Stephen Curry

Crystal structure of human serum albumin complexed with fatty acid reveals an asymmetric distribution of binding sites.

Human serum albumin (HSA) is the most abundant protein in the circulatory system. Its principal function is to transport fatty acids, but it is also capable of binding a great variety of metabolites and drugs. Despite intensive efforts, the detailed structural basis of fatty acid binding to HSA has remained elusive. We have now determined the crystal structure of HSA complexed with five molecules of myristate at 2.5 Å resolution. The fatty acid molecules bind in long, hydrophobic pockets capped by polar side chains, many of which are basic. These pockets are distributed asymmetrically throughout the HSA molecule, despite its symmetrical repeating domain structure.

The extraordinary ligand binding properties 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, accounts for most of the antioxidant capacity of human serum, and acts as a NO‐carrier. The globular domain structural organization of monomeric HSA is at the root of its allosteric properties which are reminiscent of those of multimeric proteins. Here, structural, functional, biotechnological, and biomedical aspects of ligand binding to HSA are summarized.

Fatty acid binding to human serum albumin: new insights from crystallographic studies

Human serum albumin possesses multiple fatty acid binding sites of varying affinities, but the precise locations of these sites have remained elusive. The determination of the crystal structure of human serum albumin complexed with myristic acid recently revealed the positions and architecture of six binding sites on the protein. While the structure of the complex is consistent with a great deal of the biochemical and biophysical data on fatty acid binding, it is not yet possible to provide a completely rigorous correlation between the structural and binding data. The challenge now is to use the new structural information to design experiments that will identify the physiologically important binding sites on HSA and provide a much richer description of fatty acid interactions with the protein.

Crystal structural analysis of human serum albumin complexed with hemin and fatty acid.

BackgroundHuman serum albumin (HSA) is an abundant plasma protein that binds a wide variety of hydrophobic ligands including fatty acids, bilirubin, thyroxine and hemin. Although HSA-heme complexes do not bind oxygen reversibly, it may be possible to develop modified HSA proteins or heme groups that will confer this ability on the complex.ResultsWe present here the crystal structure of a ternary HSA-hemin-myristate complex, formed at a 1:1:4 molar ratio, that contains a single hemin group bound to subdomain IB and myristate bound at six sites. The complex displays a conformation that is intermediate between defatted HSA and HSA-fatty acid complexes; this is likely to be due to low myristate occupancy in the fatty acid binding sites that drive the conformational change. The hemin group is bound within a narrow D-shaped hydrophobic cavity which usually accommodates fatty acid; the hemin propionate groups are coordinated by a triad of basic residues at the pocket entrance. The iron atom in the centre of the hemin is coordinated by Tyr161.ConclusionThe structure of the HSA-hemin-myristate complex (PDB ID 1o9x) reveals the key polar and hydrophobic interactions that determine the hemin-binding specificity of HSA. The details of the hemin-binding environment of HSA provide a structural foundation for efforts to modify the protein and/or the heme molecule in order to engineer complexes that have favourable oxygen-binding properties.

The structure and antigenicity of a type C foot-and-mouth disease virus.

BACKGROUND Picornaviruses are responsible for a wide range of mammalian diseases and, in common with other RNA viruses, show considerable antigenic variation. Foot-and-mouth disease viruses (FMDVs) constitute one genus of the picornavirus family and are classified into seven serotypes, each of which shows considerable intratypic variation. This antigenic variation leads to continuing difficulties in controlling the disease. To date the structure of only one serotype, O, has been reported. RESULTS The three-dimensional structure of a serotype C (isolate C-S8c1) FMDV, has been determined crystallographically at 3.5 A resolution. The main chain conformation of the virion is very similar to that of type O1 virus. The immunodominant G-H loop of VP1, the presumed site of cell attachment, is disordered in both types of virus indicating a functional role for flexibility of this region. There are significant changes in the structure of other antigenic loops and in some internal regions involved in protomer-protomer contacts, including the entire amino-terminal portion of VP2, described here for the first time for a picornavirus. Antigenic sites have been identified by genetic and peptide mapping methods, and located on the capsid. The data reveal a major new discontinuous antigenic site (site D) which is located near to the three-fold axis and involves residues of VP1, VP2 and VP3 which lie adjacent to each other on the capsid. CONCLUSION In FMDV type C, amino acid substitutions seen in mutants that are resistant to neutralization by monoclonal antibodies (MAbs) map to predominantly surface-oriented residues with solvent-accessible side-chains not involved in interactions with other amino acids, whereas residues which are accessible but not substituted are found to be more frequently involved in protein-protein interactions. This provides a molecular interpretation for the repeated isolation of the same amino acid substitutions in MAb-resistant variants, an observation frequently made with RNA viruses. This first comparison of two FMDV serotypes shows how subtle changes at antigenic sites are sufficient to cause large changes in antigenic specificity between serotypes.

Molecular Tectonic Model of Virus Structural Transitions: the Putative Cell Entry States of Poliovirus

ABSTRACT Upon interacting with its receptor, poliovirus undergoes conformational changes that are implicated in cell entry, including the externalization of the viral protein VP4 and the N terminus of VP1. We have determined the structures of native virions and of two putative cell entry intermediates, the 135S and 80S particles, at ∼22-Å resolution by cryo-electron microscopy. The 135S and 80S particles are both ∼4% larger than the virion. Pseudoatomic models were constructed by adjusting the beta-barrel domains of the three capsid proteins VP1, VP2, and VP3 from their known positions in the virion to fit the 135S and 80S reconstructions. Domain movements of up to 9 Å were detected, analogous to the shifting of tectonic plates. These movements create gaps between adjacent subunits. The gaps at the sites where VP1, VP2, and VP3 subunits meet are plausible candidates for the emergence of VP4 and the N terminus of VP1. The implications of these observations are discussed for models in which the externalized components form a transmembrane pore through which viral RNA enters the infected cell.

Crystallographic Analysis of Human Serum Albumin Complexed with 4Z,15E-Bilirubin-IXα

Bilirubin, an insoluble yellow-orange pigment derived from heme catabolism, accumulates to toxic levels in individuals with impaired or immature liver function. The resulting jaundice may be managed with phototherapy to isomerize the biosynthetic 4Z,15Z-bilirubin-IXα to more soluble and excretable isomers, such as 4Z,15E-bilirubin. Bilirubin and its configurational isomers are transported to the liver by human serum albumin (HSA) but their precise binding location(s) on the protein have yet to be determined. To investigate the molecular details of their interaction, we co-crystallised bilirubin with HSA. Strikingly, the crystal structure—determined to 2.42 Å resolution—revealed the 4Z,15E-bilirubin-IXα isomer bound to an L-shaped pocket in sub-domain IB. We also determined the co-crystal structure of HSA complexed with fusidic acid, an antibiotic that competitively displaces bilirubin from the protein, and showed that it binds to the same pocket. These results provide the first crystal structure of a natural bilirubin pigment bound to serum albumin, challenge some of the present conceptions about HSA–bilirubin interactions, and provide a sound structural framework for finally resolving the long-standing question of where 4Z,15Z-bilirubin-IXα binds to the protein.

Structural basis of albumin–thyroxine interactions and familial dysalbuminemic hyperthyroxinemia

Human serum albumin (HSA) is the major protein component of blood plasma and serves as a transporter for thyroxine and other hydrophobic compounds such as fatty acids and bilirubin. We report here a structural characterization of HSA–thyroxine interactions. Using crystallographic analyses we have identified four binding sites for thyroxine on HSA distributed in subdomains IIA, IIIA, and IIIB. Mutation of residue R218 within subdomain IIA greatly enhances the affinity for thyroxine and causes the elevated serum thyroxine levels associated with familial dysalbuminemic hyperthyroxinemia (FDH). Structural analysis of two FDH mutants of HSA (R218H and R218P) shows that this effect arises because substitution of R218, which contacts the hormone bound in subdomain IIA, produces localized conformational changes to relax steric restrictions on thyroxine binding at this site. We have also found that, although fatty acid binding competes with thyroxine at all four sites, it induces conformational changes that create a fifth hormone-binding site in the cleft between domains I and III, at least 9 Å from R218. These structural observations are consistent with binding data showing that HSA retains a high-affinity site for thyroxine in the presence of excess fatty acid that is insensitive to FDH mutations.

Structure of tandem RNA recognition motifs from polypyrimidine tract binding protein reveals novel features of the RRM fold

Polypyrimidine tract binding protein (PTB), an RNA binding protein containing four RNA recognition motifs (RRMs), is involved in both pre‐mRNA splicing and translation initiation directed by picornaviral internal ribosome entry sites. Sequence comparisons previously indicated that PTB is a non‐canonical RRM protein. The solution structure of a PTB fragment containing RRMs 3 and 4 shows that the protein consists of two domains connected by a long, flexible linker. The two domains tumble independently in solution, having no fixed relative orientation. In addition to the βαββαβ topology, which is characteristic of RRM domains, the C‐terminal extension of PTB RRM‐3 incorporates an unanticipated fifth β‐strand, which extends the RNA binding surface. The long, disordered polypeptide connecting β4 and β5 in RRM‐3 is poised above the RNA binding surface and is likely to contribute to RNA recognition. Mutational analyses show that both RRM‐3 and RRM‐4 contribute to RNA binding specificity and that, despite its unusual sequence, PTB binds RNA in a manner akin to that of other RRM proteins.

Structural analysis of cooperative RNA binding by the La motif and central RRM domain of human La protein.

The La protein is a conserved component of eukaryotic ribonucleoprotein complexes that binds the 3′ poly(U)-rich elements of nascent RNA polymerase III (pol III) transcripts to assist folding and maturation. This specific recognition is mediated by the N-terminal domain (NTD) of La, which comprises a La motif and an RNA recognition motif (RRM). We have determined the solution structures of both domains and show that the La motif adopts an α/β fold that comprises a winged-helix motif elaborated by the insertion of three helices. Chemical shift mapping experiments show that these insertions are involved in RNA interactions. They further delineate a distinct surface patch on each domain—containing both basic and aromatic residues—that interacts with RNA and accounts for the cooperative binding of short oligonucleotides exhibited by the La NTD.