Biochemistry Exams - 2008/2009 3rd year Undenominated Science Semester I - Question 6

Hi ya all,

Today we carry on with the 3rd year Biochemistry Exam Paper which you can find here: 2008/2009 3rd year Undenominated Science Exam Paper Semester I.

Question 6. Describe the main structural features of glycoproteins and proteoglycans and, by giving examples, outline some of the key biological functions of these glycoconjugates.

Super short background:)

Glycoproteins - are proteins which carry oligosacharide (sugar) chains covalently attached to aminoacid sidechains (N-glycosylation at Asparagine and O-glycosylation on Serine, Threonine, hydroxylysine and hydroxyproline).

 Picture taken from http://www.ideacenter.org/stuff/contentmgr/files/e27b080d92450837e43d44bf73780847/misc/glycoprotein.jpg

Process of sugar attachment to protein is called glycosylation. Glycosylation is a example of either post-translational (after protein being synthesized) or cotranslational (during protein synthesis) modification.

 

Most of the glycoproteins are targeted to the cell membrane where they are responsible for cell to cell interactions. The best example of the glycosylated proteins are mucins. Mucins are proteins found on the surface of epithelial cells of the respiratory and digestive tracts. They play very important protective role. They are part of the mucus that prevents from antigens like bacteria, viruses or other particles. They play similar role in the digestive tract where they protect epithelial cells from harmful action of acids or proteolytic enzymes.

Proteoglycans - are also glycoproteins but they can carry form one to few long chains of sugars (glycosaminoglycans). Proteoglycans are usually negatively charged as the sugar residues contain acid moieties.

Picture taken from http://medinfo.ufl.edu/pa/chuck/summer/handouts/images/gag.jpg

One of the proteoglycan function is to "fill" spaced between the cells (extracellular matrix). They are responsible for strength and flexibility of the connective tissues. It is also involved in cell to cell interactions and cell signaling pathways (they regulate movement of molecules through extracellular matrix). Because proteoglycans bear negative charge their also involved in sequestering metal ions (like sodium, potasium or calcium) and water.

Good example of such proteoglycan is a aggrecan which is a major component of the cartilage. Aggrecans bears very high negative charge which function is to regulate osmotic potential of the cartilage. This gives cartilage its strength and flexibility.

Different examples of glycosylated proteins:

- immunoglobulins (anitbodies),

- glycoproteins are responsible for different blood types,

- Glycoprotein-41  and glycoprotein-120 are HIV viral coat proteins and play essential role in the HIV infection of T4 helper cells.

That would be it:)

I hope it was helpful:)

Maciek GGSTEAM