A glycosylated form of the human cardiac hormone pro B-type natriuretic peptide is an intrinsically unstructured monomeric protein

Abstract

The N-terminal fragment of pro B-type natriuretic peptide (NT-proBNP) and proBNP are used as gold standard clinical markers of myocardial dysfunction such as cardiac hypertrophy and left ventricle heart failure. The actual circulating molecular forms of these peptides have been the subject of intense investigation particularly since these analytes are measured in clinical assays. Conflicting data has been reported and no firm consensus on the exact nature of the molecular species exists. Because these clinical assays are immunoassay-based, specific epitopes are detected. It is conceivable then that certain epitopes may be masked and therefore unavailable for antibody binding, thus the importance of determining the nature of the circulating molecular forms of these analytes. This situation is an unavoidable Achilles' heel of immunoassays in general.A recombinant O-linked glycosylated form of proBNP has been show to mimic some of the properties of extracted plasma from a heart failure patient. In particular the recombinant and native material co-migrated as diffuse Western-immunostained bands on SDS-PAGE and each band collapsed to an apparent homogeneous band following deglycosylation. Thus, glycosylated-proBNP may be one such circulating form. Here we provide extensive physiochemical characterization for this O-linked protein and compare these results to other described circulating species, non-glycosylated-proBNP and NT-proBNP. It will be shown that glycosylation has no influence on the secondary and quaternary structure of proBNP. In fact, at moderate concentration in benign physiological neutral pH buffer, all three likely circulating species are essentially devoid of major secondary structure, i.e., are intrinsically unstructured proteins (IUPs). Furthermore, all three proteins exist as monomers in solution. These results may have important implications in the design of NT-proBNP/BNP immunoassays. Immunoassays utilize antibodies that react with specific analyte (usually protein) epitope(s). If these epitopes become unreactive due to say proteolytic processing, or masked by interaction with other molecule(s), or are post-or co-translationally modified, then the immunoassay can generate incorrect results. It becomes important then to identify the circulating forms of the analytes. This analysis can include primary sequence, post-translational modifications, and quaternary structure as homo-and hetero-interaction partners. Knowledge of such can aid in the development of immunoassays that accurately detect the circulating species. Keywords GlycosylatedPrevious characterization of human NT-proBNP whether as a recombinant product or synthetic peptide showed this N-terminal fragment to be a largely unfolded, flexible protein, termed an IUP, which exists as a monomer in solution [7,8]. No comparable data were hitherto available for proBNP and a recently discovered [9] glycosylated form of proBNP. Here we use a combination of CD to assess the global average secondary structure of proBNP and glycosylated-proBNP...

Effect of Sialylated O-Glycans in Pro–Brain Natriuretic Peptide Stability

“…Our results indicated that human pro-ANP and pro-CNP did not contain any detectable amounts of either N- or O-glycans under our experimental conditions. In contrast, human pro-BNP contained substantial amounts of O-glycans but no detectable amounts of N-glycans, consistent with previous reports of O-glycans present in native human pro-BNP (15–17, 25). We extended these findings and showed that O-glycans on pro-BNP were terminally sialylated and that the presence of sialic acids protected O-glycans from O-glycosidase digestion.…”

“…To date, however, the biological significance of O-glycans on the other residues, which are away from the activation site, remains unclear. A possible role of glycosylation in pro-BNP to prevent its oligomerization has been proposed (15, 25). …”

Corin in clinical laboratory diagnostics

“…Furin also cleaves pro-C-type natriuretic peptide (pro-CNP) but not pro-ANP [54]. Recent studies show that human pro-BNP contains abundant O -glycans that are terminally sialylated [55–59]. This posttranslational modification is unusual, because no N - or O -linked glycosylation was detected in human pro-ANP and pro-CNP [56].…”

Characterization of Molecular Forms of N-Terminal B-Type Natriuretic Peptide In Vitro

“…Deamidation decreases the hydrophobicity of peptides, but a single deamidation increases the mass by only 1, a change that may go unnoticed in the mass spectrometric analysis. The recent evidence suggests that the recombinant NTproBNP as well as the glycosylated form of proBNP are intrinsically unstructured proteins (18,19 ), making them susceptible to this kind of modification. However, the epitope sequence of anti-NT-proBNP 57-76 antiserum does not contain asparagine or glutamine residues, refuting the hypothesis of deamidation for this part of the peptide.…”