Hans Paulsen: Contributions to the Investigations of Glycoprotein Biosynthesis

Abstract

Hans Paulsen was one of the first scientists who believed that chemistry should be applied to biology and medicine. His interest in natural products and their roles solidified in the 1970s. He passed on his knowledge to hundreds of students and coworkers and advanced science with many national and international collaborators. No matter where he was, at home or travelling, he was always curious and keen to learn, from chemistry to enzymes, their roles in diseases, and the possible applications of synthetic compounds. His creative chemistry and synthesis of novel compounds made essential contributions to elucidating the mechanisms and pathways of glycoprotein biosynthesis. This review describes the biosynthetic pathways of the O- and N-glycans of glycoproteins and studies of novel substrates and inhibitors developed by Hans Paulsen's group.

Keywords: N-glycosylation; O-glycosylation; glycopeptides; glycoproteins; glycosyltransferases; inhibitors; mucins; substrates.

Figure 1

Early pathways of N-glycan biosynthesis in the ER.

Figure 2

Golgi pathways of N-glycosylation. N-glycans are processed to hybrid or complex N-glycans and extension of the antennae in the Golgi. Glycans are trimmed by mannosidase, followed by the transfer of GlcNAc in β1-2 linkage to Manα1-3 of the Man3-arm by MGAT1. This facilitates further synthesis by α6-fucosylation of core GlcNAc attached to Asn, trimming of Man residues on the Man6-arm, and synthesis of further antennae by MGAT2,4,5 and bisecting structures by MGAT3. The antennae are then extended by a wealth of different enzymes that introduce repeating Gal-GlcNAc units and many possible branched and linear epitopes, Lewis antigens, blood groups, and tissue-specific structures. Glycoproteins are then transported to their preferred locations.

Figure 3

Specificity of GlcNAc-transferase V. GnT V transfers a GlcNAcβ residue to the 6-hydroxyl of the Manα1-6 residue of the N-glycan core. The acceptor substrate for purified GnT V from the hamster kidney was a derivative of the 6-arm, GlcNAcβ1-2Manα1-6Manβ-R. The 3-arm of the N-glycans is not essential for GnT V activity. R can be an oligosaccharide or a hydrophobic group such as octyl. The Manβ-residue can be replaced by Glcβ having full activity. To determine acceptor specificity, hydroxyls were deleted or replaced by a larger group. The rectangular boxes indicate an absolute requirement for the hydroxyl, while the circles indicate that the hydroxyl is important for the activity. Replacement of the 4- and 6-hydroxyls of the Manα1-6 residue with an O-methyl group yielded the GnT V inhibitor GlcNAcβ1-2(4,6-di-O-methyl)Manαl-6Glcβ-pnp.

Figure 4

O-glycosylation pathways, synthesis of cores 1 to 4. All mucin-type O-glycans are linked through GalNAcα to Ser or Thr. This linkage is formed by one or more of 20 polypeptide GalNAc-transferases (GALNTs) that are expressed in a cell type-specific fashion. GalNAc can be converted to core 1 by β1,3-Gal-transferase C1GALT1, or to core 3 by β1,3-GlcNAc-transferase B3GNT6. Unmodified GalNAc is recognized as the cancer-associated Tn antigen. If sialic acid is added to GalNAc by ST6GalNAc1, the O-glycan becomes the Sialyl-Tn antigen, which is not further extended. Core 1 can be converted to core 2 by GCNT1, GCNT3, or GCNT4. Core 3 is converted to core 4 only by GCNT3. Unmodified core 1 is named the T antigen, which is often found in cancer. Sialylation of the T antigen also stops further extension to complex chains.

Figure 5

Hans Paulsen at the International Carbohydrate Symposium in Whistler, Canada (2006).