(m) and overestimation of V(max).

Evidence for co-operativity in coenzyme binding to tetrameric Sulfolobus solfataricus alcohol dehydrogenase and its structural basis: fluorescence, kinetic and structural studies of the wild-type enzyme and non-co-operative N249Y mutant

The interaction of coenzyme with thermostable homotetrameric NAD(H)-dependent alcohol dehydrogenase from the thermoacidophilic sulphur-dependent crenarchaeon Sulfolobus solfataricus (SsADH) and its N249Y (Asn-249-->Tyr) mutant was studied using the high fluorescence sensitivity of its tryptophan residues Trp-95 and Trp-117 to the binding of coenzyme moieties. Fluorescence quenching studies performed at 25 degrees C show that SsADH exhibits linearity in the NAD(H) binding [the Hill coefficient (h) approximately 1) at pH 9.8 and at moderate ionic strength, in addition to positive co-operativity (h=2.0-2.4) at pH 7.8 and 6.8, and at pH 9.8 in the presence of salt. Furthermore, NADH binding is positively co-operative below 20 degrees C (h approximately 3) and negatively co-operative at 40-50 degrees C (h approximately 0.7), as determined at moderate ionic strength and pH 9.8. Steady-state kinetic measurements show that SsADH displays standard Michaelis-Menten kinetics between 35 and 45 degrees C, but exhibits positive and negative co-operativity for NADH oxidation below (h=3.3 at 20 degrees C) and above (h=0.7 at 70-80 degrees C) this range of temperatures respectively. However, N249Y SsADH displays non-co-operative behaviour in coenzyme binding under the same experimental conditions used for the wild-type enzyme. In loop 270-275 of the coenzyme domain and segments at the interface of dimer A-B, analyses of the wild-type and mutant SsADH structures identified the structural elements involved in the intersubunit communication and suggested a possible structural basis for co-operativity. This is the first report of co-operativity in a tetrameric ADH and of temperature-induced co-operativity in a thermophilic enzyme.

The binding of 1,8 ANS congeners to I-FABP and comparison of some hypotheses about ANS' spectral sensitivity to environment

The ANS congeners 1-anilinonaphthalene and 1-amino,8-sulfonato naphthalene were investigated as analogs of 1,8 anilinonaphthalene sulfonate. Like 1,8 ANS, they also bind to I-FABP, and the fully-bound spectra reveal interesting similarities and differences with respect to ANS binding. The nature of these differences suggests that certain hypotheses in the literature about ANS photophysics ought to be revised. The conceptual decomposition of energetic effects in the thermodynamics of ANS binding proposed earlier [W. Kirk, E. Kurian, F. Prendergast, Characterization of the sources of protein-ligand affinity: 1-sulfonato-8-(1')anilinonaphthalene binding to intestinal fatty acid binding protein. Biophys. J. (1996) 70 69-83.] is extended further in this report.

Vibrio cholerae hemolysin. Implication of amphiphilicity and lipid-induced conformational change for its pore-forming activity

Vibrio cholerae hemolysin (HlyA), a water-soluble protein with a native monomeric relative molecular mass of 65 000, forms transmembrane pentameric channels in target biomembranes. The HlyA binds to lipid vesicles nonspecifically and without saturation; however, self-assembly is triggered specifically by cholesterol. Here we show that the HlyA partitioned quantitatively to amphiphilic media irrespective of their compositions, indicating that the toxin had an amphiphilic surface. Asialofetuin, a beta1-galactosyl-terminated glycoprotein, which binds specifically to the HlyA in a lectin-glycoprotein type of interaction and inhibits carbohydrate-independent interaction of the toxin with lipid, reduced effective amphiphilicity of the toxin significantly. Resistance of the HlyA to proteases together with the tryptophan fluorescence emission spectrum suggested a compact structure for the toxin. Fluorescence energy transfer from the HlyA to dansyl-phosphatidylethanolamine required the presence of cholesterol in the lipid bilayer and was synchronous with oligomerization. Phospholipid bilayer without cholesterol caused a partial unfolding of the HlyA monomer as indicated by the transfer of tryptophan residues from the nonpolar core of the protein to a more polar region. These observations suggested: (a) partitioning of the HlyA to lipid vesicles is driven by the tendency of the amphiphilic toxin to reduce energetically unfavorable contacts with water and is not affected significantly by the composition of the vesicles; and (b) partial unfolding of the HlyA at the lipid-water interface precedes and promotes cholesterol-induced oligomerization to an insertion-competent configuration.

Cleavage properties of an archaeal site-specific recombinase, the SSV1 integrase

SSV1 is a virus infecting the extremely thermophilic archaeon Sulfolobus shibatae. The viral-encoded integrase is responsible for site-specific integration of SSV1 into its host genome. The recombinant enzyme was expressed in Escherichia coli, purified to homogeneity, and its biochemical properties investigated in vitro. We show that the SSV1 integrase belongs to the tyrosine recombinases family and that Tyr(314) is involved in the formation of a 3'-phosphotyrosine intermediate. The integrase cleaves both strands of a synthetic substrate in a temperature-dependent reaction, the cleavage efficiency increasing with temperature. A discontinuity was observed in the Arrhenius plot above 50 degrees C, suggesting that a conformational transition may occur in the integrase at this temperature. Analysis of cleavage time course suggested that noncovalent binding of the integrase to its substrate is rate-limiting in the cleavage reaction. The cleavage positions were localized on each side of the anticodon loop of the tRNA gene where SSV1 integration takes place. Finally, the SSV1 integrase is able to cut substrates harboring mismatches in the binding site. For the cleavage step, the chemical nature of the base in position -1 of cleavage seems to be more important than its pairing to the opposite strand.

Interaction of camel lens zeta-crystallin with quinones: portrait of a substrate by fluorescence spectroscopy

Interaction of camel lens zeta-crystallin, an NADPH:quinone oxidoreductase, with several quinone derivatives was examined by fluorescence spectroscopy and activity measurements. Fluorescence of zeta-crystallin was quenched to different levels by the different quinones:juglone (5-OH, 1,4 naphthoquinone), 1,4 naphthoquinone (1,4-NQ), and 1,2 naphthoquinone (1,2-NQ) considerably quenched the fluorescence of zeta-crystallin, where as the commonly used substrate, 9,10-phenanthrenequinone (PQ) did not induce significant quenching. Activity measurements showed only PQ served as a substrate for camel lens zeta-crystallin, while juglone, 1,4-NQ, and 1,2-NQ were inhibitors. Thus quinones that interacted with zeta-crystallin directly inhibited the enzyme, whereas the substrate had very low affinity for the enzyme in the absence of NADPH. Another substrate, dichlorophenol indophenol (DCIP), conformed to the same pattern; DCIP did not quench the fluorescence of the enzyme significantly, but served as a substrate. This pattern is consistent with an ordered mechanism of catalysis with quinone being the second substrate. All three naphthoquinones were uncompetitive inhibitors with respect to NADPH and noncompetitive with respect to PQ. These kinetics are similar to those exhibited by cysteine- and/or lysine-modifying agents. Juglone, 1,4-NQ, and 1,2-NQ interacted with and quenched the fluorescence of camel lens alpha-crystallin, but to lesser extent than that of zeta-crystallin.

Hydrophobicity of the NADPH binding domain of camel lens zeta-crystallin

Interaction of camel lens zeta-crystallin with the hydrophobic probe 1-anilinonaphthalene-8-sulfonic acid (ANS) enhanced the ANS fluorescence and quenched the protein fluorescence. Both of these events were concentration-dependent and showed typical saturation curves suggesting specific ANS-zeta-crystallin binding. Quantitative analysis indicated that 1 mole zeta-crystallin bound at most 1 mole ANS. NADPH but not 9,10-phenanthrenequinone (PQ) was able to displace zeta-crystallin-bound ANS. These results suggested the presence of a hydrophobic domain in zeta-crystallin, possibly at the NADPH binding site. alpha-Crystallin as well as NADPH protected zeta-crystallin against thermal inactivation suggesting the importance of this site for enzyme stability. The NADPH:quinone oxidoreductase activity of zeta-crystallin was inhibited by ANS with NADPH as electron donor and PQ as electron acceptor. Lineweaver-Burk plots indicated mixed-type inhibition with respect to NADPH, with a K(i) of 2.3 microM. Secondary plots of inhibition with respect to NADPH indicated a dissociation constant (K'I) of 12 microM for the zeta-crystallin-NADPH-ANS complex. The K(i) being smaller than K'I suggested that competitive inhibition at the NADPH binding site was predominant over non-competitive inhibition. Like ANS-zeta-crystallin binding, inhibition was dependent on ANS concentration but independent of incubation time.

Thermal unfolding and refolding of beta-lactoglobulin. An intrinsic andextrinsic fluorescence study

The conformational features of beta-lactoglobulin, refolded by cooling from a thermally perturbed state, has been characterized by intrinsic and extrinsic fluorescence measurements on the protein. It is found that even at 85-90 degrees C, beta-lactoglobulin does not completely lose its folded structure. The unfolding and refolding of beta-lactoglobulin as observed through intrinsic tryptophan fluorescence is nearly reversible because the native beta-lactoglobulin and its refolded form, following heating and cooling, show nearly identical tryptophan fluorescence properties. However, the fluorescence properties of an extrinsic probe 1-anilino 8-naphthalene sulfonic acid (ANS) for the native and refolded forms are quite different from each other. Significant increase in fluorescence intensity and blue shifts in emission maxima of ANS bound to refolded beta-lactoglobulin is observed compared to that of the native form. Our results indicate that beta-lactoglobulin, refolded after heating to above 70 degrees C, has deep hydrophobic pockets which can be accessed by ANS. These pockets are either nonexistent or inaccessible to ANS in native beta-lactoglobulin. The opening of the central cavity collapses at pH close to the isoelectric pH of the protein. This indicates that electrostatic repulsion is necessary to keep this access open.

In vitro folding, purification and characterization of Escherichia coli outer membrane protease ompT

OmpT is a protease present in the outer membrane of Escherichia coli. The enzyme was overexpressed without its signal sequence in E. coli using a T7 system, resulting in the accumulation of OmpT as inclusion bodies. After solubilization of the inclusion bodies in urea, the protein could be folded in vitro by dilution in the presence of detergent n-dodecyl-N, N-dimethyl-1-ammonio-3-propanesulphonate. The addition of lipopolysaccharide to the protein was essential to obtain active enzyme. The correctly folded protein was purified to homogeneity by ion exchange chromatography with a 57% overall yield. Autoproteolysis between Lys217-Arg218 was a major problem during purification, but degradation could be abolished by introducing the mutations G216K and K217G. A novel fluorimetric assay using the internally quenched substrate Abz-Ala-Arg-Arg-Ala-Tyr(NO2)-NH2 (where Abz is o-aminobenzoyl and Tyr(NO2) is 3-nitrotyrosine) enabled the determination of the kinetic parameters. The wild-type enzyme has an affinity Km of 0.4 microM for the substrate and a turnover number kcat of 40 s-1. The Km and kcat for the double variant were 1.1 microM and 1.6 s-1, respectively. The pH profiles of the wild type and variant were identical, showing optimal activity at pH 6.5 and pKa values of 5.6 and 7.5, respectively. Circular dichroism spectra of both enzymes indicated a high content of beta-strand conformation, and on that basis a beta-barrel topology model is proposed.

Binding of 1-anilinonaphthalene-8-sulfonic acid to alpha-crystallin

alpha-Crystallin was found to exhibit a time-dependent uptake of the hydrophobic probe, 1-anilinonaphthalene-8-sulfonic acid (ANS), similar to that typically observed with lipid membranes. Analysis of the interaction of ANS with alpha-crystallin revealed two types of interactive processes, partitioning and binding. The predominant process involved partitioning, with a coefficient of 300 M-1. The binding component had the following characteristics: 1 binding site/24 subunits and a Kd of about 9 microM. The binding was unaffected by the number of subunits used in the assembly of the alpha-aggregate, since both the alpha m- and alpha c-forms had similar binding characteristics. No discernible differences were observed in the binding of ANS to homopolymers of alpha A and alpha B subunits, suggesting that the hydrophobic sites to which ANS bound were similar in both the A and B subunits. The majority of the fluorescence was lost when the protein was incubated in 3 M urea, a concentration of denaturant where the protein is still intact, suggesting that the ANS binding sites are located near the surface of the protein. The decrease was attributed to a decrease in the quantum yield of the bound dye.

The role of glutamate 87 in the kinetic mechanism of Thermus thermophilus isopropylmalate dehydrogenase

The kinetic mechanism of the oxidative decarboxylation of 2R,3S-isopropylmalate by the NAD-dependent isopropylmalate dehydrogenase of Thermus thermophilus was investigated. Initial rate results typical of random or steady-state ordered sequential mechanisms are obtained for both the wild-type and two mutant enzymes (E87G and E87Q) regardless of whether natural or alternative substrates (2R-malate, 2R,3S-tartrate and/or NADP) are utilized. Initial rate data fail to converge on a rapid equilibrium-ordered pattern despite marked reductions in specificity (kcat/Km) caused by the mutations and alternative substrates. Although the inhibition studies alone might suggest an ordered kinetic mechanism with cofactor binding first, a detailed analysis reveals that the expected noncompetitive patterns appear uncompetitive because the dissociation constants from the ternary complexes are far smaller than those from the binary complexes. Equilibrium fluorescence studies both confirm the random binding of substrates and the kinetic estimates of the dissociation constants of the substrates from the binary complexes. The latter are not distributed markedly by the mutations at site 87. Mutations at site 87 do not affect the dissociation constants from the binary complexes, but do greatly increase the Michaelis constants, indicating that E87 helps stabilize the Michaelis complex of the wild-type enzyme. The available structural data, the patterns of the kinetics results, and the structure of a pseudo-Michaelis complex of the homologous isocitrate dehydrogenase of Escherichia coli suggest that E87 interacts with the nicotinamide ring.

Limited proteolysis of cytochrome c in trifluoroethanol

Horse heart cytochrome c is cleaved by thermolysin in 50% aqueous TFE (v/v) at neutral pH (25 degrees C, 24 h) at the Gly56-Ile57 peptide bond of the 104-residue chain of the protein. Additional, but anyway minor, fragmentation at the Gly45-Phe46 and Met80-Ile81 peptide bonds is also observed. On the other hand, in buffer only and in the absence of TFE, cytochrome c is digested by thermolysin to numerous small peptides. Considering the broad substrate specificity of the TFE-resistant thermolysin, clearly the conformational state of the protein substrate dictates the observed selective proteolysis. It is proposed that the highly helical secondary structure acquired by cytochrome c when dissolved in aqueous TFE hampers binding and adaptation of the protein substrate at the active site of the protease and that peptide bond fission occurs at flexible chain segments characterized by a low alpha-helix propensity.

Synthesis and refolding of human TIMP-2 from E. coli, with specific activity for MMP-2

Tissue inhibitors of metalloproteinase (TIMPs) are inhibitory counterparts of collagenases, containing 12 cysteine residues paired to six internal disulphide bridges. TIMP-2, an inhibitory protein of 72 kDa gelatinase/type IV collagenase (MMP-2), was expressed in Escherichia coli as a fusion protein with a 34 amino acid NH2-linked tail containing six consecutive histidine residues. The protein was purified in a single-step using an ion metal affinity column (IMAC) in denaturing conditions. The immobilized fusion TIMP-2 protein was refolded at a high concentration in the column, producing about 5 mg of protein per litre of bacterial cells. It shows specific binding and inhibitory activity against MMP-2, but has no effect against 92 and 45 kDa gelatinases.

Heat denaturation of human orosomucoid in water/methanol mixtures

Heat denaturation of orosomucoid in solutions of methanol concentrations ranging from 0 to 70% (v/v) has been studied by using circular dichroism, intrinsic protein fluorescence and thermal difference absorption spectroscopy. Regardless of its high saccharide content (40%), the highly cooperative denaturation transition of orosomucoid is fully reversible in neutral water solution. A two-state model has been successfully applied; the numerical analysis results in thermodynamical parameter values that are in close agreement with previously reported experimental data from calorimetric measurements. However, in solutions containing even minute concentrations of methanol (5%) the heat denaturation is irreversible. After cooling of the denatured protein the refolded molecules exhibit a higher alpha-helical content than the native one. Possibilities of methanol interaction with native and denatured protein molecule are discussed.

Cercospora beticola toxins. VII. Fluorometric study of their interactions with biological membranes

The interactions of two beticolins, Cercospora beticola toxins, and of their magnesium complexes with liposomes or plasma membrane were studied. The fluorometric pH titration curves of beticolins in liposomes and in plasma membranes reveal the presence of the dissociated form of beticolins. The concentration of the magnesium complex in these membranes increases at high pH. The partition coefficient of beticolin-1 on liposomes is 3-fold higher than that of beticolin-2 and the fluorescence of both compounds on liposomes is similar. The addition of magnesium to liposomes causes a 40-fold and 20-fold increase in the partition coefficient of beticolin-1 and -2, respectively, as a result of the interactions between membrane, magnesium and beticolins. Beticolins react to a delta pH across the liposome membrane but the formation of the magnesium complex completely abolishes this effect.

An oxygenation-sensitive dye binding to Carcinus maenas hemocyanin

The interaction of 4',6-diamidino-2-phenylindole (DAPI) with Carcinus maenas hemocyanin has been investigated by steady state fluorescence, dynamic fluorescence and circular dichroism measurements. The dye binds to apohemocyanin (without copper) as well as to oxygenated hemocyanin and to deoxygenated hemocyanin with very similar affinities (kd approximately equal to 1 microM ) and number of binding sites (one per subunit). In contrast, the fluorescence quantum yield enhancement of DAPI bound to oxygenated hemocyanin is nearly 60% lower than that observed for deoxygenated and apo forms. The decrease of fluorescence of the dye bound to deoxygenated hemocyanin is a sigmoidal function of the oxygen partial pressure, specular to that observed by following the absorbance of the copper-oxygen charge transfer band at 340 nm. This result provides preliminary evidence that DAPI may be used as a functional probe to monitor the cooperative binding of oxygen to the protein. The higher fluorescence quantum yield of DAPI bound to either apohemocyanin or deoxygenated protein is characterized by a single fluorescence decay with lifetime of about 3 ns, while with the oxygenated protein two components of about 1 ns and 3.0 ns are observed. This result is interpreted assuming the existence of two rotamers of DAPI in solution (Szabo et al. Photochem. Photobiol. 44 (1986) 143-150) both able to interact with oxygenated hemocyanin but only one to deoxygenated and apo forms. We conclude that the different fluorescence behaviour of the dye induced by the presence of oxygen bound to the protein is probably due to a structural change of hemocyanin in cooperative interaction with oxygen. Furthermore, the interaction is confirmed by the induced negative ellipticity of DAPI bound to apohemocyanin and deoxy-hemocyanin and by the increase of fluorescence anisotropy of DAPI bound to all forms of protein investigated.

Fluorescence studies of spectrin and its subunits

To better understand the solution structure of spectrin, the environment of its tryptophan residues have been examined by fluorescence spectroscopy. The spectra and the extent of quenching by several quenching agents have been determined for intact spectrin and its alpha and beta subunits. The arsenal of quenchers used in the study represented both hydrophilic and hydrophobic species including anionic, cationic and neutral compounds. Effects on spectrin fluorescence of ethanol and ionic strength, which extend and/or rigidify spectrin, and of glycerol, which is commonly used in electron microscopy of the protein, have also been assessed in the presence and absence of quenchers. Most of the tryptophans of spectrin are either internally quenched or are sequestered, hindering the approach of hydrophilic quenching agents. Both the spectral shape and the extent of quenching by acrylamide indicate that some tryptophans of the beta subunit are slightly more exposed in the isolated chain than in the dimer. Similar effects on spectra and on quenching of the intact dimer and of the isolated beta chain are seen when the ionic strength is reduced. Ethanol and glycerol reduce spectrin tryptophan accessibility to 2-p-toluidinyl napthalene-6-sulfonic acid (TNS). It therefore appears that low ionic strength, alpha-beta association and neutral solute (or lowered dielectric constant) all induce a similar, but modest conformational change in the domain structure. The extent of TNS binding is not increased by lowering the ionic strength, suggesting that the expansion and/or stiffening of the molecule in low electrolyte solution does not involve exposure of significant numbers of hydrophobic sites.

Unfolding and refolding of hen egg-white riboflavin binding protein

The unfolding and refolding of riboflavin-binding protein (RfBP) from hen egg-white induced by addition of guanidinium chloride (GdnHCl), and its subsequent removal by dialysis have been studied by c.d. and fluorescence for both the native and reduced protein. The reduction of its nine disulphide bonds causes a reduction in the secondary structure (alpha-helix plus beta-sheet) from 63% to 33% of the amino acid residues. Unfolding of the native protein occurred in two phases; the first involving a substantial loss of tertiary structure, followed by a second phase involving loss of secondary structure at higher GdnHCl concentrations. By contrast this biphasic behaviour was not discernible in the reduced protein. The loss of ability to bind riboflavin occurred after the first phase of unfolding. Comparison of unfolding of the holoprotein and apoprotein suggested that riboflavin has only a small stabilizing effect on the unfolding process. After removal of GdnHCl, the holoprotein, apoprotein and reduced protein assumed their original conformation. The significance of the results in relation to various models for protein folding is discussed.

The aromatic amino acid content of the bacterial chaperone protein groEL (cpn60). Evidence for the presence of a single tryptophan

Studies of the absorption and fluorescence properties of the chaperone protein groEL (cpn60) from Escherichia coli show that tryptophan is present, in contrast to the proposed amino acid sequence of the protein (Hemmingsen, S.M. et al. (1988) Nature 333, 330-334). By determining a suitable value for the specific absorption coefficient of the protein at 280 nm, it has been shown that the content of the aromatic amino acids corresponds to a single tryptophan and (most probably) seven tyrosines per subunit (Mr 57,200).

Glucose dehydrogenase from Bacillus subtilis expressed in Escherichia coli. I: Purification, characterization and comparison with glucose dehydrogenase from Bacillus megaterium

Escherichia coli containing the Bacillus subtilis glucose dehydrogenase gene on a plasmid (prL7) was used to produce the enzyme in high quantities. Gluc-DH-S was purified from the cell extract by (NH4)2SO4-precipitation, ion-exchange chromatography and Triazine-dye chromatography to a specific activity of 375 U/mg. The enzyme was apparently homogenous on SDS-PAGE with a subunit molecular mass of 31.5 kDa. Investigation of Gluc-DH-S was performed for comparison with the corresponding properties of Gluc-DH-M. The limiting Michaelis constant at pH 8.0 for NAD+ is Ka = 0.11 mM and for D-glucose Kb = 8.7 mM. The dissociation constant for NAD+ is Kia = 17.1 mM. Similar to Gluc-DH-M, Gluc-DH-S is inactivated by dissociation under weak alkaline conditions at pH 9.0. Complete reactivation is attained by readjustment to pH 6.5. Ultraviolet absorption, fluorescence and CD-spectra of native Gluc-DH-S, as well as fluorescence- and CD-backbone-spectra of the dissociated enzyme were nearly identical to the corresponding spectra of Gluc-DH-M. The aromatic CD-spectrum of dissociated Gluc-DH-S was different, representing a residual ellipticity of tryptophyl moieties in the 290-310 nm region. Density gradient centrifugation proved that this behaviour is due to the formation of inactive dimers in equilibrium with monomers after dissociation. In comparison to Gluc-DH-M, the kinetics of inactivation as well as the time-dependent change of fluorescence intensity at pH 9.0 of Gluc-DH-S showed a higher velocity and a changed course of the dissociation process.

Enzyme modification by MPEG with an amino acid or peptide as spacer arms

A method for the modification of enzymes by MPEG carrying an amino acid or peptide as a spacer arm is described and tested with aliphatic or aromatic side chains amino acids. The procedure involves MPEG activation by p-nitrophenylchloroformate for the amino acid or peptide coupling that is in turn activated for the protein binding. The advantage of the method resides in the possibility to introduce proper reporter groups between the polymer and the protein as norleucine for a direct evaluation of the bound polymer chains, tryptophan for structural studies of the polymer-protein adduct, and radioactive amino acid for pharmacokinetic investigations. The method was positively tested with arginase, ribonuclease, and superoxide dismutase as enzymes of therapeutic value.