Shown following is the first 30 amino acid region of this putative amino acid se

Question

Shown following is the first 30 amino acid region of this putative amino acid sequence for p77:

1YFNPT     6DSWPT    11LAPAN    16YSFLF    21FLARY    26WYINL30

Based on the partial amino acid sequence provided for p77,

a. which amino acids in the sequence provide may be glycosylated and how can you identify these potential sites?

b. and with which type of glycosylation might they be modified?

c. and explain how these modifications are made,

d. and explain where these modifications are made in the cell,

e. and finally, explain what effects glycosylation might have on the protein.

Explanation / Answer

a. The given protein sequence contain the following amino acids that can be glycosylated (numbers in backet indicate the amino acid position in the polypeptide chain)

Asparagine (3, 29)

Threonine (5, 10)

Serine (7, 17)

Tryptophan (8, 26)

b. Each of these amino acids are glycosylated through the addition of a glycan or sugars. The linkage is named after the type of attachment:

Asparagine is linked to sugars through its –NH2 (amino group), the linkage is called N-linked glycosidic bond

Serine and threonine use their alcoholic –OH group to link the sugars. It is called O-linked glycosylation

Tryptophan specifically gets glycosylated by binding to Mannose, it is called a C-linked glycosylation

Glypitation involves substitution of a –COOH group with a lipoprotein

c. The modifications are done by specific enzymes called glycosyl transferases. Each type of carbohydrate is specific for a glycosyl transferase. The tranferases are non-template directed but are highly specific

d. Glycosylation occurs in the endoplasmic reticulum and golgi complex

e. Glycosylation determines the binding properties of proteins to their ligands. They give stability to the protein. Non-glycosylated proteins are degraded faster than glycosylated proteins, in the cells. Glycosylation aids in proper protein folding also. Glycosylation may target proteins to specific target sites, for eg; Mannose-6-phospate residues target proteins to lysosomes.

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