Myristic acid binding to human serum albumin investigated by dialytic exchange rate

Stable Mammalian Serum Albumins Designed for Bacterial Expression

Albumin is the most abundant protein in the blood serum of mammals and has essential carrier and physiological roles. Albumins are also used in a wide variety of molecular and cellular experiments and in the cultivated meat industry. Despite their importance, however, albumins are challenging for heterologous expression in microbial hosts, likely due to 17 conserved intramolecular disulfide bonds. Therefore, albumins used in research and biotechnological applications either derive from animal serum, despite severe ethical and reproducibility concerns, or from recombinant expression in yeast or rice. We use the PROSS algorithm to stabilize human and bovine serum albumins, finding that all are highly expressed in E. coli. Design accuracy is verified by crystallographic analysis of a human albumin variant with 16 mutations. This albumin variant exhibits ligand binding properties similar to those of the wild type. Remarkably, a design with 73 mutations relative to human albumin exhibits over 40 °C improved stability and is stable beyond the boiling point of water. Our results suggest that proteins with many disulfide bridges have the potential to exhibit extreme stability when subjected to design. The designed albumins may be used to make economical, reproducible, and animal-free reagents for molecular and cell biology. They also open the way to high-throughput screening to study and enhance albumin carrier properties.

Molecular analysis and therapeutic applications of human serum albumin-fatty acid interactions

Human serum albumin (hSA) is the major carrier protein for fatty acids (FAs) in plasma. Its ability to bind multiple FA moieties with moderate to high affinity has inspired the use of FA conjugation as a safe and natural platform to generate long-lasting therapeutics with enhanced pharmacokinetic properties and superior efficacy. In this frame, the choice of the FA is crucial and a comprehensive elucidation of the molecular interactions of FAs with hSA cannot be left out of consideration. To this intent, we report here a comparative analysis of the binding mode of different FA moieties with hSA. The choice among different albumin-binding FAs and how this influence the pharmacokinetics properties of a broad spectrum of therapeutic molecules will be discussed including a critical description of some clinically relevant FA conjugated therapeutics.

Design, synthesis and biological evaluation of novel aryldiketo acids with enhanced antibacterial activity against multidrug resistant bacterial strains

Antimicrobial resistance (AMR) is a major health problem worldwide, because of ability of bacteria, fungi and viruses to evade known therapeutic agents used in treatment of infections. Aryldiketo acids (ADK) have shown antimicrobial activity against several resistant strains including Gram-positive Staphylococcus aureus bacteria. Our previous studies revealed that ADK analogues having bulky alkyl group in ortho position on a phenyl ring have up to ten times better activity than norfloxacin against the same strains. Rational modifications of analogues by introduction of hydrophobic substituents on the aromatic ring has led to more than tenfold increase in antibacterial activity against multidrug resistant Gram positive strains. To elucidate a potential mechanism of action for this potentially novel class of antimicrobials, several bacterial enzymes were identified as putative targets according to literature data and pharmacophoric similarity searches for potent ADK analogues. Among the seven bacterial targets chosen, the strongest favorable binding interactions were observed between most active analogue and S. aureus dehydrosqualene synthase and DNA gyrase. Furthermore, the docking results in combination with literature data suggest that these novel molecules could also target several other bacterial enzymes, including prenyl-transferases and methionine aminopeptidase. These results and our statistically significant 3D QSAR model could be used to guide the further design of more potent derivatives as well as in virtual screening for novel antibacterial agents.

Fatty acid binding into the highest affinity site of human serum albumin observed in molecular dynamics simulation

Multiple molecular dynamics simulations were performed to investigate the association of stearic acid into the highest affinity binding site of human serum albumin. All binding events ended with a rapid (<10 ps) lock-in of the fatty acid due to formation of a hydrogen bond with Tyr401. The kinetics and energetics of the penetration process both depended linearly on the positional shift of the fatty acid, with an average insertion time and free energy reduction of, respectively, 32 ± 20 ps and 0.70 ± 0.15 kcal/mol per methylene group absorbed. Binding events of longer duration (tbind>1 ns) were characterized by a slow exploration of the pocket entry and, frequently, of a nearby protein crevice corresponding to a metastable state along the route to the binding site. Taken all together, these findings reconstruct the following pathway for the binding process of stearic acid: (i) contact with the protein surface, possibly facilitated by the presence of an intermediate location, (ii) probing of the site entry, (iii) insertion into the protein, and (iv) lock-in at the final position. This general description may also apply to other long-chain fatty acids binding into any of the high-affinity sites of albumin, or to specific sites of other lipid-binding proteins.

Identification of high affinity fatty acid binding sites on human serum albumin by MM-PBSA method

Human serum albumin (HSA) has seven common fatty acid (FA) binding sites. In this study, we used the molecular mechanics Poisson-Boltzmann surface area method to identify high affinity FA binding sites on HSA in terms of binding free energy. Using multiple HSA-FA (myristate, palmitate) complex models constructed by molecular dynamics simulations, two methods were performed in molecular mechanics Poisson-Boltzmann surface area, the "three-trajectory method" and the "single-trajectory method". The former, which is less precise than the latter but may be more accurate as it includes the effects of conformational change upon binding, was used to classify high and low affinity sites. As a result, Sites 2, 4, and 5 were identified as high affinity sites for both FAs. The latter method, which is precise because energies are calculated from snapshots of the same trajectory for HSA-FA complex, was performed to compare the magnitude of binding free energy for these sites. The order of magnitude was 5 > 4 > 2, identical to that of a previous publication by others. In this way, a combination of the two methods was effectively used to identify high affinity sites. This study therefore provides an insight into the quantitative identification of high affinity FA binding sites on HSA.

Chain Length-dependent Binding of Fatty Acid Anions to Human Serum Albumin Studied by Site-directed Mutagenesis

Human serum albumin is the most abundant protein in the circulatory system, and one of its principal functions is to transport fatty acids. Binding of octanoate, decanoate, laurate and myristate was studied by a rate-of-dialysis technique. The primary association constants increased, but not linearly, with chain length. The number of high-affinity sites also increased with chain length; octanoate and decanoate bind to one such site, whereas laurate and myristate most probably bind to two sites. Albumin is composed of three homologous helical domains (I-III), which can be subdivided into two subdomains (A and B). For getting information about the positions of the high-affinity sites we produced 13 recombinant isoforms mutated in four different subdomains. Results obtained with these albumins are in accordance with the following model: octanoate and decanoate bind to a single site in subdomain IIIA, laurate binds to sites in subdomains IIIA and IIIB, whereas myristate binds in subdomains IB and IIIB. The results also showed that primary fatty acid binding is sensitive to amino acid substitutions in other parts of the protein. This is in contrast to the effect of amino acid substitutions of genetic albumin variants (alloalbumins). Usually these substitutions, which are situated at the surface of the protein, have no effect on fatty acid binding. Binding of fatty acid anions to different high-affinity sites and the sensitivity of these sites to amino acid substitutions elsewhere in the protein (and perhaps also to other types of modifications) are important factors that could effect simultaneous binding of other ligands, e.g. in patients treated with albumin-binding drugs.

Detergent binding as a sensor of hydrophobicity and polar interactions in the binding cavities of proteins

To evaluate the role of hydrophobic and electrostatic or other polar interactions for protein-ligand binding, we studied the interaction of human serum albumin (HSA) and beta-lactoglobulin with various aliphatic (C10-C14) cationic and zwitterionic detergents. We find that cationic detergents, at levels that do not cause unfolding, interact with a single site on beta-lactoglobulin and with two primary and five to six secondary sites on HSA with an affinity that is approximately the same as that with which zwitterionic (dimethylamineoxide) detergents interact, suggesting the absence of significant electrostatic interactions in the high-affinity binding of these compounds. The binding affinity for all of the groups of compounds was dependent upon hydrocarbon chain length, suggesting the predominant role of hydrophobic forces, supported by polar interactions at the protein surface. A distinct correlation between the binding energy and the propensity for micelle formation within the group of cationic or noncharged (nonionic and zwitterionic) detergents indicated that the critical micellar concentration (CMC) for each of these detergent groups, rather than the absolute length of the hydrocarbon chain, can be used to compare their hydrophobicities during their interaction with protein. Intrinsic fluorescence data suggest that the two primary binding sites on serum albumin for the zwitterionic and cationic compounds are located in the C-terminal part of the albumin molecule, possibly in the Sudlow II binding region. Comparisons with previous binding data on anionic amphiphiles emphasize the important contribution of ion bond formation and other polar interactions in the binding of fatty acids and dodecyl sulfate (SDS) by HSA but not by beta-lactoglobulin. Electrostatic interactions by cationic detergents played a significant role in destabilizing the protein structure at high binding levels, with beta-lactoglobulin being more susceptible to unfolding than HSA. Zwitterionic detergents, in contrast to the cationic detergents, had no tendency to unfold the proteins at high concentrations.

Expression of recombinant psoriasis-associated fatty acid binding protein in Escherichia coli: gel electrophoretic characterization, analysis of binding properties and comparison with human serum albumin

The psoriasis-associated fatty acid binding protein (PA-FABP, also known as FABP5) is a novel keratinocyte protein that is highly up-regulated in psoriatic plaques (P. Madsen, H. H. Rasmussen, H. Leffers, B. Honoré and J. E. Celis, J. Invest. Dermatol. 1992, 99, 299-305). Here we have expressed PA-FABP in Escherichia coli as a fusion protein containing an NH2-terminal hexa-His tag followed by a factor Xa cleavage site. The recombinant protein was expressed at a level of about 30% of the soluble proteins and was purified to homogeneity using a simple two-step protocol consisting of affinity chromatography on Ni2+-nitrilotriacetic acid agarose followed by gel filtration. The recombinant protein was then digested with factor Xa and characterized by two-dimensional gel electrophoresis. The ability of PA-FABP to bind saturated fatty acids ranging from 6 to 16 carbons was determined directly by dialysis and compared to human serum albumin (HSA). The results showed that PA-FABP binds multiple molecules of the fatty acids hexanoate (C6:0), octanoate (C8:0), decanoate (C10:0) and laurate (C12:0), all with a K1 of about 10(4) M(-l), and myristate (C14:0) with a K1 of 4.4 X 10(5) M(-l). Palmitate (C16:0) also bound strongly with multiple molecules. Due to the very low solubility of palmitate its affinity to PA-FABP was measured relatively to HSA and found to be 8.1 times lower. At ligand/protein ratios below 1, all fatty acids bound to PA-FABP with about one to three orders of magnitude lower affinity than to HSA. The difference in the fatty acid binding properties of the two proteins may reflect differences in their three-dimensional structures, which in the case of PA-FABP consists mainly of beta-sheets while HSA contains predominantly alpha-helices.

Palmitate and stearate binding to human serum albumin. Determination of relative binding constants

Multiple binding equilibria of two apparently insoluble ligands, palmitate and stearate, to defatted human serum albumin were studied in a 66 mM sodium phosphate buffer (pH 7.4) at 37 degrees C, by determination of dialytic exchange rates of ligands among identical equilibrium solutions. The experimental data were analysed by a computerised curve fitting procedure using equilibrium equations for multiple binding of ligands, containing relative binding constants, valid whether the ligands are truly insoluble or are slightly soluble and irrespective of aggregation in aqueous solution. A best-fit set of relative binding constants was found, and subsequently 30 sets of acceptable constants for each set of data in order to evaluate the variation. The data were first fitted by the relative Scatchard's equation, then by the relative, stoichiometric equation. Scatchard's equation is deduced on the presumption that cooperativity is absent while the stoichiometric equation is valid even when cooperativity is present. It was found with palmitate as well as with stearate that the two equations fitted the data equally well, and it was concluded that the observations were compatible with absence of cooperativity. The relative Scatchard binding constants were converted to relative, stoichiometric constants and it was found that the variations of the latter were slight.

Effects of ionic strength and pH on the binding of medium-chain fatty acids to human serum albumin

Binding equilibria for the interactions of the medium-chain fatty acid anions, laurate and myristate, with defatted human serum albumin have been investigated under varying environmental conditions such as ionic strength and pH. Since these ligands bind strongly to albumin (Kass approximately 10(7) M-1), conventional equilibrium dialysis is not a feasible method for these investigations. Consequently, we employed a dialysis method, allowing determination of very low concentrations of unbound ligand by measuring the rate of exchange of labelled ligand across a dialysis membrane under conditions of chemical equilibrium. Over a range of ionic strength, 8-68 mM, the binding of the first few molecules of laurate to albumin was weakened with increasing ionic strength, whereas the binding of subsequent molecules seemed to proceed independently of ionic strength. The binding of myristate by albumin, however, appeared to be independent of ionic strength in the observed range of concentrations. The influence of pH in the range 5.1-9.0 on the binding of the two fatty acid anions by albumin was more complicated. The first molecule of laurate appeared to bind with a slightly weaker affinity to albumin at low pH, compared to pH 7 and high pH, while the trends for the following molecules varied. The binding of myristate (irrespective of concentration) seemed to strengthen monotonously with pH, but this conclusion depends critically on the interpretation of the kinetic behaviour of the myristate anion. We have previously shown [Pedersen, A. O., Honoré, B. & Brodersen, R. (1990) Eur. J. Biochem. 190, 497-502] that the strength of binding of the first few molecules of the two fatty acid anions to albumin decreases with increasing temperature, whereas binding of subsequent molecules seems to proceed independently of temperature. We explain these findings as follows. The binding of the first few (3 or 4) molecules of the C12 laurate anion is clearly driven by formation of ionic bonds between the fatty acid anion and positively charged groups, such as lysine residues, in the albumin molecule, whereas the binding of subsequent molecules of laurate seems to depend more on hydrophobic interactions. In the case of the C14 myristate anion, the binding of the first few (only 1 or 2) molecules may depend on ionic forces, but binding of the following molecules of myristate seems to depend on hydrophobic interactions only.(ABSTRACT TRUNCATED AT 250 WORDS)

IGF-II receptors in luminal and basolateral membranes isolated from pars convoluta and pars recta of rabbit proximal tubule

The binding of 125I-labeled insulin-like growth factor-II (125I-IGF-II) to luminal and basolateral membrane vesicles isolated from pars convoluta and the straight part (pars recta) of rabbit proximal tubule was investigated. Analyses of the binding data by use of the general stoichiometric binding equation revealed, that in all preparations IGF-II was bound to one high-affinity binding site and other sites with lower affinities. The specificity of the high-affinity 125I-IGF-II binding to the membrane vesicles assessed by displacement by unlabeled IGF-II, IGF-I and insulin showed that IGF-I displaced 125I-IGF-II in the range 22.5-47.9 nM (IC50) whereas insulin did not effect 125I-IGF-II binding at all. beta-Galactosidase inhibited the 125I-IGF-II binding with half-maximal inhibition of 20-30 nM beta-galactosidase. D-Mannose 6-phosphate increased the binding of 125I-IGF-II and reversed the inhibitory effect of beta-galactosidase. Analyses of 125I-IGF-II binding curves in the presence of beta-galactosidase or D-mannose 6-phosphate demonstrated that none of these compounds changed the binding affinity of 125I-IGF-II for the membrane vesicles. The IGF-II/M6P receptor content in the luminal membranes was in the range 0.21-0.34 pmol IGF-II/M6P receptor per mg protein and very low compared to 2.27-2.86 pmol IGF-II/M6P receptor per mg protein in basolateral membranes.

Long-chain fatty acid-binding to albumin: re-evaluation with directly measured concentrations

In studies on uptake of fatty acids (FA) into organs, the unbound (or free) fatty acid fraction is commonly calculated from the concentration bound to albumin and from published binding constants. However, there is some dispute on the methods used for determining those binding constants. We developed a method allowing direct measurement of unbound FA by extending the previous studies of Svenson et al. [1] and Reed et al. [2]. Albumin was coupled to a solid phase (Sepharose 4B), loaded with FA and equilibrated with an aqueous solution. Laurate, palmitate and oleate concentrations in the aqueous phase were determined at different molar ratios of FA to albumin (r) and at different temperatures. FA albumin-binding constants (Ki) increase with chain length and decrease with temperature, in accordance with data obtained by others. However, the unbound concentrations measured are markedly lower than those obtained from binding constants, and the resulting Ki values markedly higher. This difference is presumed to result from (1) our direct measurement of unbound FA and (2) utilizing different more physiological conditions. Recalculating kinetic parameters from published FA uptake data, we found considerably different Km and Vmax values compared to the original data. Thus, the FA-binding characteristics measured in this study may influence the interpretation of FA uptake substantially.

Thermodynamic characterization of drug binding to human serum albumin by isothermal titration microcalorimetry

Binding sites on human serum albumin (HSA) for anionic drugs and fatty acids have been thermodynamically characterized by microcalorimetry. The binding and the thermodynamic parameters were directly computed from the calorimetric titration data at 37 degrees C in a phosphate buffer (pH 7.4) using one- and two-class binding models. From compensation analyses plotting the molar enthalpy change (delta Hm,i) versus those of the molar free energy (delta Gm,i) and molar entropy (delta Sm,i) for each class of binding sites, HSA binding sites were classified into groups S1, S2, and S3. Group S1 included high-affinity binding sites for site II-bound drugs, such as ibuprofen, flufenamic acid, and ethacrynic acid, and short- or medium-length alkyl-chain fatty acids; group S2 included low-affinity binding sites of site II-bound drugs and long-length alkyl-chain fatty acids; and group S3 contained the high-affinity binding sites for site I-bound drugs, such as phenylbutazone, oxphenbutazone, and warfarin, and long-length alkyl-chain fatty acids. High- and low-affinity bindings sites for salicylic acid and acetylaslicylic acid agreed with the regions of groups S3 and S2, respectively. Groups S1 and S2 were characterized by large negative values of delta Hm,i and delta Sm,i, reflecting van der Waals interaction and hydrogen-bonding formation in low dielectric media, and the main force to stabilize the binding complex in group S3 was a hydrophobic interaction, characterized by a small negative delta Hm,i and minor or positive values of delta Sm,i (entropy-driven).

Biothermodynamic characterization of monocarboxylic and dicarboxylic aliphatic acids binding to human serum albumin: a flow microcalorimetric study

Thermodynamic parameters have been evaluated for the binding of unbranched monocarboyxlic aliphatic acids (MCAs) of 4 to 16 carbons (MC4 to MC16) and dicarboxylic aliphatic acids (DCAs) of 4 to 16 carbons (DC4 to DC16) to human serum albumin (HSA) on the basis of microcalorimetric measurement at pH 7.4 and 37 degrees C by computer-fitting to single- and two-class binding models. Long-chain MCAs (MC10 to MC16) and DCAs (DC14 and DC16) had the first class of binding sites with high affinity (large binding constant) of 10(5) to 10(6) M-1 and the second class with lower affinity and high capacity (large numbers of binding sites). Short- or medium-chain MCAs and DCAs bound to HSA at some low affinity binding sites. The binding constants of MCAs were ten times larger than those of DCAs. All the relationships between the thermodynamic parameters and alkyl-chain length of the acids showed clear-cut inflections in their plots around eight or nine methylene units. The free energy change of the first class of binding sites (- delta G1) became more negative with an increment of -1.0 kJ mol-1 CH2(-1) as the alkyl-chain length increased, but there were steep rises between MC9 and MC11 with -2.90 kJ mol-1 CH2(-1) and between DC9 and DC12 with -2.02 kJ mol-1 CH2(-1). The enthalpy change (- delta H) increased at the rate of -7.4 kJ mol-1 CH2(-1) to the maximum at MC9 and DC10, then decreased due to hydrophobicity of the alkyl-chains. From compensation analyses (delta H vs. delta S and delta G), HSA binding sites were characterized into three groups.

Fatty acid and drug binding to a low-affinity component of human serum albumin, purified by affinity chromatography

Binding equilibria for decanoate to a defatted, commercially available human serum albumin preparation were investigated by dialysis exchange rate determinations. The binding isotherm could not be fitted by the general binding equation. It was necessary to assume that the preparation was a mixture of two albumin components about 40% of the albumin having high affinity and about 60% having low affinity. By affinity chromatography we succeeded in purifying the low-affinity component from the mixture. The high-affinity component, however, could not be isolated. We further analyzed the fatty acid and drug binding abilities of the low-affinity component. The fatty acids decanoate, laurate, myristate and palmitate were bound with higher affinity to the mixture than to the low-affinity component. Diazepam was bound with nearly the same affinity to the low-affinity component as to the albumin mixture, whereas warfarin was not bound at all to the low-affinity component.

Solubility of long-chain fatty acids in phosphate buffer at pH 7.4

The solubility of the saturated fatty acids lauric, myristic, palmitic, and stearic acid and the unsaturated oleic acid at 37 degrees C in phosphate buffer (pH 7.4) was estimated by using two independent methods. The one was a conventional solubility technique measuring the concentration of dissolved fatty acid in buffer by using radioactive compounds. The other was a dialysis exchange technique monitoring possible aggregation of solvated fatty acid anions by measuring the rate of diffusion of labelled compound across a dialysis membrane under conditions of chemical equilibrium. It was found that the results were strongly dependent on the radiochemical purity of the fatty acids. Using highly purified samples of radioactively labelled fatty acids, the solubility of monomeric laurate was shown to be greater than 500 microM, whereas the solubility of monomeric myristate was found to be 20-30 microM. Palmitate, stearate, and oleate solutions, on the other hand, showed a tendency to aggregation even at concentrations below 1 microM. Special attention was given to palmitate, as a reference compound for long-chain fatty acids, and the solubility of monomeric palmitate was estimated to be lower than 10(-10) M.

Thermodynamic parameters for binding of fatty acids to human serum albumin

Binding of laurate and myristate anions to human serum albumin has been studied over a range of temperatures, 5-37 degrees C, at pH 7.4. The binding curves indicate that the strength of binding of the first few molecules of fatty acid to albumin (r less than 5) decreases with increasing temperature, whereas binding of the following molecules seems to proceed independently of temperature. Binding data were analyzed according to the general binding equation yielding several sets of acceptable binding constants within a probability limit of 0.75. From the temperature dependence of the first step constant, it was possible to calculate values for the changes in enthalpy and entropy during the initial binding step. For the medium-chain fatty acids, laurate and myristate, binding of the first molecule to albumin appeared to be enthalpic, with a tendency to an increasing contribution of entropy to binding energy with increasing chain length of the fatty acid.

Serum albumin binding of palmitate and stearate. Multiple binding theory for insoluble ligands

In usual studies of ligand binding to a carrier, free and bound ligand concentrations are measured in equilibrium mixtures with varying carrier and ligand concentrations. The observed data are then analyzed by a binding equation such as Scatchard's or the general binding equation. With palmitic, stearic and oleic acids as ligands we found that the aqueous solubility is too low to allow this procedure. We have consequently transformed the general binding equation so that it does not contain parameters related to aqueous solutions of the ligand. While the classical binding equations describe affinities of transfer of a ligand from an aqueous solution to the carrier, the new equation is valid for transfer of a ligand from one bound state to another, i.e. for relative binding description. The relative binding constants, L1, L2, L3 ... Li, in the new equation thus define the transfer affinity for the ligand from a 1:1 complex with a standard carrier to an i:1 complex of the ligand with the carrier investigated. Binding of palmitate and stearate to human serum albumin was studied by determination of dialytic exchange rates between identical fatty acid/albumin solutions. The results were analyzed by the new equation without reference to ligands in aqueous solution.