Structural study of the glycosylated and unglycosylated forms of a genetic variant of human serum albumin (63 Asp-->Asn)

Posttranslational modifications of intact proteins detected by NMR spectroscopy: application to glycosylation

Posttranslational modifications (PTMs) are an integral part of the majority of proteins. The characterization of structure and function of PTMs can be very challenging especially for glycans. Existing methods to analyze PTMs require complicated sample preparations and suffer from missing certain modifications, the inability to identify linkage types and thus chemical structure. We present a direct, robust, and simple NMR spectroscopy method for the detection and identification of PTMs in proteins. No isotope labeling is required, nor does the molecular weight of the studied protein limit the application. The method can directly detect modifications on intact proteins without sophisticated sample preparation. This approach is well suited for diagnostics of proteins derived from native organisms and for the quality control of biotechnologically produced therapeutic proteins.

Human serum albumin isoforms: genetic and molecular aspects and functional consequences

BACKGROUND

At present, 67 different genetic variants of human serum albumin and proalbumin have been molecularly characterized at the protein and/or gene level.

SCOPE OF REVIEW

This review summarizes present knowledge about genetic and molecular aspects, functional consequences and potential uses of the variants.

MAJOR CONCLUSIONS

The frequency of bisalbuminemia in the general population is probably about 1:1000, but it can be much higher in isolated populations. Mutations are often due to hypermutable CpG dinucleotides, and in addition to single-amino acid substitutions, glycosylated variants and C-terminally modified alloalbumins have been found. Some mutants show altered stability in vivo and/or in vitro. High-affinity binding of Ni(++) and Cu(++) is blocked, or almost so, by amino acid changes at the N-terminus. In contrast, substitution of Leu90 and Arg242 leads to strong binding of triiodothyronine and l-thyroxine, respectively, resulting in two clinically important syndromes. Variants often have modified plasma half-lives and organ uptakes when studied in mice.

GENERAL SIGNIFICANCE

Because alloalbumins do not seem to be associated with disease, they can be used as markers of migration and provide a model for study of neutral molecular evolution. They can also give valuable molecular information about albumins binding sites, antioxidant and enzymatic properties, as well as stability. Mutants with increased affinity for endogenous or exogenous ligands could be therapeutically relevant as antidotes, both for in vivo and extracorporeal treatment. Variants with modified biodistribution could be used for drug targeting. In most cases, the desired function can be further elaborated by producing site-directed, recombinant mutants. This article is part of a Special Issue entitled Serum Albumin.

Human serum albumin: from bench to bedside

Human serum albumin (HSA), the most abundant protein in plasma, is a monomeric multi-domain macromolecule, representing the main determinant of plasma oncotic pressure and the main modulator of fluid distribution between body compartments. HSA displays an extraordinary ligand binding capacity, providing a depot and carrier for many endogenous and exogenous compounds. Indeed, HSA represents the main carrier for fatty acids, affects pharmacokinetics of many drugs, provides the metabolic modification of some ligands, renders potential toxins harmless, accounts for most of the anti-oxidant capacity of human plasma, and displays (pseudo-)enzymatic properties. HSA is a valuable biomarker of many diseases, including cancer, rheumatoid arthritis, ischemia, post-menopausal obesity, severe acute graft-versus-host disease, and diseases that need monitoring of the glycemic control. Moreover, HSA is widely used clinically to treat several diseases, including hypovolemia, shock, burns, surgical blood loss, trauma, hemorrhage, cardiopulmonary bypass, acute respiratory distress syndrome, hemodialysis, acute liver failure, chronic liver disease, nutrition support, resuscitation, and hypoalbuminemia. Recently, biotechnological applications of HSA, including implantable biomaterials, surgical adhesives and sealants, biochromatography, ligand trapping, and fusion proteins, have been reported. Here, genetic, biochemical, biomedical, and biotechnological aspects of HSA are reviewed.

Cryoglobulin/albumin complexes in a patient with severe autoimmune syndrome

We describe the case of a 30-year-old man with a severe autoimmune disease characterized by cryoglobulinaemia, pulmonary hypertension, Raynaud's phenomenon, lymphadenopathy, and glomerulonephritis. Despite initial remission following autologous stem cell transplantation, his disease relapsed and he died from pulmonary hypertension. At presentation the patient had hypergammaglobulinaemia and a number of autoantibodies, including rheumatoid factor (1:10240). The most striking feature was the extremely high level of cryoglobulins. The cryoprecipitate consisted of polyclonal IgM, IgG and albumin. Interestingly, the albumin in the cryoprecipitate was exclusively present in SS-bonded oligomeric forms, and contained an abnormal acidic component as judged by 2D gel electrophoresis. Oxidized albumin was also present in serum, and represented a small but significant fraction. None of the many known albumin variants have so far been associated with a particular disease; thus our results may represent the first description of an altered albumin associated with severe disease.

Effect of genetic variation on the thermal stability of human serum albumin

Reversible thermal denaturation of 33 genetic variants of human serum albumin (HSA) appeared to be a two-state process when studied by circular dichroism (CD). Fourteen single-residue variants have Tm values (midpoint of denaturation) higher than, and nine have Tm values lower than, their endogenous, wild-type counterpart. Nine single-residue variants have DeltaHv values (van't Hoff enthalpy) higher than, and 14 have DeltaHv values lower than, normal albumin. All types of combinations of positive and negative DeltaTm values and Delta(DeltaHv) values were found. Good linear correlations between mutation-induced changes of alpha-helical content and Delta(DeltaHv) values, but not DeltaTm values, were found especially for the variants mutated in domains I and III. The effect of altered chain length and glycosylation on Tm and DeltaHv was also studied. For all variants, no clear relationship was found between the changes in the thermodynamic parameters and the type of substitution, changes in protein charge or hydrophobicity. However, the protein changes taking place in domain I have a rather uniform effect (almost all of the nine variants have positive DeltaTm values and negative Delta(DeltaHv) values, i.e., they denature more easily than normal albumin but they do so at a higher temperature). The present results can be of both protein chemical relevance and of clinical interest, because they could be useful when designing stable, recombinant HSAs for clinical applications.

Structural analysis and fatty acid-binding properties of two Croatian variants of human serum albumin

BACKGROUND

The aim of the present work was to characterize the molecular defects of a slow-migrating (albumin Zagreb) and a fast-migrating (albumin Krapina) genetic variant of human serum albumin detected in heterozygous persons living in Croatia and to elucidate the fatty acid-binding properties of the two alloalbumins.

METHODS

Purification and structural identification of the variants were performed by conventional protein chemistry methods, whereas types and amounts of albumin-bound, endogenous fatty acids were determined by gas chromatography.

RESULTS

Protein sequencing established that albumin Zagreb is a proalbumin variant (-1Arg-->Gln), and that albumin Krapina is due to a mutation within the mature polypeptide chain (573Lys-->Glu). The gas chromatographic results showed that the fatty acid-binding properties of the proalbumin variant are normal, while the amino acid substitution in position 573 resulted in a general decrease of fatty acid binding.

CONCLUSIONS

The structural defects of the first alloalbumins, detected by routine clinical electrophoresis among the Croatian population, were characterized. Albumin Zagreb is caused by a hot-spot mutation occurring in a CpG sequence in the albumin gene. It is commonly assumed that bisalbuminaemia has no direct clinical relevance. However, the present study suggests that naturally occurring mutations can affect the ligand-binding properties of human serum albumin.

Structural analysis, fatty acid and thyroxine binding properties of Vancouver and Naskapi variants of human serum albumin

OBJECTIVES

To purify and structurally identify two albumin variants found in the Canadian population of native Amerindian origin. To assess the ability of variant albumins to bind lauric acid and L-thyroxine.

METHODS

The structural characterization of the alloalbumins was performed by conventional protein chemistry methods and by mass spectrometric analysis. Lauric acid and L-thyroxine affinities to variant albumins were assessed by kinetic dialysis and equilibrium dialysis techniques, respectively.

RESULTS

The sequence investigations proved the two variants to be albumin Naskapi [372Lys --> Glu] and albumin Vancouver [501Glu --> Lys], respectively. Among the carriers of albumin Naskapi, we found a rare case of homozygosity. Furthermore, this is the first reported case of the 501Glu-->Lys mutation in the native North American population. Scatchard plot analysis revealed that the association constants for lauric acid and L-thyroxine to the two variants were indistinguishable from the endogenous form of albumin.

CONCLUSION

We show that albumin variants Vancouver and Naskapi have normal fatty acid and L-thyroxine binding capabilities. These findings support the assumption that bisalbuminemias associated with these albumin variants are benign conditions.

The glycan structure of albumin Redhill, a glycosylated variant of human serum albumin

Although human serum albumin is synthesized without carbohydrate, glycosylated variants of the protein can be found. We have determined the structure of the glycan bound to the double-mutant albumin Redhill (-1 Arg, 320 Ala-->Thr). The oligosaccharide was released from the protein using anhydrous hydrazine, and its structure was investigated using neuraminidase and a reagent array analysis method, which is based on the use of specific exoglycosidases. The glycan was shown to be a disialylated biantennary complex type oligosaccharide N-linked to 318 Asn. However, a minor part (11 mol%) of the glycan was without sialic acid. The structure is principally the same as that of glycans bound to two other types of glycosylated albumin variants. Glycosylation can affect, for example, the fatty acid binding properties of albumin. Taking the present information into account, it is apparent that different effects on binding are caused not by different glycan structures but by different locations of attachment, with the possible addition of local conformational changes in the protein molecule.

A nucleotide insertion and frameshift cause albumin Kénitra, an extended and O-glycosylated mutant of human serum albumin with two additional disulfide bridges

Albumin Kenitra is a new type of genetic variant of human serum albumin that has been found in two members of a family of Sephardic Jews from Kenitra (Morocco). The slow-migrating variant and the normal protein were isolated by anion-exchange chromatography and, after treatment with CNBr, the digests were analyzed by two-dimensional electrophoresis in a polyacrylamide gel. The CNBr peptides of the variant were purified by reverse-phase high performance liquid chromatography and submitted to sequence analysis. Albumin Kenitra is peculiar because it has an elongated polypeptide chain, 601 residues instead of 585, and its sequence is modified beginning from residue 575. DNA structural studies showed that the variant is caused by a single-base insertion, an adenine at nucleotide position 15 970 in the genomic sequence, which leads to a frameshift with the subsequent translation to the first termination codon of exon 15. Mass spectrometric analyses revealed that the four additional cysteine residues of the variant form two new S-S bridges and showed that albumin Kenitra is partially O-glycosylated by a monosialylated HexHexNAc structure. This oligosaccharide chain has been located to Thr596 by amino-acid sequence analysis of the tryptic fragment 592-597.

Structure of a major oligosaccharide of PASII/PMP22 glycoprotein in bovine peripheral nerve myelin

The amino acid sequence of the glycopeptide obtained from bovine PASII/PMP22 protein in the PNS myelin was determined to be Gln-Asn-Cys-Ser-Thr, where the asparagine was glycosylated. To eliminate all the contaminated P(o) glycopeptides from the PASII/PMP22 glycopeptide preparation, we used a fluorescent probe, N-[2-(2-pyridylamino)ethyl]maleimide, which reacts with the cysteine of the PASII/PMP22 glycopeptides. The labeled PASII/PMP22 glycopeptides were isolated by HPLC and were digested further with glycopeptidase A. The resultant oligosaccharides were conjugated with 2-aminopyridine (PA) as a fluorescent tag. One major PA-oligosaccharide, OPPE1, was purified by HPLC. The structure of OPPE1 was elucidated by fast atom bombardment mass spectrometry and (1)H-NMR studies and comparing the derivatives of PA-OPPE1 and PA-oligosaccharides of gamma-globulin on HPLC. The structure, SO(4)-3GlcAbeta1-3Galbeta1-4GlcNAcbeta1-2Manalpha1+ ++-6(GlcNAcbeta1-4) (GlcNAcbeta1-2Manalpha1-3)Manbeta1-4GlcNAcbeta1- 4(Fucalpha1-6)GlcNAc- PA, was identical to the pyridylaminated form of the major oligosaccharide D8 of bovine P(o) previously reported.

Genetic variants showing apparent hot-spots in the human serum albumin gene

The molecular defects of three different slow-migrating genetic variants of human serum albumin, albumins Kamloops (formerly RIH), Stirling and Amsterdam, previously characterized only by electrophoretic and dye-binding studies, are now reported. Two of them are proalbumin variants: sequential analysis of the purified whole proteins has established the mutation responsible for albumin Kamloops as -1Arg-->Gln, and for albumin Stirling as -2Arg-->His. A Glu-->Lys substitution in position 570 of the mature albumin molecule was determined in albumin Amsterdam by sequential analysis of two abnormal tryptic fragments. The three alloalbumins are caused by single-base changes all of which seem to represent hot-spots in the albumin gene. The possible functional consequences of the presence of a circulating alloalbumin are discussed.

Structural characterization, stability and fatty acid-binding properties of two French genetic variants of human serum albumin

Four bisalbuminemic, unrelated persons were found in Bretagne, France, and their variant and normal albumins were isolated by DEAE ion exchange chromatography, reduced, carboxymethylated and treated with CNBr. Comparative two-dimensional electrophoresis of the CNBr digests showed that three of the variants were modified in fragment CB4, whereas the fourth had an abnormal fragment CB1. These fragments were isolated, digested with trypsin and mapped by reverse-phase HPLC. Sequencing of altered tryptic peptides showed that the three variants modified in CB4 were caused by the same, previously unreported, amino acid substitution: Asp314-->Val (albumin Brest). The fourth, however, was a proalbumin variant with the change Arg-2-->Cys (albumin Ildut). Both amino acid substitutions can be explained by point mutations in the structural gene: GAT-->GTT (albumin Brest) and CGT-->TGT (albumin Ildut). The proalbumin Ildut is very unstable and already in vivo it is to a large extent cleaved posttranslationally to Arg-Albumin and normal albumin. Furthermore, we observed that during a lengthy isolation procedure the remaining proalbumin was changed to Arg-Albumin or proalbumin lacking Arg-6. In addition, part of normal albumin had lost Asp1. Gas chromatographic investigations using isolated proteins indicated that albumin Brest has improved in vivo fatty acid-binding properties, whereas the structural modification(s) of albumin Ildut does not affect fatty acid binding.

Effect of genetic variation on the fatty acid-binding properties of human serum albumin and proalbumin

In the circulation, non-esterified fatty acids are transported by albumin which also facilitates their removal from donor cells and uptake into receptor cells. We have studied whether genetic variations in the albumin molecule can affect its in vivo fatty acid-binding properties. The fatty acids bound to 25 structurally different variants and to their wildtype counterparts, isolated from heterozygous carriers, were determined gas chromatographically. The variants were proalbumins, albumins with single amino acid substitutions and glycosylated or truncated albumins. In eight cases the total amount bound to the variants was diminished (0.4-0.8-fold), and in seven cases the load was increased to 1.3 or more of normal. Twenty-one fatty acids were quantitated, and for 19 alloalbumins significant deviations from normal were found. Usually, changes in total and individual fatty acid binding were of the same type, but several exceptions to this rule was found. The glycosylated albumin Casebrook showed the largest changes, the total load and the amount of bound palmitate was 8.6 and 14 times, respectively, the normal. The most pronounced changes and the majority of cases of increased binding were caused by molecular changes in domain III. Mutations in domain I, II and the propeptide resulted in smaller effects, if any, and these were often reductions in binding.

Structural characterization of four genetic variants of human serum albumin associated with alloalbuminemia in Italy

A long-term electrophoretic survey on plasma proteins, which was carried out in several clinical laboratories in Italy, identified 28 different genetic variants of human serum albumin and four cases of analbuminemia. We have previously characterized 16 point mutations, 3 C-terminal mutants, and the genetic defects in two analbuminemic subjects. Here, we report the molecular defects of four alloalbumins that have been characterized by protein structural analysis. Of these, three represent new single-point mutations: albumins Tregasio, Val122-->Glu, Bergamo, Asp314-->Gly, and Maddaloni, Val533-->Met. The fourth, albumin Besana Brianza, has the same Asp494-->Asn mutation that introduces a glycosylation site which has been previously reported in a variant from New Zealand, albumin Casebrook. However, in contrast to albumin Casebrook, albumin Besana Brianza is only partially glycosylated and the oligosaccharide is heterogeneous, consisting of a biantennary complex type N-glycan with either two or one sialic acid residue(s) on the antennae. Both albumin Maddaloni and Besana Brianza represent mutations at hypermutable CpG dinucleotide sites; albumin Maddaloni is a mutant that does not involve a charged amino acid.