Glycoproteins Flashcards
What is a glycoprotein?
a protein with covalently attached carbohydrate
Where does glycosylation occur?
RER lumen
When does glycosylation occur?
as the polypeptide is being synthesized, aka cotranslationally
How does the glycosylation occur?
a preassembled oligosaccharide is transferred by dolichol phosphate to an appropriate Asn in the recognition sequence
What is Dolichol phosphate?
an intermembrane protein in the RER membrane that catalyzes the N-glycosylation of proteins as they are translated/synthesized
Where on the protein are the N-linked sugars attached?
to the nitrogen group of an appropriate asparagine in the recognition sequence of the protein
Where on the protein are the O-linked sugars attached?
to the oxygen of the hydroxylated side chain of amino acids serine, threonine, or hydroxylysine
Collagen and fibronectin are both __-linked glycoproteins.
O-linked
Will most glycoproteins, if glycosylation is inhibited, be more or less soluble, and why?
less soluble, because the sugars have lots of hydrophilic hydroxyl groups that increase solubility
What are the major recognition functions of glycoproteins?
hormone receptor, cell surface receptor, integrins, cell surface adhesion molecules
What is the main function of integrins?
to anchor cells to the extracellular matrix (necessary for Viability of most cells)
What mediates the aggregation of like cell types?
adhesion molecules on the cell surface (glycoproteins), such as cadherins
What is the immune function of glycoproteins?
host defense and pathogen presentation; WBC traveling through endothelium
What is the role of selectins in the extravasation of WBCs?
selectins are on the endothelial cells and they adhere to the WBCs, recognize, and then they weakly stick and roll along endothelium with further help of integrins
How are sugars linked together to form oligosaccharides?
Linked by enzymes through many of their hydroxyl groups to form oligosaccharides
Fibronectin binds both cells and ___________ molecules.
Extracellular matrix
Describe the structure of Fibronectin.
A dimer of two similar subunits joined by disulfide bonds at the C terminal.
True or false: glycoproteins protect proteins from degradation by proteases.
True
What cell surface components are necessary for cell-cell recognition and interaction and aggregation?
Glycoproteins
What glycoprotein mediates diapedesis of WBCs along the endothelium of vessels?
Selectin
What glycoprotein mediates margination (extravasation) of WBCs through the endothelium of vessels?
Integrin
What is unique about albumin?
it’s an extracellular protein that is not glycosylated
What are proteoglycans?
a protein core that is heavily glycosylated with several GAG (glycosaminoglycan) residues
What sugars are used in the formation of glycoproteins?
mannose, GlcNac/NAG, sialic acid
What’s the difference between a glycoprotein and a proteoglycan?
glycoproteins have lower carbohydrate content, whereas proteoglycans have a higher protein content
When do N-linked oligosaccharides undergo modifications?
prior to secretion of the glycoprotein from the RER, as well as in the ER–>Golgi processing
Dolichol phosphate attaches what to the nascent protein?
a standard, preassembled oligosaccharide
Describe the structure and function of fibronectin.
it binds both cells and ECM molecules; dimer joined by disulfide bonds, 3 forms from one gene, and it’s multifunctional with modules
Inhibiting a selectin may inhibit what important cell-cell interaction?
WBC extravasation
How do glycoproteins come in with regard to lysosomal function?
a phospho-mannose tag is needed to target/keep the protein/enzyme in the lysosome
What is I-cell disease?
a lysosomal enzyme disorder where the enzymes are glycosylated and not taken to the lysosome, therefore there’s a build-up of waste in the lysosome and cell death; resulting in mental retardation and skeletal abnormalities
How are oligosaccharides involved in protein life spans?
If the sialic acid is lost from the oligosaccharide, the half life is decreased x700 because it is targeted for uptake by liver cells.
What is the modular function of fibronectin?
globular polypeptide that functions for a specific binding purpose/anchor
