The polypeptide part of human chorionic gonadotrophin affects the kinetics of alpha 6-sialylation of its N-linked glycans but does not alter the branch specificity of CMP-NeuAc:Gal beta 1----4GlcNAc-R alpha 2----6-sialyltransferase

The N-glycan acceptor specificity of a glucuronyltransferase, GlcAT-P, associated with biosynthesis of the HNK-1 epitope

The acceptor specificity of a rat brain glucuronyltransferase, GlcAT-P, associated with biosynthesis of the HNK-1 epitope on glycoproteins, was investigated using asialoorosomucoid as a model acceptor substrate. Structural analysis of N-linked oligosaccharides, to which glucuronic acid was transferred by GlcAT-P, by means of two-dimensional mapping of pyridylamino-oligosaccharides and MS spectrometry, demonstrated that the enzyme transferred glucuronic acid to bi-, tri-, and tetra-antennary complex type sugar chains, with almost equal efficiency, indicating that the enzyme has no preference as to the number of acceptor sugar branches. Next, we studied the branch specificity of this enzyme by means of the selective branch scission method involving two step exoglycosidase digestion using authentic pyridylamino-oligosaccharides. The GlcAT-P is highly specific for the terminal N-acetyllactosamine structure and no glucuronic acid was incorporated into a Galbeta1-3GlcNAc moiety. The GlcAT-P transferred glucuronic acid to the galactose residues in the N-acetyllactosamine branches of bi-, tri-, and tetra-antennary oligosaccharide chains, with different efficiencies and most preferentially to those in the Galbeta1-4GlcNAcbeta1-4Manalpha1-3 branch.

Structural analysis of the carboxyl terminal peptide from human chorionic gonadotropin beta-subunit by two-dimensional nuclear magnetic resonance spectroscopy

PROBLEM

Vaccines that target human chorionic gonadotropin (hCG) might be made more effective through greater understanding of the solution three-dimensional structure and behavior of the hormone.

METHOD

A 37-amino acid carboxyl terminal peptide of the hCG beta subunit was synthesized, purified, and analyzed by two-dimensional nuclear magnetic resonance spectroscopy.

RESULTS

Double-quantum filtered correlated spectroscopy data on the peptide in water at 4 degrees C reveals 27 multiplets in the peptide fingerprint region, as expected. A nuclear Overhauser effect spectroscopy spectrum shows several intraresidual peaks in the amide region but lacks clearly assignable interresidual signals.

CONCLUSION

By itself in water the carboxyl terminal peptide of hCG lacks defined secondary structure elements and is thus likely a random coil. The presence of beta turns appears possible although neither their existence nor their localization can be confirmed.

Enzymic remodelling of the N- and O-linked carbohydrate chains of human chorionic gonadotropin. Effects on biological activity and receptor binding

The effects of altered terminal sequences in human chorionic gonadotropin (hCG) N- and O-linked glycans on receptor binding and signal transduction were analyzed using forms of hCG with remodelled carbohydrate chains. hCG derivatives were obtained by enzymic removal of the alpha 3-linked sialic acid residues followed by alpha 6-sialylation, alpha 3-galactosylation or alpha 3-fucosylation of uncovered Gal beta 1-->4GlcNAc (LacNAc) termini, or alpha 3-sialylation of Gal beta 1-->3GalNAc sequences. Also a form that carried GalNAc beta 1-->4-GlcNAc units, which are typical for pituitary hormone oligosaccharides, was derived by enzymic desialylation and degalactosylation followed by beta 4-N-acetylgalactosaminylation. The potency to stimulate testosterone production and the binding to the lutotropin/choriogonadotropin receptor of the preparations were compared with those of native and desialylated hCG (as-hCG). The decrease in bioactivity caused by desialylation of hCG was only restored upon alpha 6-sialylation of the Gal beta 1-->4GlcNAc beta 1-->-2Man alpha 1-->3Man branch of the N-linked glycans. This was without a major effect on receptor binding. Further alpha 6-sialylation, occurring at the Gal beta 1-->4GlcNAc beta 1-->2Man alpha 1-->6Man branch, resulted in a bioactivity below a level found with as-hCG, concomitant with a decreased receptor binding affinity. Similarly alpha 3-galactosylation of the Gal beta 1-->4GlcNAc beta 1-->2-Man alpha 1-->6Man branch yielded a hCG derivative that showed decreased bioactivity and receptor binding. alpha 3-Fucosylation of native as well as as-hCG also led to a decreased activity. Re-alpha 3-sialylation of the O-linked chains on as-hCG had little effect on the bioactivity and receptor binding. Hormone preparations with GalNAc beta 1-->4GlcNAc termini showed lower bioactivity and receptor affinity than as-hCG. It is concluded that the Gal beta 1-->4GlcNAc beta 1-->2Man alpha 1-->3Man- rather than the Gal beta 1-->4GlcNAc beta 1-->2-Man alpha 1-->6Man branch of the N-linked glycans on hCG plays an essential role in signal transduction, whereas the latter branch can potentially interfere with receptor binding. Furthermore attachment of sialic acid, but not of other sugars, to the first branch fulfils the requirement for the full expression of bioactivity, while sialylation of the O-linked chains is of minor importance.

Enzymatic characterization of CMP-NeuAc:Gal beta 1-4GlcNAc-R alpha(2-3)-sialyltransferase from human placenta

In this report we present the enzymatic characterization of CMP-NeuAc:Gal beta 1-4GlcNAc-R alpha(2-3)-sialyltransferase from human placenta using placenta membranes as an enzyme preparation. This sialyltransferase is highly sensitive to detergents and prefers type 2 chain (Gal beta 1-4GlcNAc) over type 1 chain (Gal beta 1-3GlcNAc) acceptors. Oligosaccharides and glycopeptides were better acceptor substrates than glycoproteins. Of the branched oligosaccharides, those with a bisected N-acetylglucosamine (GlcNAc) structure appeared to be poorer substrates, while triantennary structures containing a Gal beta 1-4GlcNAc beta 1-4Man alpha 1-3Man branch were preferred. Product characterization, using 400 MHz 1H-NMR spectroscopy, confirmed that sialic acid was introduced into the Gal beta 1-4GlcNAc-R units of the acceptor substrates in an alpha (2-3) linkage, and revealed that this sialyltransferase does not prefer either of the two branches of a complex type di-antennary glycopeptide acceptor for sialic acid attachment. These properties distinguish this enzyme from all other sialyltransferases characterized to date.

Human liver and human placenta both contain CMP-NeuAc:Gal beta 1-->4GlcNAc-R alpha 2-->3- as well as alpha 2-->6-sialyltransferase activity

A high pH anion exchange chromatographic (HPAEC) system for the separation of isomeric sialo-oligosaccharide products was developed. Employing this system, using Gal beta 1-->4GlcNAc beta 1-->2Man alpha 1-->6Man beta 1-->4GlcNAc as a substrate, a Gal beta 1-->4GlcNAc-R alpha 2-->3-sialyltransferase activity was detected for the first time in human liver. This activity is expressed together with the prevalent alpha 2-->6-sialyltransferase. Furthermore, in addition to the major alpha 2-->3-sialyltransferase, a low but distinct activity of alpha 2-->6-sialyltransferase was detected in human placenta. This activity could not be found by methods based on methylation analysis or high resolution NMR spectroscopy. It is concluded that HPAEC, in combination with the use of the pentasaccharide as an acceptor substrate, is suited for the specific detection of minor, Gal beta 1-->4GlcNAc-specific sialyltransferase activities.