A comparative study of the conformational properties of Escherichia coli-derived rat intestinal and liver fatty acid binding proteins

Excreta biomarkers in response to different gut barrier dysfunction models and probiotic supplementation in broiler chickens

Increased intestinal permeability (IP) and inflammation are both linked with functionality of the intestinal barrier and in particular enterocytes. Currently, almost all assessment methods of the intestinal barrier function are invasive. The present study aimed to quantify selected proteins as novel biomarkers in excreta of broiler chickens to facilitate non-invasive assessment of gut barrier function using enzyme-linked immunosorbent assays (ELISA). It was further hypothesised that probiotics as feed additives may counteract gut barrier dysfunction. A 3 × 2 factorial arrangement of treatments was used with the main factors being gut barrier dysfunction models (control, rye-based diet, and dexamethasone-DEX) with and without probiotic supplementation (a three-strain Bacillus) using 72 male Ross 308 day-old chickens. Each of the 6 experimental treatments was replicated 12 times. On d 21 of age, fluorescein isothiocyanate dextran (FITC-d) uptake into serum was examined to test IP. Fresh excreta samples were collected on d 20. The biomarkers included alpha-1 antitrypsin (A1AT), intestinal fatty acid binding protein (I-FABP), lipocalin-2 (LCN2), fibronectin (FN), intestinal alkaline phosphatase (IAP), ovotransferrin (OVT) and superoxide dismutase [Cu-Zn] (SOD1). Only DEX increased (P<0.001) FITC-d passage to the blood on d 21 of age, indicating a greater IP. The excreta concentrations of A1AT, I-FABP and SOD1 were unaltered by the experimental treatments. DEX increased (P<0.05) FN concentration in excreta compared with control birds. Conversely, inclusion of rye in the diet reduced (P<0.05) FN but increased (P<0.001) OVT in excreta. Independently, DEX decreased IAP (P<0.05) in excreta compared with control and rye-fed birds. The excreta concentration of LCN2 tended (P = 0.086) to increase in birds injected by DEX. There was no demonstrable effect of probiotic addition on any of the studied parameters. Among the tested biomarkers, FN, IAP, and LCN2 revealed promise as biomarkers of intestinal barrier function quantified by ELISA kits.

Characterization of fatty acid binding and transfer from Δ98Δ, a functional all-β abridged form of IFABP

Intestinal fatty acid binding protein (IFABP) is an intracellular lipid binding protein whose specific functions within the cell are still uncertain. An abbreviated version of IFABP encompassing residues 29-126, dubbed Δ98Δ is a stable product of limited proteolysis with clostripain of holo-IFABP. Cumulative evidence shows that Δ98Δ adopts a stable, monomeric and functional fold, with compact core and loose periphery. In agreement with previous results, this abridged variant indicates that the helical domain is-not necessary to preserve the general topology of IFABP's β-barrel and that the helix-turn-helix motif is a fundamental element of the portal region involved in ligand binding and protein-membrane interactions. Results presented here suggest that Δ98Δ binds fatty acids with affinities lower than IFABP but higher than those shown by previous helix-less variants, shows a 'diffusional' fatty acid transfer mechanism and it interacts with artificial membranes. This work highlights the importance of the β-barrel of IFABP for its specific functions.

Liver fatty acid-binding protein and obesity

While low levels of unesterified long chain fatty acids (LCFAs) are normal metabolic intermediates of dietary and endogenous fat, LCFAs are also potent regulators of key receptors/enzymes and at high levels become toxic detergents within the cell. Elevated levels of LCFAs are associated with diabetes, obesity and metabolic syndrome. Consequently, mammals evolved fatty acid-binding proteins (FABPs) that bind/sequester these potentially toxic free fatty acids in the cytosol and present them for rapid removal in oxidative (mitochondria, peroxisomes) or storage (endoplasmic reticulum, lipid droplets) organelles. Mammals have a large (15-member) family of FABPs with multiple members occurring within a single cell type. The first described FABP, liver-FABP (L-FABP or FABP1), is expressed in very high levels (2-5% of cytosolic protein) in liver as well as in intestine and kidney. Since L-FABP facilitates uptake and metabolism of LCFAs in vitro and in cultured cells, it was expected that abnormal function or loss of L-FABP would reduce hepatic LCFA uptake/oxidation and thereby increase LCFAs available for oxidation in muscle and/or storage in adipose. This prediction was confirmed in vitro with isolated liver slices and cultured primary hepatocytes from L-FABP gene-ablated mice. Despite unaltered food consumption when fed a control diet ad libitum, the L-FABP null mice exhibited age- and sex-dependent weight gain and increased fat tissue mass. The obese phenotype was exacerbated in L-FABP null mice pair fed a high-fat diet. Taken together with other findings, these data suggest that L-FABP could have an important role in preventing age- or diet-induced obesity.

New insights on the protein-ligand interaction differences between the two primary cellular retinol carriers

The main retinol carriers in the cytosol are the cellular retinol-binding proteins types I and II (CRBP-I and CRBP-II), which exhibit distinct tissue distributions. They play different roles in the maintenance of vitamin A homeostasis and feature a 100-fold difference in retinol affinity whose origin has not been described in detail. NMR-based hydrogen/deuterium exchange measurements show that, while retinol binding endows both proteins with a more rigid structure, many amide protons exchange much faster in CRBP-II than in CRBP-I in both apo and holo form, despite the conserved three-dimensional fold. The remarkable difference in intrinsic stability between the two homologs appears to modulate their binding properties: the stronger retinol binder CRBP-I displays a reduced flexibility of the backbone structure with respect to CRBP-II. This difference must derive from specific evolution-based amino acid substitutions, resulting in additional stabilization of the CRBP-I scaffold: in fact, we identified a number of potential salt bridges on the protein surface as well as several key interactions inside the binding cavity. Furthermore, our NMR data demonstrate that helix alphaII of the characteristic helix-turn-helix motif in the ligand portal region exists in both apo and holo CRBP-II. Hence, the previously proposed model of retinol binding needs to be revised.

Localization, function and regulation of the two intestinal fatty acid-binding protein types

Although intestinal (I) and liver (L) fatty acid binding proteins (FABP) have been widely studied, the physiological significance of the presence of the two FABP forms (I- and L-FABP) in absorptive cells remains unknown as do the differences related to their distribution along the crypt-villus axis, regional expression, ontogeny and regulation in the human intestine. Our morphological experiments supported the expression of I- and L-FABP as early as 13 weeks of gestation. Whereas cytoplasmic immunofluorescence staining of L-FABP was barely detectable in the lower half of the villus and in the crypt epithelial cells, I-FABP was visualized in epithelial cells of the crypt-villus axis in all intestinal segments until the adult period in which the staining was maximized in the upper part of the villus. Immunoelectron microscopy revealed more intense labeling of L-FABP compared with I-FABP, accompanied with a heterogeneous distribution in the cytoplasm, microvilli and basolateral membranes. By western blot analysis, I- and L-FABP at 15 weeks of gestation appeared predominant in jejunum compared with duodenum, ileum, proximal and distal colon. Exploration of the maturation aspect documented a rise in L-FABP in adult tissues. Permanent transfections of Caco-2 cells with I-FABP cDNA resulted in decreased lipid export, apolipoprotein (apo) biogenesis and chylomicron secretion. Additionally, supplementation of Caco-2 with insulin, hydrocortisone and epidermal growth factor differentially modulated the expression of I- and L-FABP, apo B-48 and microsomal triglyceride transfer protein (MTP), emphasizing that these key proteins do not exhibit a parallel modulation. Overall, our findings indicate that the two FABPs display differences in localization, regulation and developmental pattern.

FTIR reveals structural differences between native beta-sheet proteins and amyloid fibrils

The presence of beta-sheets in the core of amyloid fibrils raised questions as to whether or not beta-sheet-containing proteins, such as transthyretin, are predisposed to form such fibrils. However, we show here that the molecular structure of amyloid fibrils differs more generally from the beta-sheets in native proteins. This difference is evident from the amide I region of the infrared spectrum and relates to the distribution of the phi/psi dihedral angles within the Ramachandran plot, the average number of strands per sheet, and possibly, the beta-sheet twist. These data imply that amyloid fibril formation from native beta-sheet proteins can involve a substantial structural reorganization.

Structural and thermal stability characterization of Escherichia coli D-galactose/D-glucose-binding protein

The effect of temperature and glucose binding on the structure of the galactose/glucose-binding protein from Escherichia coli was investigated by circular dichroism, Fourier transform infrared spectroscopy, and steady-state and time-resolved fluorescence. The data showed that the glucose binding induces a moderate change of the secondary structure content of the protein and increases the protein thermal stability. The infrared spectroscopy data showed that some protein stretches, involved in alpha-helices and beta strand conformations, are particularly sensitive to temperature. The fluorescence studies showed that the intrinsic tryptophanyl fluorescence of the protein is well represented by a three-exponential model and that in the presence of glucose the protein adopts a structure less accessible to the solvent. The new insights on the structural properties of the galactose/glucose-binding protein can contribute to a better understanding of the protein functions and represent fundamental information for the development of biotechnological applications of the protein.

New insights into intracellular lipid binding proteins: The role of buried water

The crystal structures of most intracellular lipid binding proteins (LBPs) show between 5 and 20 internally bound water molecules, depending on the presence or the absence of ligand inside the protein cavity. The structural and functional significance of these waters has been discussed for several LBPs based on studies that used various biophysical techniques. The present work focuses on two very different LBPs, heart-type fatty acid binding protein (H-FABP) and ileal lipid binding protein (ILBP). Using high-resolution nuclear magnetic resonance spectroscopy, certain resonances belonging to side-chain protons that are located inside the water-filled lipid binding cavity were observed. In the case of H-FABP, the pH- and temperature-dependent behavior of selected side-chain resonances (Ser82 OgH and the imidazole ring protons of His93) indicated an unusually slow exchange with the solvent, implying that the intricate hydrogen-bonding network of amino-acid side-chains and water molecules in the protein interior is very rigid. In addition, holo H-FABP appeared to display a reversible self-aggregation at physiological pH. For ILBP, on the other hand, a more solvent-accessible protein cavity was deduced based on the pH titration behavior of its histidine residues. Comparison with data from other LBPs implies that the evolutionary specialization of LBPs for certain ligand types was not only because of mutations of residues directly involved in ligand binding but also to a refinement of the internal water scaffold.

Effect of acidic phospholipids on the structural properties of recombinant cytosolic human glyoxalase II

A peculiar characteristic of highly concentrated cytosolic recombinant human glyoxalase II (GII) solutions is to undergo partial precipitation. Previous work indicated that anionic phospholipids (PLs) exert a noncompetitive inhibition on the enzymatic activity of the soluble enzyme. In this study, FTIR spectroscopy was used to analyze the structural properties and the thermal stability of the soluble protein in the absence and in the presence of liposomes made of different phospholipids (PLs). The structural analysis was performed on the precipitate as well. The interaction of acidic PLs with GII lowered the thermal stability of the enzyme and inhibited protein intermolecular interactions (aggregation) brought about by thermal denaturation. Infrared data indicated that ionic and hydrophobic interactions occur between GII and acidic PLs causing small changes in the secondary structure of the enzyme. No interactions of the protein with egg phosphatidylcholine liposomes were detected. The results are consistent with the destabilization of the protein tertiary structure, and indicate that GII possesses hydrophobic part(s) that interact with the acyl chains of PLs. Data on precipitated GII did not show remarkable modification of secondary structure, suggesting that hydrophobic stretches of the enzyme may also be involved in the protein-protein association (precipitation) at high GII concentration. The alterations in the GII structure and the noncompetitive inhibition exerted by acidic PLs are strictly related.

Modulation of intestinal and liver fatty acid-binding proteins in Caco-2 cells by lipids, hormones and cytokines

Intestinal and liver fatty acid binding proteins (I- and L-FABP) are thought to play a role in enterocyte fatty acid (FA) trafficking. Their modulation by cell differentiation and various potential effectors was investigated in the human Caco-2 cell line. With the acquisition of enterocytic features, Caco-2 cells seeded on plastic progressively increased L-FABP quantities, whereas I-FABP was not detectable even very late in the maturation process. On permeable filters that improved differentiation markers (sucrase, alkaline phosphatase, transepithelial resistance), Caco-2 cells furthered their L-FABP content and expressed I-FABP. Western blot analysis showed a significant increase in I- and L-FABP expression following an 8-hour incubation period with butyric acid, oleic acid, and phosphatidylcholine. However, in all cases, I-FABP levels were higher than L-FABP concentrations regardless of the lipid substrates added. Similarly, hydrocortisone and insulin enhanced the cellular content of I- and L-FABP whereas leptin triggered I-FABP expression only after an 8-hour incubation. Finally, tumor necrosis factor-alpha was more effective in increasing the cytosolic amount of I-FABP levels. In conclusion, our data demonstrate that I-FABP expression is limited to fully differentiated Caco-2 cells and can be more easily regulated than L-FABP by lipids, hormones, and cytokines.

Porcine odorant-binding protein: structural stability and ligand affinities measured by fourier-transform infrared spectroscopy and fluorescence spectroscopy

Infrared spectra show that the binding of the odorants 2-isobuthyl-3-methoxypyrazine (PYR) and 3,7-dimethyl-1-octanol (DMO) stabilises the tertiary structure of porcine OBP-I against thermal denaturation. The fluorescence emission spectrum of the single tryptophan shows a lambdamax at 337 nm, indicating that the residue is not directly exposed to the solvent. Tryptophan does not appear to be involved in the odorant binding process and it is not accessible to the fluorescence quenchers NaI, CsCl and acrylamide. The binding of the fluorescent dye 1-aminoanthracene (1-AMA), a strong ligand, does not modify the tryptophan fluorescence spectrum. In contrast, the lambdamax of 1-AMA bound to OBP-I is shifted from 537 to 481 nm, with a lambdamax intensity increase by a factor of 80. Bound 1-AMA is displaced by odorant molecules in competitive binding assays and can be employed in simple and rapid binding assay, avoiding the use of radioactive ligands. The Scatchard plot shows that 1-AMA binds to OBP-I with a dissociation constant of 1.3 microM and an equimolar stoichiometry.

Isolation and characterization of two distinct forms of liver fatty acid binding protein from the rat

Liver fatty acid binding protein (L-FABP) appears to contain several different forms that may result from post-translational modification or bound ligand. To further assess this possibility, L-FABP was purified from rat liver homogenate and two putative isoforms separated using a sulfonyl column, a strong cation exchange resin. Fraction I eluted at 0.2 M NaCl, had a pI of 7.59, and following a final size exclusion step contained > 98% L-FABP. Fraction II eluted at 1.0 M NaCl, had a pI of 7.59, and following a final size exclusion step contained > 99% L-FABP. Both fractions contained approx. 0.15 moles of endogenous bound fatty acid per mole of protein, while L-FABP not subjected to the cation exchange step contained 0.75 moles of fatty acid per mole of protein. Fractions I and II had a greater proportion of saturated and monounsaturated fatty acids with a large reduction in polyunsaturated fatty acids compared to L-FABP not fractionated by cation exchange. Mass spectral analysis indicated the molecular mass of Fraction I was 14,315.02 +/- 0.35 Da and Fraction II was 14,315.86 +/- 0.34 Da. The peptide map for each fraction was determined by limited digestion of each fraction with either trypsin, Asp-N, or chymotrypsin to yield overlapping peptide fragments. Mass spectral analysis of these digests indicated the two proteins had identical amino acid fragments and that Cys69 was reduced and there were no Asn to Asp exchanges. Hence, these two forms of L-FABP were not isoforms and were not the result of differences in bound fatty acid. It is proposed that these two distinct forms of rat L-FABP were structural conformers based on two alternative folding pathways.

Fatty acid binding protein isoforms: structure and function

Although structural aspects of cytosolic fatty acid binding proteins (FABPs) in mammalian tissues are now well understood, significant advances regarding the physiological function(s) of these proteins have been slow in forthcoming. Part of the difficulty lies in the complexity of the multigene FABP family with nearly twenty identified members. Furthermore, isoelectric focusing and ion exchange chromatography operationally resolve many of the mammalian native FABPs into putative isoforms. However, a more classical biochemical definition of an isoform, i.e. proteins differing by a single amino acid, suggests that the operational definition is too broad. Because at least one putative heart H-FABP isoform, the mammary derived growth inhibitor, was an artifact (Specht et al. (1996) J. Biol. Chem. 271: 1943-49), the ensuing skepticism and confusion cast doubt on the existence of FABP isoforms in general. Yet, increasing data suggest that several FABPs, e.g. human intestinal I-FABP, bovine and mouse heart H-FABP, rabbit myelin P2 protein and bovine liver L-FABP may exist as true isoforms. In contrast, the rat liver L-FABP putative isoforms may actually be due either to bound ligand, post-translational S-thiolation and/or structural conformers. In any case, almost nothing is known regarding possible functions of either the true or putative isoforms in vitro or in vivo. The objective of this article is to critically evaluate which FABPs form biochemically defined or true isoforms versus FABPs that form additional forms, operationally defined as isoforms. In addition, recent developments in the molecular basis for FABP true isoform formation, the processes leading to additional operationally defined putative isoforms and insights into potential function(s) of this unusual aspect of FABP heterogeneity will be examined.

Structure of yeast plasma membrane H(+)-ATPase: comparison of activated and basal-level enzyme forms

Plasma membrane H(+)-ATPase of the yeast Saccharomyces cerevisiae was isolated and purified in its two forms, the activated A-ATPase from glucose-metabolising cells, and the basal-level B-ATPase from cells with endogenous metabolism only. Structure of the two enzyme forms and the effects of beta, gamma-imidoadenosine 5'-triphosphate (AMP-PNP) and of diethylstilbestrol (DES) thereon were analysed by FT-IR spectroscopy. IR spectra revealed the presence of two populations of alpha-helices with different exposure to the solvent in both the A-ATPase and B-ATPase. AMP-PNP did not affect the secondary structure of A-ATPase while DES affected the ratio of the two alpha-helix populations. Thermal denaturation experiments suggested a more stable structure in the B-form than in the A-form. AMP-PNP stabilised the A-ATPase structure while DES destabilised both enzyme forms. IR spectra showed that 60% of the amide hydrogens were exchanged for deuterium in both forms at 20 degrees C. The remaining 40% were exchanged at higher temperatures. The maximum amount of H/D exchange was observed at 50-55 degrees C for both enzyme forms, while in the presence of DES it was observed at lower temperatures. The data do not contradict the possibility that the activation of H(+)-ATPase is due to the C-terminus of the enzyme dissociating from the ATP-binding site which is covered by it in the less active form.

The crystal structure of the liver fatty acid-binding protein. A complex with two bound oleates

The crystal structure of the recombinant form of rat liver fatty acid-binding protein was completed to 2.3 A and refined to an R factor of 19.0%. The structural solution was obtained by molecular replacement using superimposed polyalanine coordinates of six intracellular lipid-binding proteins as a search probe. The entire amino acid sequence of rat liver fatty acid-binding protein along with an amino-terminal formyl-methionine was modeled in the crystal structure. In addition, the crystal was obtained in the presence of oleic acid, and the initial electron density clearly showed two fatty acid molecules bound within a central cavity. The carboxylate of one fatty acid molecule interacts with arginine 122 and is shielded from free solvent. It has an overall bent conformation. The more solvent-exposed carboxylate of the other oleate is located near the helix-turn-helix that caps one end of the beta-barrel, while the acyl chain lies in the interior. The cavity contains both polar and nonpolar residues but also shows extensive hydrophobic character around the nonpolar atoms of the ligands. The primary and secondary oleate binding sites appear to be totally interdependent, mainly because favorable hydrophobic interactions form between both aliphatic chains.

Time-resolved fluorescence of intestinal and liver fatty acid binding proteins: role of fatty acyl CoA and fatty acid

The effect of fatty acyl CoA and fatty acid on the solution structure and dynamics of two intestinal enterocyte fatty acid binding proteins, intestinal (I-FABP) and liver (L-FABP), was examined by time-resolved fluorescence of FABP aromatic amino acid residues. I-FABP Trp displayed two rotational correlation times, 6.6 and 0.4 ns. reflecting motion of the protein as a whole and segmental mobility of Trp. Neither oleoyl CoA, oleic acid, nor CoASH altered overall I-FABP rotational correlation time. However, oleic acid and CoASH increased I-FABP Trp segmental mobility, while oleoyl CoA and CoASH decreased I-FABP Trp limiting anisotropy (order). The angle of I-FABP Trp "wobbling in a cone" was increased by ligands in the order oleoyl CoA > CoASH > oleic acid. L-FABP Trp segmental mobility. L-FABP overall rotational motion, in contrast to that of I-FABP, was significantly increased by ligands in the order oleoyl CoA > oleic acid > CoASH. cis-Parinaric acid and cis-parinaroyl CoA bound to L-FABP also reflected overall L-FABP motion but yielded longer rotational correlation times, 8.2 and 10.7 ns, than the respective apo-FABPs. Such effects were not observed with I-FABP. Finally, both cis-parinaric acid and cis-parinaroyl CoA were much less ordered in the I-FABP ligand binding site than with L-FABP. These observations suggest that the rotational dynamics of L-FABP and its conformation are more sensitive to ligands than I-FABP. Further, ligands such as fatty acids, fatty acyl CoAs, and/or CoASH differentially modulate the I-FABP and L-FABP dynamics, and the ligand binding sites of these proteins differ in their ability to order the ligands.

Flexibility is a likely determinant of binding specificity in the case of ileal lipid binding protein

BACKGROUND

The family of lipid binding proteins (LBPs) includes a large number of fatty acid binding proteins (FABPs) but only two proteins (ileal lipid binding protein, ILBP, and liver fatty acid binding protein) that can bind both fatty acids and bile acids. Bile acid transport is medically and pharmacologically important, but is poorly understood. To understand the binding properties of ILBP, we studied its solution structure with and without bound lipids and compared these with known structures of FABPs.

RESULTS

The sequence-specific 1H resonance assignments for porcine ILBP have been determined by homonuclear two-dimensional (2D) NMR spectroscopy for the apo-protein as well as for ILBP complexes with fatty acid and bile acid ligands. From NOE spectra and hydrogen exchange data, similar secondary structure elements were identified for all three protein forms. ILBP is composed of ten antiparallel beta strands arranged in two nearly orthogonal beta sheets (a fold seen in other FABPs, and dubbed the "beta-clam shell'), covered on one side by two short, nearly parallel alpha helices. Binding of fatty acids or bile acids to ILBP alters mainly the side-chain proton resonances of amino acids within the protein cavity, indicating that both bile acids and fatty acids can bind in the interior of the protein between the two beta sheets; binding of bile acids stabilizes the protein backbone by a small amount. Fast hydrogen exchange rates for the backbone amide protons of ILBP indicate that the hydrogen-bonding network of the beta sheet in ILBP is weaker than the corresponding network in rat intestinal and bovine heart FABPs.

CONCLUSIONS

The tertiary structure of ILBP is similar to that of other LBPs, but appears to be unusually flexible, with a relatively weak hydrogen-bonding network. It is likely that this flexibility is important in allowing bile acids, which are larger and more rigid than fatty acids, to enter the central cavity of the protein.

Fatty acid transfer from liver and intestinal fatty acid-binding proteins to membranes occurs by different mechanisms

Intestinal absorptive cells contain high levels of expression of two homologous fatty acid-binding proteins (FABP), liver FABP (L-FABP), and intestinal FABP (I-FABP). Both bind long chain fatty acids with relatively high affinity. The functional distinction, if any, between these two proteins remains unknown. It is often hypothesized that FABP are important in intracellular transport of fatty acids. To assess whether fatty acid transport properties might differ between the two enterocyte FABPs, we examined the rate and mechanism of transfer of fluorescent anthroyloxy fatty acids (AOFA) from these proteins to model membranes using a resonance energy transfer assay. The results show that the absolute rate of AOFA transfer from I-FABP is faster than from L-FABP. Moreover, the apparent mechanism of fatty acid transfer is different between the two proteins. The rate of AOFA transfer from I-FABP is independent of ionic strength, directly dependent on the concentration of acceptor membrane vesicles, and dramatically regulated by the lipid composition of the membranes. These data strongly suggest that fatty acid transfer from I-FABP to membranes occurs by direct collisional interaction of the protein with the phospholipid bilayer. In contrast, the characteristics of fatty acid transfer from L-FABP are consistent with an aqueous diffusion-mediated process. Thus the two enterocyte FABPs may perform different functions within the intestinal absorptive cell in the regulation of fatty acid transport and utilization. It is hypothesized that L-FABP may act as a cytosolic buffer for fatty acids, maintaining the unbound fatty acid concentration, whereas I-FABP may be involved in the uptake and/or specific targeting of fatty acid to subcellular membrane sites.

Mutations of recombinant rat liver fatty acid-binding protein at residues 102 and 122 alter its structural integrity and affinity for physiological ligands

Rat liver fatty acid-binding protein (FABP) is able to accommodate a wide range of non-polar anions in addition to long-chain fatty acids. This property means that the liver protein is functionally different from other FABPs from intestine, muscle and adipose tissue that have a more restricted ligand specificity and stoichiometry. The availability of crystal structures for the latter proteins has highlighted the importance of two arginine residues that are involved in the binding of the fatty acid carboxylate. Only one of these arginine residues, arginine-122, is conserved in liver FABP, whereas the other arginine, at position 102, is replaced by a threonine. In order to gain further insight into the nature of ligand interactions with liver FABP these key residues (102 and 122) have been changed by site-directed mutagenesis. The results with an R122Q mutant highlight the critical role of this arginine in determining ligand affinity, while similar but less dramatic effects were observed with the T102Q mutant. The double mutant T102Q/R122Q was expressed but had lost the ability to bind fluorescent ligands. It is concluded that Arg-122 plays a role in accommodating the carboxylate group of at least one fatty acid. It is proposed that physiological ligands with more bulky headgroups, such as lysophospholipids, acyl-CoA and mono-olein, bind with the headgroups in a solvent-exposed location near the portal region of the protein. The portal region is suggested to be more flexible in the mutants (R122Q and to a lesser extent T102Q). The net result is that the ligand specificity of the R122Q mutant changes to that of a protein with enhanced affinity for acyl-CoA, lysophospholipids and mono-olein.

Structural properties and thermal stability of human liver and heart fatty acid binding proteins: a Fourier transform IR spectroscopy study

The secondary structure and the thermal stability of human liver (L-FABP) and heart (H-FABP) fatty acid-binding proteins were analyzed, in the absence and in the presence of oleic acid, by Fourier transform ir spectroscopy. The study was done in order to gain information on the secondary as well three-dimensional structure of L-FABP and to check the possible H-FABP self-association that has been found to occur in rat and pig H-FABP. Comparison of human L-FABP and H-FABP ir spectra reveals that, in spite of the low sequence homology, the two proteins have similar secondary and probably tertiary structures. The ir data indicates that a larger amount of beta-strands are exposed to the solvent in H-FABP as compared to L-FABP, suggesting minor differences in the three-dimensional structures of these proteins. The binding of oleic acid to L-FABP and H-FABP stabilizes their structures and does not modify their secondary structure. The ir spectra neither confirm nor exclude self-association of human H-FABP.

Influence of ADP, AMP-PNP and of depletion of nucleotides on the structural properties of F1ATPase: a Fourier transform infrared spectroscopic study

Mitochondrial F1ATPase from beef heart was treated with different buffers in order to modulate the nucleotide content of the enzyme and then analysed by FT-IR spectroscopy. Treatment of F1ATPase with a buffer lacking nucleotides and glycerol led to the formation of two fractions consisting of an inactive aggregated enzyme deprived almost completely of bound nucleotides and of an active enzyme containing ATP only in the tight sites and having a structure largely accessible to the solvent and a low thermal stability. Treatment of F1ATPase with saturating ADP, which induced the hysteretic inhibition during turnover, or AMP-PNP did not affect remarkably the secondary structure of the enzyme complex but significantly increased its compactness and thermal stability. It was hypothesised that the formation of the inactive aggregated enzyme was mainly due to the destabilisation of the alpha-subunits of F1ATPase and that the induction of the hysteretic inhibition is related to a particular conformation of the enzyme, which during turnover becomes unable to sustain catalysis.

Prediction of the structure of GroES and its interaction with GroEL

The three-dimensional structure of the GroES monomer and its interaction with GroEL has been predicted using a combination of prediction tools and experimental data obtained by biophysical [electron microscope (EM), Fourier transform infrared (FTIR), and nuclear magnetic resonance (NMR)] and biochemical techniques. The GroES monomer, according to the prediction, is composed of eight beta-strands forming a beta-barrel with loose ends. In the model, beta-strands 5-8 run along the outer surface of GroES, forming an antiparallel beta-sheet with beta 4 loosely bound to one of the edges. beta-strands 1-3 would then be parallel and placed in the interior of the molecule. Loops 1-3 would face the internal cavity of the GroEL-GroES complex, and together with conserved residues in loops 5 and 7, would form the active surface interacting with GroEL.

Comparison of the structure of wild-type HtrA heat shock protease and mutant HtrA proteins. A Fourier transform infrared spectroscopic study

The HtrA protease of Escherichia coli, identical with the DegP protease, is a 48-kDa heat shock protein, indispensable for bacterial survival only at temperatures above 42 degrees C. Proteolytic activity of HtrA is inhibited by diisopropyl fluorophosphate, suggesting that HtrA is a serine protease. We have recently found that mutational alteration of serine in position 210 of the mature HtrA or of histidine in position 105 totally eliminated proteolytic activity of HtrA. However, little was known about the consequences of the mutations on HtrA conformation. In this work, Fourier transform infrared spectroscopy has been used to examine the conformation in aqueous solution of wild-type HtrA and mutant HtrAS210 and HtrAH105 proteins. The spectra were collected at different temperatures in order to gain information also on the thermal stability of the three proteins. The analysis of HtrA protein spectrum, by resolution-enhancement methods, revealed that beta-sheet is the major structural element of the conformation of HtrA. Deconvoluted as well as second derivative spectra of wild-type HtrA and mutant HtrAS210 and HtrAH105 collected at 20 degrees C were identical, indicating no differences in the secondary structure of these proteins. The analysis of spectra obtained at different temperatures revealed a maximum of protein denaturation within 65-70 degrees C for wild-type HtrA as well as for the HtrAS210 and HtrAH105 mutant proteins. However, the thermal denaturation pattern of wild-type HtrA revealed a lower cooperativity in the denaturation process as compared to the mutant proteins which instead behaved similarly. These data suggest that the mutations in HtrA protein induced minor changes in the tertiary structure of the protein (most likely located at the mutation sites). Our results strongly support the idea that Ser210 and His105 may represent two elements of the active-site triad (Ser, His, and Asp), found in most serine proteases. We have also found that in vitro, in the range from 37 to 55 degrees C, the proteolytic activity of HtrA rapidly increased with temperature and that HtrA activity remained unchanged for at least 4 h at 45 degrees C.