Influence of glycerol on the structure and stability of ferric horse heart myoglobin: a SAXS and circular dichroism study

Structural insights into the effects of glycerol on ligand binding to cytochrome P450

New antitubercular drugs are vital due to the spread of resistant strains. Carbethoxyhexyl imidazole (CHImi) inhibits cytochrome P450 CYP124, which is a steroid-metabolizing enzyme that is important for the survival of Mycobacterium tuberculosis in macrophages. The available crystal structure of the CYP124-CHImi complex reveals two glycerol molecules in the active site. A 1.15 Å resolution crystal structure of the glycerol-free CYP124-CHimi complex reported here shows multiple conformations of CHImi and the CYP124 active site which were previously restricted by glycerol. Complementary molecular dynamics simulations show coherence of the ligand and enzyme conformations. Spectrophotometric titration confirmed the influence of glycerol on CHImi binding: the affinity decreases more than tenfold in glycerol-containing buffer. In addition, it also showed that glycerol has a similar effect on other azole and triazole CYP124 ligands. Together, these data show that glycerol may compromise structural-functional studies and impede rational drug-design campaigns.

Polyol additives modulate the in vitro stability and activity of recombinant human phenylalanine hydroxylase

Phenylketonuria (PKU; OMIM 261600), the most common disorder of amino acid metabolism, is caused by a deficient activity of human phenylalanine hydroxylase (hPAH). Although the dietetic treatment has proven to be effective in preventing the psycho-motor impairment, much effort has been made to develop new therapeutic approaches. Enzyme replacement therapy with hPAH could be regarded as a potential form of PKU treatment if the reported in vitro hPAH instability could be overcome. In this study, we investigated the effect of different polyol compounds, e.g. glycerol, mannitol and PEG-6000 on the in vitro stability of purified hPAH produced in a heterologous prokaryotic expression system. The recombinant human enzyme was stored in the presence of the studied stabilizing agents at different temperatures (4 and -20 degrees C) during a 1-month period. Protein content, degradation products, specific activity, oligomeric profile and conformational characteristics were assessed during storage. The obtained results showed that the use of 50% glycerol or 10% mannitol, at -20 degrees C, protected the enzyme from loss of its enzymatic activity. The determined DeltaG(0) and quenching parameters indicate the occurrence of conformational changes, which may be responsible for the observed increase in catalytic efficiency.

Local compressibilities of proteins: comparison of optical experiments and simulations for horse heart cytochrome-c

Spectroscopy with probe molecules yields local information on the environment of the probe. In this article we compare local compressibilities of cytochrome-c as obtained from molecular dynamics simulations with experimental results as obtained from spectroscopic measurements. The simulations show that the protein-core around the heme is much less compressible in a glycerol/water solvent than in pure water. The pocket is also much less compressible than the protein as a whole, although the compressibility of the water inside the rather incompressible protein-core is almost liquidlike. We show that the local compressibility values capture the collective correlations of local volume fluctuations with volume fluctuations in the surrounding protein-solvent system. The decoupling of the volume fluctuations of the core from the solvent shell explains the reduction of the heme-core-compressibility in glycerol/water solvent. This decoupling could be traced back to the suppression of the exchange between pocket-water and hydration-shell-water upon addition of glycerol as co-solvent.

Stability of proteins: Temperature, pressure and the role of the solvent

We focus on the various aspects of the physics related to the stability of proteins. We review the pure thermodynamic aspects of the response of a protein to pressure and temperature variations and discuss the respective stability phase diagram. We relate the experimentally observed shape of this diagram to the low degree of correlation between the fluctuations of enthalpy and volume changes associated with the folding-denaturing transition and draw attention to the fact that one order parameter is not enough to characterize the transition. We discuss in detail microscopic aspects of the various contributions to the free energy gap of proteins and put emphasis on how a cosolvent may either enlarge or diminish this gap. We review briefly the various experimental approaches to measure changes in protein stability induced by cosolvents, denaturants, but also by pressure and temperature. Finally, we discuss in detail our own molecular dynamics simulations on cytochrome c and show what happens under high pressure, how glycerol influences structure and volume fluctuations, and how all this compares with experiments.

An electrochemical study of hemoglobin in water–glycerol solutions

The effect the composition of a water-glycerol mixture has on the electrochemical properties of hemoglobin (Hb) is studied. With the increased glycerol concentrations, the peak-to-peak separation of hemoglobin is found to increase from approximately 40 to 200 mV, with the apparent standard potential of hemoglobin negatively shifted, which demonstrate that the electron-transfer activity of hemoglobin will decrease at relatively high glycerol concentrations and the oxidized state of hemoglobin will be more stable with the increasing glycerol concentrations. Meanwhile, the electrocatalytic activity of hemoglobin to hydrogen peroxide, as well as the binding of ligands or effectors to hemoglobin in the presence of glycerol, are also been investigated. Our studies indicate that glycerol will decrease the electrocatalytic activity of hemoglobin, while have little effect on the microenvironment around the heme site.

Conformational substates of the oxyheme centers in alpha and beta subunits of hemoglobin as disclosed by EPR and ENDOR studies of cryoreduced protein

Exposure of frozen solutions of oxyhemoglobin to gamma-irradiation at 77 K yields EPR- and ENDOR-active, one-electron-reduced oxyheme centers which retain the conformation of the diamagnetic precursor. EPR spectra have been collected for the centers produced in human HbO(2) and isolated alphaO(2) and betaO(2) chains, as well as alphaO(2)beta(Zn), alpha(Zn)betaO(2), and alphaO(2)beta(Fe(3+)) hybrids, each in frozen buffer and in frozen glasses that form in the presence of glycols and sugars and also in the presence of IHP. These reveal two spectroscopically distinct classes of such ferriheme centers (g(1) <or= 2.25), denoted A and B. Averaged over many similar sites, the A-center has a rhombic EPR signal with a g-tensor, g(A) = [2.248(4), 2.146(1), 1.966(1)]; the B-center exhibits a less anisotropic EPR signal, g(B) = [2.216(3), 2.118(2), 1.966(1)]. Early measurement had suggested that, in the cryoreduced HbO(2) tetramer, the two centers corresponded to the two different chains [Symons, M. C. R., and Petersen, R. L. (1978) Proc. R. Soc. London, Ser. B 201, 285-300]. However, the present EPR and ENDOR results show that the two signals instead reflect the fact that the parent oxyhemes exist in two major conformational substates and that this is true for both alphaO(2) and betaO(2) subunits: alphaO(2)(A) (minor species) and alphaO(2)(B) (major species); betaO(2)(A)(major species) and betaO(2)(B) (minor species). Similar behavior is seen for MbO(2) [Kappl, R., Höhn-Berlage, M., Hüttermann, J., Bartlett, N., and Symons, M. C. R. (1985) Biochim. Biophys. Acta 827, 327-343]. The A/B g-tensors of alphaO(2) and betaO(2) chains vary little with the environment of the chains, while the relative populations of the substates depend greatly on glycols and IHP. These results suggest a quaternary influence on the oxyheme distal pocket of alpha chains and that the glycol-induced changes in the substate populations of the R-state HbO(2) tetramer are largely associated with the alphaO(2) subunit. (1)H ENDOR spectra from the distal histidine proton hydrogen-bonded to the peroxo ligand show very different isotropic coupling for the A- and B-centers. Analysis of the spectroscopic data suggests that the A- and B-centers represent different orientations of the oxyheme O(2) ligand relative to the distal histidine. It is likely that the A and B conformational substates in the alphaO(2) and betaO(2) subunits differ not only in their tertiary structures but in their affinities for O(2).

The optical interconversion of the P-450 and P-420 forms of neuronal nitric oxide synthase: effects of sodium cholate, mercury chloride and urea

We investigated whether or not neuronal nitric oxide synthase (nNOS) (EC 1.14.13.39) was converted to the P-420 form on exposure to sodium cholate, mercury chloride or urea, and the reconversion of the P-420 to the P-450 form. Sodium cholate and mercury chloride induced the conversion of nNOS from the P-450 to the P-420 form in concentration- and incubation time-dependent manners, and the nNOS activity decreased. In the presence of glycerol, L-arginine and/or tetrahydrobiopterin, the sodium cholate-treated P-420 form could be reconverted to the P-450 form under constant experimental conditions, and the nNOS activity could also be restored. The mercury chloride-treated P-420 form of nNOS could be reconverted to the P-450 form on incubation with reduced glutathione (GSH) or L-cysteine, and the nNOS activity was recovered. However, no reconversion of the mercury chloride-treated P-420 form to the P-450 form was observed in the presence of glycerol, L-arginine, or tetrahydrobiopterin. Urea (4.0 M) dissociated nNOS into its subunits, but nNOS remained in the P-450 form. The nNOS monomer was more susceptible to sodium cholate. After removing the urea by dialysis, and supplementation of the nNOS solution with glycerol, L-arginine or BH(4), the P-420 was reconverted to the P-450 form, and the reassociation of nNOS monomers was also observed. These results suggested that nNOS was more stable as to exposure to sodium cholate, mercury chloride or urea in comparison to microsomal cytochrome P-450, which may be due to the different heme environment and protein structure.

Adsorbed protein secondary and tertiary structures by circular dichroism and infrared spectroscopy with refractive index matched emulsions

The secondary structure of protein adsorbed at the emulsion interface has been studied in refractive index matched emulsions using the techniques of circular dichroism (CD) and Fourier transform infrared spectroscopy. Bovine serum albumin (BSA) and bovine beta-lactoglobulin (betalg) stabilized emulsions were studied, and the refractive index was altered by the addition of glycerol or polyethylene glycol. The effect of additive on the solution and adsorbed protein structure in addition to the effect of adsorption time was considered. Both adsorption and glycerol addition alter protein secondary structure; however, the majority of secondary structure remains. Small changes are observed in the secondary structure of adsorbed protein with time. Near-ultraviolet CD studies showed the effect of glycerol and adsorption on the aromatic groups. BSA showed small changes both upon the addition of glycerol to protein in solution and upon adsorption. betalg showed slightly larger changes upon the addition of glycerol to protein in solution and a larger change upon adsorption.

Instability, stabilization, and formulation of liquid protein pharmaceuticals

One of the most challenging tasks in the development of protein pharmaceuticals is to deal with physical and chemical instabilities of proteins. Protein instability is one of the major reasons why protein pharmaceuticals are administered traditionally through injection rather than taken orally like most small chemical drugs. Protein pharmaceuticals usually have to be stored under cold conditions or freeze-dried to achieve an acceptable shelf life. To understand and maximize the stability of protein pharmaceuticals or any other usable proteins such as catalytic enzymes, many studies have been conducted, especially in the past two decades. These studies have covered many areas such as protein folding and unfolding/denaturation, mechanisms of chemical and physical instabilities of proteins, and various means of stabilizing proteins in aqueous or solid state and under various processing conditions such as freeze-thawing and drying. This article reviews these investigations and achievements in recent years and discusses the basic behavior of proteins, their instabilities, and stabilization in aqueous state in relation to the development of liquid protein pharmaceuticals.

Structural properties of human glycodelin A in water and in water-alcohol mixtures: a comparison with bovine beta-lactoglobulin A

Human glycodelin A (GdA) is a glycoprotein that is highly homologous to bovine beta-lactoglobulin A (beta-LgA) because the amino acid sequences are 50-60% identical. The structural characteristics of human GdA and beta-LgA were compared in water and 2-propanol/water solutions. Circular dichroism spectra reveal that in water the two proteins have a very similar beta-sheet secondary structure. In the presence of 2-propanol/water mixtures (up to 50% v/v) the alpha-helix structure of both proteins increases. A further increase in the alcohol percentage of the solvent (up to 80% v/v 2-propanol) causes the formation of a new folded tertiary structure containing mainly beta-sheet features. Synchrotron radiation small angle X-ray scattering indicates that, in a neutral pH aqueous solution, GdA is a dimer. Its radius of gyration value (Rg), 25.1+/-0.4 A, is greater than that of beta-LgA (21.1+/-0.3 A), probably because of the contribution of polysaccharides bound to Asn-28 and Asn-63 residues of GdA. Conversely, small angle X-ray scattering and gel permeation chromatography data on GdA in 2-propanol have revealed a massive aggregation of the protein.

New stable folding of beta-lactoglobulin induced by 2-propanol

beta-lactoglobulin A has been studied in 2-propanol-water mixtures by means of circular dichroism, fluorescence and small angle X-ray scattering. At a low ionic strength, 2-propanol induces an increase in alpha-helix structure followed by a further transformation which gives rise to a new feature, rich of beta-sheet fragments. The second step of the secondary structure transformation is time-dependent and depressed at high ionic strength. As a consequence, the tertiary structure is completely modified and a new stable protein folding may be hypothesized. Small angle X-ray scattering measurements reveal that 2-propanol induces a diffuse protein aggregation, but the complex equilibria among intra- and inter-molecular hydrophobic and electrostatic interactions may be modulated by balancing the ionic strength and/or alcohol percentage.

Influence of glycerol on the structure and redox properties of horse heart cytochrome c. A circular dichroism and electrochemical study

The effect of glycerol on the structure and redox properties of horse heart cytochrome c was investigated by absorption spectroscopy, circular dichroism, and dc cyclic voltammetry techniques. The results show that the organic solvent increases the alpha-helix structure of the protein and induces slight changes at the active-site environment: however, the overall tertiary structure does not appear to be significantly perturbed. Glycerol stabilizes cytochrome c, the free energy of denaturation (delta G0) being approximately 0.7 kcal/mol larger than that determined in phosphate buffer under the same conditions, and influences the heterogeneous electron transfer kinetics at a chemically modified gold electrode: on the other hand, the redox potential of the protein is unaltered. On the whole, the results obtained indicate that glycerol acts as a suitable stabilizing agent of cytochrome c, which is of interest for application in biotechnology: the organic solvent does not alter the tertiary structure significantly or the redox properties of the protein. This has to be interpreted not only in terms of the glycerol-induced solvent ordering around the protein surface, but also as due to the specific features of the protein matrix.