PubMed Health⌕ Search

Biomedical subjects

A M Klibanov

Publications and source records attributed to A M Klibanov.

At least 19 recordsLinked to original sources
PubMed1997

The crystal structure of subtilisin Carlsberg in anhydrous dioxane and its comparison with those in water and acetonitrile.

The x-ray crystal structure of the serine protease subtilisin Carlsberg in anhydrous dioxane has been determined to 2.6-A resolution. The enzyme structure is found to be nearly indistinguishable from the structures previously determined in water and acetonitrile. Small changes in the side-chain conformations between the dioxane and water structures are of the same magnitude as those observed between two structures in different aqueous systems. Seven enzyme-bound dioxane molecules have been detected, each potentially forming at least one hydrogen bond with a subtilisin hydrogen-bond donor or bound water. Two of the bound dioxane molecules are in the active-site region, one in the P2 and another bridging the P1' and P3' pockets. The other five dioxane molecules are located on the surface of subtilisin at interprotein crystal contacts. The locations of the bound solvent in the dioxane structure are distinct from those in the structures in acetonitrile and in water.

Acetonitriles↗
PubMed1997

Why are enzymes less active in organic solvents than in water?

In order to exploit fully the biotechnological opportunities afforded by nonaqueous enzymology, the issue of often drastically diminished enzymatic activity in organic solvents compared with that in water must be addressed and resolved. Recent studies have made great strides towards elucidating causes of this phenomenon of activity loss. None of these causes is insurmountable; by designing strategies that systematically target them, enzymatic activity in organic solvents can be readily enhanced by multiple orders of magnitude and ultimately brought to the aqueous-like level.

Biomechanical Phenomena↗
PubMed1996

The secondary structure and aggregation of lyophilized tetanus toxoid.

Tetanus toxoid (TT), the vaccine for tetanus, is an important protein antigen and candidate for sustained release from polymeric matrices. During administration from the latter, the solid (e.g., lyophilized) protein will be exposed to elevated levels of temperature and moisture, conditions which trigger its aggregation. To examine the connection between this aggregation and the structure of the TT molecule in the solid state, Fourier-transform infrared (FTIR) spectroscopy was employed to determine the secondary structure of TT in the presence of various excipients. We found that excipient-free TT undergoes a significant alteration (mostly reversible) in the secondary structure during lyophilization. Specifically, more than half the total alpha-helix content was lost with a concomitant increase in beta-sheet structure. The extent of structural alterations in the presence of 1:5 (g:g protein) NaCl, sorbitol, or poly-(ethylene glycol), did not correlate with stability conferred towards moisture-induced aggregation. These results suggest that the degree of retention of the native protein structure in the dry state is not a general predictor of stability for the "wetted" solid within polymer controlled-release vehicles.

Excipients↗
PubMed1995

Lyophilization-induced reversible changes in the secondary structure of proteins.

Changes in the secondary structure of some dozen different proteins upon lyophilization of their aqueous solutions have been investigated by means of Fourier-transform infrared spectroscopy in the amide III band region. Dehydration markedly (but reversibly) alters the secondary structure of all the proteins studied, as revealed by both the quantitative analysis of the second derivative spectra and the Gaussian curve fitting of the original infrared spectra. Lyophilization substantially increases the beta-sheet content and lowers the alpha-helix content of all proteins. In all but one case, proteins become more ordered upon lyophilization.

Amides↗
PubMed1995

Stabilization of tetanus and diphtheria toxoids against moisture-induced aggregation.

The progress toward single-dose vaccines has been limited by the poor solid-state stability of vaccine antigens within controlled-release polymers, such as poly(lactide-co-glycolide). For example, herein we report that lyophilized tetanus toxoid aggregates during incubation at 37 degrees C and elevated humidity--i.e., conditions relevant to its release from such systems. The mechanism and extent of this aggregation are dependent on the moisture level in the solid protein, with maximum aggregation observed at intermediate moisture contents. The main aggregation pathway is consistent with formaldehyde-mediated cross-linking, where reactive electrophiles created and stored in the vaccine upon formalinization (exposure to formaldehyde during vaccine preparation) react with nucleophiles of a second vaccine molecule to form intermolecular cross-links. This process is inhibited by the following: (i) succinylating the vaccine to block reactive amino groups; (ii) treating the vaccine with sodium cyanoborohydride, which presumably reduces Schiff bases and some other electrophiles created upon formalinization; and (iii) addition of low-molecular-weight excipients, particularly sorbitol. The moisture-induced aggregation of another formalinized vaccine, diphtheria toxoid, is also retarded by succinylation, suggesting the generality of this mechanism for formalinized vaccines. Hence, mechanistic stability studies of the type described herein may be important for the development of effective single-dose vaccines.

Bacterial Vaccines↗
PubMed1995

Fourier-transform infrared spectroscopic investigation of protein stability in the lyophilized form.

Upon the removal of water, proteins undergo a major, reversible rearrangement of their secondary structure, as revealed by FTIR spectroscopy. We have found herein that for recombinant human albumin (rHA) the extent of this structural change does not depend significantly either on the composition of the aqueous solution prior to lyophilization (protein concentration, pH, and the presence of excipients such as dextran or NaCl) or on the mode of dehydration (lyophilization, spray drying, or rotary evaporation), even though these factors profoundly affect rHA's solid-state stability against moisture-induced aggregation. In all cases, the alpha-helix content of rHA drops from 58% in solution to 25-35% in the dehydrated state, the beta-sheet content rises from 0 to 10-20%, and unordered structures increase from 40% to 50-60%. We have also investigated another model protein, hen egg-white lysozyme, and confirmed that it too undergoes a significant alteration of the secondary structure upon lyophilization. The extent of this structural reorganization has been found to be insensitive to the pH of the aqueous solution prior to lyophilization from pH 1.9 to 5.1, even though the thermal transition temperature (Tm) in aqueous solution over this range varies by 30 degrees C.

Dextrans↗
PubMed1995

Cloning of an organic solvent-resistance gene in Escherichia coli: the unexpected role of alkylhydroperoxide reductase.

Although bacterial strain able to grow in the presence of organic solvents have been isolated, little is known about the mechanism of their resistance. In the present study, 1,2,3,4-tetrahydronaphthalene (tetralin), a solvent with potential applications in industrial biocatalysis, was used to select a resistant mutant of Escherichia coli. The resultant mutant strain was tested for resistance to a wide range of solvents of varying hydrophobicities and was found to be resistant not only to tetralin itself but also to cyclohexane, propylbenzene, and 1,2-dihydronaphthalene. A recombinant library from mutant DNA was used to clone the resistance gene. The sequence of the cloned locus was determined and found to match the sequence of the previously described alkylhydroperoxide reductase operon ahpCF. The mutation was localized to a substitution of valine for glycine at position 142 in the coding region of ahpC, which is the gene encoding the catalytic subunit of the enzyme. The ahpC mutant was found to have an activity that was three times that of the wild type in reducing tetralin hydroperoxide to 1,2,3,4-tetrahydro-1-naphthol. We conclude that the toxicity of such solvents as tetralin is caused by the formation of toxic hydroperoxides in the cell. The ahpC mutation increases the activity of the enzyme toward hydrophobic hydroperoxides, thereby conferring resistance. The ahpC mutant was sensitive to the more hydrophilic solvents xylene and toluene, suggesting that there are additional mechanisms of solvent toxicity. Mutants resistant to a mixture of xylene and tetralin were isolated from the ahpC mutant but not from the wild-type strain.

Base Sequence↗
PubMed1995

Transport of proteins dissolved in organic solvents across biomimetic membranes.

Using lipid-impregnated porous cellulose membranes as biomimetic barriers, we tested the hypothesis that to afford effective transmembrane transfer of proteins and nucleic acids, the vehicle solvent should be able to dissolve both the biopolymers and the lipids. While the majority of solvents dissolve one or the other, ethanol and methanol were found to dissolve both, especially if the protein had been lyophilized from an aqueous solution of a pH remote from the protein's isoelectric point. A number of proteins, as well as RNA and DNA, dissolved in these alcohols readily crossed the lipidized membranes, whereas the same biopolymers placed in nondissolving solvents (e.g., hexane and ethyl acetate) or in those unable to dissolve lipids (e.g., water and dimethyl sulfoxide) exhibited little transmembrane transport. The solubility of biopolymers in ethanol and methanol was further enhanced by complexation with detergents and poly(ethylene glycol); significant protein and nucleic acid transport through the lipidized membranes was observed from these solvents but not from water.

Biological Transport↗
PubMed1994

The solvent dependence of enzyme specificity.

The discovery that enzymes possess catalytic activity in organic solvents has made it possible to address the question of the influence of the reaction medium on enzymatic specificity. Recently, the substrate specificity, enantioselectivity, prochiral selectivity, regioselectivity, and chemoselectivity of enzymes have been found to dramatically depend on the nature of the solvent. This review discusses the scope, possible mechanisms, and implications of this phenomenon, as well as directions of future research in the area.

Animals↗
PubMed1994

X-ray crystal structure of cross-linked subtilisin Carlsberg in water vs. acetonitrile.

The crystal structure of subtilisin Carlsberg lightly cross-linked with glutaraldehyde was solved in aqueous solution by X-ray crystallography at 2.3 A resolution. It was found to be virtually identical to the recently determined (Fitzpatrick, P.A., Steinmetz, A.C.U., Ringe, D.A. & Klibanov, A.M. (1993) Proc. Natl. Acad. Sci. USA 90, 8653) structure of the cross-linked enzyme in anhydrous acetonitrile. The latter structure was found to be significantly more rigid than in water, as reflected by their average B factors. The numbers of subtilisin-bound water molecules in the two structures are similar (114 and 99 in water and in acetonitrile, respectively), but the locations of some half of these bound waters are distinct.

Acetonitriles↗
PubMed1994

Detergent-enabled transport of proteins and nucleic acids through hydrophobic solvents.

It is demonstrated that proteins and nucleic acids can be transported through hydrophobic organic solvents (liquid membranes) via nonspecific complex formation with detergents, whereas no macromolecule transport is observed without the latter. A protein (or a nucleic acid) first interacts with an oppositely charged detergent due to hydrophobic ion pairing in the aqueous feed phase. The resultant hydrophobic complex readily partitions into an organic solvent and then into the aqueous receiver phase, where it dissociates. Experiments with (i) different detergent/protein molar ratios, (ii) a range of unrelated organic solvents as liquid membranes, and (iii) homologous detergents with hydrophobic tails of varying lengths indicate that the protein flux through the membrane directly correlates with the partitioning of the protein-detergent complexes from the aqueous feed into the organic phase. Very little protein transport was detected at detergent concentrations above the critical micelle concentration, suggesting that individual detergent molecules, rather than micelles, play the key role. The rate of the detergent-enabled protein transport is not a function of the protein molecular weight, provided that enough detergent molecules bind to make the complex sufficiently hydrophobic; e.g., bovine serum albumin can be transported faster than insulin, which is less than 1/10th of its size.

Amino Acid Sequence↗
PubMed1994

Solid-phase aggregation of proteins under pharmaceutically relevant conditions.

In order to successfully employ proteins as pharmaceuticals, it is essential to understand mechanistically the stability issues relevant to their formulation and delivery. Various deleterious processes may occur in protein formulations, thereby diminishing their therapeutic value. This review focuses upon one aspect of this problem, namely aggregation of solid proteins under pharmaceutically relevant conditions (elevated temperature and water activity). Strategies to pursue such studies are presented with an emphasis on a mechanistic analysis of aggregate formation. Both covalent and noncovalent aggregation pathways have been elucidated. Proteins that contain disulfide bonds as well as free thiol residues may aggregate via thiol-disulfide interchange. For proteins which contain disulfides but not free thiol residues, intermolecular disulfide bonding may still occur when intact disulfides undergo beta-elimination, yielding free thiols which can catalyze disulfide scrambling. Finally, proteins containing no cysteine/cystine residues may aggregate by other covalent pathways or by noncovalent routes. On the basis of these pathways, some rational stabilization strategies have been proposed and verified. Ultimately, application of this knowledge should lead to more stable and effective pharmaceutical protein formulations.

Chemical Phenomena↗
PubMed1994

Moisture-induced aggregation of lyophilized insulin.

A critical problem in the storage and delivery of pharmaceutical proteins is aggregation in the solid state induced by elevated temperature and moisture. These conditions are particularly relevant for studies of protein stability during accelerated storage or for proteins loaded in polymeric delivery devices in vivo. In the present investigation, we have found that, when exposed to an environment simulating these conditions, lyophilized insulin undergoes both covalent and noncovalent aggregation. The covalent process has been elucidated to be intermolecular thiol-catalyzed disulfide interchange following beta-elimination of an intact disulfide bridge in the insulin molecule. This process is accelerated by increasing the temperature and water content of the insulin powder or by performing lyophilization and/or dissolution of insulin in alkaline media. The aggregation can be ameliorated by the presence of Cu2+, which presumably catalyzes the oxidization of free thiols. The water sorption isotherm for insulin reveals that the extent of aggregation directly correlates with the water uptake by the lyophilized insulin powder, thus pointing to the critical role of protein conformational mobility in the aggregation process.

Animals↗
PubMed1993

Enzyme crystal structure in a neat organic solvent.

The crystal structure of the serine protease subtilisin Carlsberg in anhydrous acetonitrile was determined at 2.3 A resolution. It was found to be essentially identical to the three-dimensional structure of the enzyme in water; the differences observed were smaller than those between two independently determined structures in aqueous solution. The hydrogen bond system of the catalytic triad is intact in acetonitrile. The majority (99 of 119) of enzyme-bound, structural water molecules have such a great affinity to subtilisin that they are not displaced even in anhydrous acetonitrile. Of the 12 enzyme-bound acetonitrile molecules, 4 displace water molecules and 8 bind where no water had been observed before. One-third of all subtilisin-bound acetonitrile molecules reside in the active center, occupying the same region (P1, P2, and P3 binding sites) as the specific protein inhibitor eglin c.

Acetonitriles↗
PubMed1992

Solid-state NMR assessment of enzyme active center structure under nonaqueous conditions.

By using solid-state NMR spectroscopy, the integrity of the active center of alpha-chymotrypsin was investigated under a variety of nonaqueous conditions. Specifically, 13C cross-polarization/magic angle spinning NMR was used to analyze the ability of alpha-chymotrypsin to stabilize a transition state intermediate analog after freezing, drying, and addition of organic solvents (both anhydrous and hydrated) to the resultant powder. Lyophilization disrupted 42 +/- 5% of the active centers; it was determined that this occurred during drying, as opposed to freezing. Seven anhydrous solvents caused 0-50% additional disruption, which occurred immediately on addition of the solvent to the enzyme powder. The extent of structural integrity loss correlated with the solvent hydrophobicity, indicating that further dehydration, i.e. stripping of water retained by the enzyme during lyophilization, was the cause. Enzyme samples prepared with lyoprotecting additives, sucrose and ammonium sulfate, exhibited varying degrees of stabilization against the drying step of lyophilization. Moreover, when hydrophilic anhydrous solvents, which had the highest propensity to strip bound water, were added to the resultant enzyme powders, no additional damage occurred.

Amino Acid Sequence↗
PubMed1992

Alterations in the structure of proteins that cause their irreversible inactivation.

A variety of changes in environmental conditions (temperature, pH, salts, solvents, etc.) can cause protein inactivation. However, the mechanisms of irreversible protein inactivation often follow common pathways. Using heat stress as an example, the conformational and covalent processes leading to the irreversible thermo-inactivation of enzymes will be described. In particular, work in our laboratory has identified several chemical reactions which contribute to enzyme thermo-inactivation: cystine destruction, thiol-catalysed disulfide interchange, oxidation of cysteine residues, deamidation of asparagine and glutamine residues, and hydrolysis of peptide bonds at aspartic acid residues. Implications of this work for the stability of proteins during, and following, the lyophilization process will be addressed.

Amino Acids↗
PubMed1991

Kinetics of insulin aggregation in aqueous solutions upon agitation in the presence of hydrophobic surfaces.

The stability of protein-based pharmaceuticals (e.g., insulin) is important for their production, storage, and delivery. To gain an understanding of insulin's aggregation mechanism in aqueous solutions, the effects of agitation rate, interfacial interactions, and insulin concentration on the overall aggregation rate were examined. Ultraviolet absorption spectroscopy, high-performance liquid chromatography, and quasielastic light scattering analyses were used to monitor the aggregation reaction and identify intermediate species. The reaction proceeded in two stages; insulin stability was enhanced at higher concentration. Mathematical modeling of proposed kinetic schemes was employed to identify possible reaction pathways and to explain greater stability at higher insulin concentration.

Adsorption↗