Allostery in a monomeric protein: the case of human serum albumin.

Human serum albumin (HSA), the most prominent protein in plasma, binds different classes of ligands at multiple sites. HSA provides a depot for many compounds, affects pharmacokinetics of many drugs, holds some ligands in a strained orientation providing their metabolic modification, renders potential toxins harmless transporting them to disposal sites. Its modular domain organization provides a variety of ligand-binding sites and the flexible modular structure involves more than the immediate vicinity of the binding site(s), affecting the ligand-binding properties of the full protein. Ferric human serum heme\u2013albumin (heme\u2013 HSA) shows a peculiar spectroscopic properties that allow to investigate structural and functional aspects of the protein. Here, a characterization of the environment of the metal binding site and of its dynamics is reported. Moreover, the effect of different endogenous and exogenous ligands on the allosteric properties of HSA and link between binding sites are investigated. As a whole, the complex mechanism modulating ligand binding to HAS represents one of the most important structure-function correlations ever reported for monomeric proteins in general

Allostery in a monomeric protein: the case of human serum albumin.

Human serum albumin (HSA), the most prominent protein in plasma, binds different classes of ligands at multiple sites. HSA provides a depot for many compounds, affects pharmacokinetics of many drugs, holds some ligands in a strained orientation providing their metabolic modification, renders potential toxins harmless transporting them to disposal sites. Its modular domain organization provides a variety of ligand-binding sites and the flexible modular structure involves more than the immediate vicinity of the binding site(s), affecting the ligand-binding properties of the full protein. Ferric human serum heme–albumin (heme– HSA) shows a peculiar spectroscopic properties that allow to investigate structural and functional aspects of the protein. Here, a characterization of the environment of the metal binding site and of its dynamics is reported. Moreover, the effect of different endogenous and exogenous ligands on the allosteric properties of HSA and link between binding sites are investigated. As a whole, the complex mechanism modulating ligand binding to HAS represents one of the most important structure-function correlations ever reported for monomeric proteins in general

The drug-dependent five- to six-coordination transition of the heme-Fe atom modulates allosterically human serum heme-albumin reactivity

Human serum albumin (HSA), the most abundant protein in plasma, displays several functions including heme transfer from high- and low-density lipoproteins to hemopexin; therefore, the HSA-heme complex acquires transiently heme-based (pseudo-)enzymatic properties. In particular, ferric human serum heme-albumin (HSA-heme) and ferrous nitrosylated HSA-heme inactivate peroxynitrite, and ferrous HSA-heme catalyzes the conversion of nitrite to nitrogen monoxide. The (pseudo-)enzymatic properties of HSA-heme are modulated allosterically by endogenous and exogenous ligands, such as drugs. The modulation of ligand binding to plasma proteins is relevant not only under physiological conditions but also in the pharmacological therapy management. Here, drug-dependent HSA-heme properties are reviewed from the functional and structural viewpoints. In particular, the drug-dependent five- to six-coordination transition of the heme-Fe atom is at the root of the allosteric modulation of the HSA-heme reactivity

Sequence analysis of serum albumins reveals the molecular evolution of ligand recognition properties

<div><p>Serum albumin (SA) is a circulating protein providing a depot and carrier for many endogenous and exogenous compounds. At least seven major binding sites have been identified by structural and functional investigations mainly in human SA. SA is conserved in vertebrates, with at least 49 entries in protein sequence databases. The multiple sequence analysis of this set of entries leads to the definition of a cladistic tree for the molecular evolution of SA orthologs in vertebrates, thus showing the clustering of the considered species, with lamprey SAs (<i>Lethenteron japonicum</i> and <i>Petromyzon marinus</i>) in a separate outgroup. Sequence analysis aimed at searching conserved domains revealed that most SA sequences are made up by three repeated domains (about 600 residues), as extensively characterized for human SA. On the contrary, lamprey SAs are giant proteins (about 1400 residues) comprising seven repeated domains. The phylogenetic analysis of the SA family reveals a stringent correlation with the taxonomic classification of the species available in sequence databases. A focused inspection of the sequences of ligand binding sites in SA revealed that in all sites most residues involved in ligand binding are conserved, although the versatility towards different ligands could be peculiar of higher organisms. Moreover, the analysis of molecular links between the different sites suggests that allosteric modulation mechanisms could be restricted to higher vertebrates.</p> </div

Warfarin inhibits allosterically the reductive nitrosylation of ferric human serum heme-albumin

Human serum heme-albumin (HSA-heme-Fe) displays heme-based ligand binding and (pseudo-)enzymatic properties. Here, the effect of the prototypical drug warfarin on kinetics and thermodynamics of NO binding to ferric and ferrous HSA-heme-Fe (HSA-heme-Fe(III) and HSA-heme-Fe(II), respectively) and on the NO-mediated reductive nitrosylation of the heme-Fe atom is reported; data were obtained between pH 5.5 and 9.5 at 20.0 \ub0C. Since warfarin is a common drug, its effect on the reactivity of HSA-heme-Fe represents a relevant issue in the pharmacological therapy management. The inhibition of NO binding to HSA-heme-Fe(III) and HSA-heme-Fe(II) as well as of the NO-mediated reductive nitrosylation of the heme-Fe(III) atom by warfarin has been ascribed to drug binding to the fatty acid binding site 2 (FA2), shifting allosterically the penta-to-six coordination equilibrium of the heme-Fe atom toward the low reactive species showing the six-coordinated metal center by His146 and Tyr161 residues. These data: (i) support the role of HSA-heme-Fe in trapping NO, (ii) highlight the modulation of the heme-Fe-based reactivity by drugs, and (iii) could be relevant for the modulation of HSA functions by drugs in vivo

Flavonoid binding to human serum albumin.

Dietary flavonoid may have beneficial effects in the prevention of chronic diseases. However, flavonoid bioavailability is often poor probably due to their interaction with plasma proteins. Here, the affinity of daidzein and daidzein metabolites as well as of genistein, naringenin, and quercetin for human serum albumin (HSA) has been assessed in the absence and presence of oleate. Values of the dissociation equilibrium constant (K) for binding of flavonoids and related metabolites to Sudlow's site I range between 3.3x10(-6) and 3.9x10(-5)M, at pH 7.0 and 20.0 degrees C, indicating that these flavonoids are mainly bound to HSA in vivo. Values of K increase (i.e., the flavonoid affinity decreases) in the presence of saturating amounts of oleate by about two folds. Present data indicate a novel role of fatty acids as allosteric inhibitors of flavonoid bioavailability, and appear to be relevant in rationalizing the interference between dietary compounds, food supplements, and drugs

All-trans-retinoic acid and retinol binding to the FA1 site of human serum albumin competitively inhibits heme-Fe(III) association

Retinoids are a class of chemicals derived from vitamin A metabolism, playing important and diverse functions. Vitamin A, also named all-trans-retinol (all-trans-ROL), is coverted into two classes of biologically active retinoids, i.e. 11-cis-retinoids and acidic retinoids. Among acidic retinoids, all-trans-retinoic acid (all-trans-RA) and 9-cis-retinoic acid (9-cis-RA) represent the main metabolic products. Specific and aspecific proteins solubilize, protect, and detoxify retinoids in the extracellular environment. The retinoid binding protein 4 (RBP4), the epididymal retinoid-binding protein (ERBP), and the interphotoreceptor matrix retinoid-binding protein (IRBP) play a central role in ROL transport, whereas lipocalin-type prostaglandin D synthase (also named β-trace) and human serum albumin (HSA) transport preferentially all-trans-RA. Here, the modulatory effect of all-trans-RA and all-trans-ROL on ferric heme (heme-Fe(III)) binding to HSA is reported. All-trans-RA and all-trans-ROL binding to the FA1 site of HSA competitively inhibit heme-Fe(III) association. Docking simulations and local structural comparison of HSA with all-trans-RA- and all-trans-ROL-binding proteins support functional data indicating the preferential binding of all-trans-RA and all-trans-ROL to the FA1 site of HSA. Present results may be relevant in vivo, in fact HSA could act as a secondary carrier of retinoids in human diseases associated with reduced levels of RBP4 and IRBP

Drugs modulate allosterically heme-fe-recognition by human serum albumin and heme-fe-mediated reactivity

Human serum albumin (HSA) represents an important determinant of plasma oncotic pressure and a relevant factor that modulates fluid distribution between the body compartments. Moreover, HSA (i) represents the depot and transporter of several compounds, both endogenous and exogenous, (ii) affects the pharmacokinetics of many drugs, (iii) regulates chemical modifications of some ligands, (iv) shows (pseudo-)enzymatic properties, (v) inactivates some toxic compounds, and (vi) displays anti-oxidant properties. HSA binding and (pseudo-)enzymatic properties are regulated competitively, allosterically, and by covalent modifications. While competitive inhibition of HSA binding properties is evident, allosteric mechanisms and covalent modifications affecting HSA reactivity are less clear. In several pathological conditions in which free heme-Fe levels increase, the buffering capacity of plasma hemopexin is overwhelmed and most of heme-Fe binds to the fatty acid site 1 of HSA. HSA-heme-Fe displays globin-like properties; in turn, heme-Fe modulates competitively and allosterically HSA binding and reactivity properties. Remarkably, heme-Fe-mediated HSA properties are time-dependent, representing a case for \u201cchronosteric effects\u201d. Here, we review the drug-based modulation of (i) heme-Fe-recognition by HSA and (ii) heme-Fe-mediated reactivity