The majority of proteins secreted by mammalian cells are glycoproteins. These proteins possess oligosaccharides covalently attached through the side chain amid of asparagein (N-linke) or through the side chain hydroxy of threonine or serine (O linked) A given glycoprotein may coontain only N-linked, O-oliknked or both. (Goochee, “Bioprocess factors affecting glycoprotein oligosaccharide structure” Develop biol standard, 76, 95-104, 1992). 

Specific Types of Glycoproteins

Antibodies: (see left hand panel)

Erythropoietin (EPO): is a human glycoprotein which stimulates the production of red blood cells.

Granulocyte colony stimulating factor (GCSF) stimulates production of neutrophils from bone marrow precurosr cells expressing its receptor GCSFR. GSCF is indicated for a range of clinical applications including administration to cancer patients undergoing chemotherpay and to other neutropenic patients. However, GCSF is rapidly depleted via receptor mediated endocytosis by bloodstream neutrophils expressing GCSFR. Sarkar (Nature Biotechnology, 20, pp. 908-913, 2002) disclose a method for designing improvements in endosomal sorting of GCSF by implementing histidine substituions in the ligan to provide pH activated binding affinity switches, resulting in increased ligand lifetime and potenency.

Human carcionembryonic antigen (CEA) family (CD66 family): is an Immunoglobulin Superfamily (IgSF) member cell surface glycoprotein used widely as a clinical tumor marker. It undergoes interactions which mediate intercellular adhesion. Evidence supports the view that deregulated overexpression of CEA has an instrumental role in tumorigenesis by the inhibition of cell differentiation and the disruption of tissue architecture.

1. CEACAM1 (biliary glycoprotein (BGP) or CD66a): is a transmembrane bound glycoprotein which is abundantly expressed in epitehlia, vessel endothelia and leukocytes. CEACAM1 triggers the dealy of spontaneous and FasL-induced apoptosis in rat granulocytes.

2. CEACAM6 (NCA or CD66c):

3. CEACAM8 (CGM6 or or CD67 or CD66b): is a glycosylphosphatidylinositol-anchored membrane glycoprotein which is a member of the CEA subfamily of the immunoglobulin superfamily. The classic form consists of a leader sequence of 34 aa with a 286aa extracellular domain, and a 29 aa hydrophobic domain replaced with a GPI anchor. It is predicted to have 1 IgG-V like domain (N-terminal) and 2 Ig C2 domains. It is heavily N-glycosylated (Skubitz, 1999, J Biolo. Regulators and Homeostatic Ag, pp. 242-3). In addition to the membrane achored form, a soluble CEACAM8 form is released extracellularly after stimulation. Both CEACAM8 versions comprise an identical amino acid sequence with the exception of a leader sequence labeling the designated membrane bound CEACAM8. Enzymes that posttranslationally modify CEACAM8 by added a GPI anchor to fix the protein to the membrane recognize the leader sequence.

Tissue Factor (TF): is a 47 transmembrane glycoprotein which enables cells to initiate the coagulation cascase. The extracellular N-terminal domain of TF contains functional sites for factor VIIa (FVIIa) binding (e.g., Lys20). The coagulation cascade is triggered wehn TF binds to FVIIa. Traditionally, TF was believed to be expressed only in tissues. However, it is now known that whole blood contains active TF.

VEGF: is a family of proteins that includes placenta growth factor (PIGF), VEGF-A, VEGF-B, VEGF-c, VEGF-D and the viral VEGF homologue VEGF-E. VEGF promotes proliferation of vascular endothelial cells derived from arteries, veines, and lympatic vessels through a variety of pathways by binding to VEGFR2, causing VEGF-receptor dimerization, tyrosine phosphorylation, and signal transduction for activation of mitogen-activated protein kinase.

1. VEGF-A: is a dimeric 36-46 kDa glycosylated protein compirsing nine different isoforms in humans generated from alternative mRNA splicing. A total of 4 well studied isoforms include VEGF121, VEGF165, VEGF189, and VEGF206 VEGF-A isoforms induce proliferation, migration and tube formation of vascular endothelial cells through a variety of pathways with the final activaiton of vascular protein kianses through 3 VEGF receptors: VEGFR-1 (Flt-1), VEGFR-2(KDR), and VEGF-3 (Flt-4). In addition, VEGF-A promotes enhanced vascular permeability through the formation of specialized regions in the plasma membrane of endothelial cells that are highly permeable to macromolecules called vesicular-vacuolar organelles and fenestra (Rodrigues, “Therapeutic monoclonal antibodies in ophthalmology” Progress in Retinal and Eye Research 28 (2009) 117-144.