Definitions

Glycan: refers to the carbohydrate portion of a glycoconjugate, such as a glycopeptide, glycoprotein, glycolipid or proteoglycan. Regnier (US8,568,993) 

N-glycans

Peptides expressed in eukaryotic cells are typically N-glycosylated on asparagine residues at sites in the peptide primary structure containing the sequence asparagine-X-serine/threonine where X can be any amino acid except proline and aspartic acid. The carbohydrate portiion of such peptides is known as an N-lniked glycan. 

N-glycans attached to glycoproteins differ with respect to the number of branches (antennae) comprising peripheral sugars (e.g., G1cNAc, galactose, fucose, and sialic acid) that are added to a G1cNac.2Man3 core structure. (Hiatt (US2013/0149300)

–biantennary N-glycans: refers to a complex oligosaccharide where the core comprises two branch terminal N-acetylglucosamine (G1cNAc), 3 mannose (man) and 2 (G1cNAc) monosaccharide residues that are attached to the asparagine residue of the glycoprotein.

O-linked glycans:

Peptides may also be modified by addition of O-linked glycans, also called “mucin-type glycans” gecasue of their prevalence on micinous glycopeptide. Unlike N-glycans that are linked to asparagine residues and are formed by en bloc transfer of oligossaccharide from lipid-bound intermediates, o-glycans are linked primarily to serine and threonine residues and are formed by the stepwise addition of guars from nucleotide sugars. (Defrees (US2008/0206808).

Sialic Acids

Sialic acid: is a generic term for the N- or O-substituted derivatives of neuraminic acid, a monosaccharide with a 9 carbon backbone. It is also the name for the most common member of this group, N-acetylneuraminic acid (Neu5Ac). Sialic acids occur at the end of sugar chains connected to the surfaces of cells or soluble proteins. 

Sialic acid on glycans are known to be important in prolonging the serum half-life of glycoproteins other than antibodies. So far the fole of sialic acid on monoclonal antibodies is not well understood (Cai, WO2007/005786). 

Sialic acids (Sias) are nine carbon backbone monosaccharides with a carboxylic function in the C-1 position and are usually present at the outermost (non-reducing) end of glycan chains in the deuterostome lineage of animals. More than 20 sialyltransferases have been described which attach different Sias onto various acceptor structures in highly specific linkages. The predominant sialic acids found on mammalian cell surfaces are Neu5Ac and Neu5Gc. Being positioned at the outermost end of glycan chains, Sias represent the receptors most frequently targeted by pathogens that use Sia binding proteins to access hot cells and specificity towards either Neu5Ac or Neu5Gc may be observed. They are also known to serve as ligands for intrinsic sialic acid binding proteins such as Siglecs. Some Siglecs even discriminate between Neu5Ac and Neu5Gc even though the two Sias differ only by the presence or absence of a single oxygen atoms. Whereas most mammals express Nue5G containing glycans on their cell surfaces, glycans on human cells primarily express the precursor molecule Neu5Ac. Indeed, humans generate immune responses against molecules carrying Neu5Gc (e.g., the “serum sickness” reaction to equine anti-thymocyte globuilin therapy). The loss of Neu5Gc occured about 3 million years ago and represented the first known genetic difference between huamns and chimpanzees that could be direclty linked to an altered phenotype. Despite the human specific loss of Neu5Gc de novo biosynthesis, Neu5Gc has been detected in several malignant human tumors. Ghaderi, Biotechnology & Genetic Engineering Reviews, 28, 147-176 (2012). 

Protein sialylation is an ezymatic process, and is the terminal reaction of glycosylation that produces matured sialylated oligosaccharides on glycoproteins. In rough endoplasmic reticulum (ER), high mannose core is added to newly synthesized protein. The protein is then transported to the GA. There are at least 18 different intracellular, Golgi membrane bound glycosyltransferases which catalyze the reaction for growing oligosaccharide chains by using nucleotide sugar precursors as substrates. For instand, sialytranferase ST3GAL4 (ST3 beta-galactoside alpha-2,3 sialytransferase 4) uses CMP sialic acid as a substrate and adds alpha-2,3 linked sialic acid to beta1,4 Galactose. (Xu, Mol Biotechno (2010) 45: 248-256)

N-acetyl neuraminic acid (Neu5Ac, NeuAc, or NANA)): is the most common Sia which serves as a biosynthetic precursor for most other Sias. The N-acetyl group of NeuAc is hydroxylated. This form is prevalent in glycoproteins from rodent and microbial sources. (Cai, WO2007/005786)

N-glycolyl-neuraminic aicd (Neu5Gc, NGNA or NeuGc): biosynthesis of Neu5Gc occurs exclusively by hydroxylation of the N-1cetyl group of CMP-Neu5Ac to yeild CMP-Neu5Gc. 

All humans have a unique inactivating homzygoud mutation in CMP-Neu5Ac hydroxylase (CMAH), eliminating the enzymatic activty that generates CMP-Neu5Gc. The CMAH mutation was caused by an Alu replacement even about 2.5 millions years ago and resulted in the absence of Neu5Gc and a secondary increased level of Neu5Ac on human cell surfaces (Sonnenburg, Glycobiology, 14(4), 339-346, 2004).