Chemical nature of sialic acid- Question of the day.

The influenza virus infects cells by binding its viral hemagglutinin to “Sialic acid” on the surface of epithelial cells. Which of the following represents the true chemical nature of sialic acid?

A. Glycolipid

B. Amino sugar

C. Amino sugar acid

D. Lipoprotein

E. Glycoprotein.

Details

The correct answer is C- Amino sugar acid.

Sialic acid is a sugar derivative.

Basic concept

Amino sugars

Amino groups may be substituted for hydroxyl group of sugars to give rise to amino sugars. Generally, the amino group is added to the second carbon of the hexoses. The most common amino sugars are Glucosamine and Galactosamine (Figure-1).The amino group in the sugar maybe further acetylated to produce N-Acetylated sugars such as N-Acetyl Glucosamine (GluNac) and N-Acetyl-Galactosamine (GalNAc), etc.

Amino sugars

Figure-1- Glucosamine and galactosamine are important constituents of glycoproteins, mucopolysaccharides and cell membrane antigens.

Amino sugar acids are produced by condensation of amino sugar with Pyruvic or lactic acid. E.g. Muramic acid is produced by the condensation of lactic acid with D- Glucosamine. Certain bacterial cell walls contain Muramic acid.

N-Acetyl Neuraminic acid is formed from the condensation of Pyruvic acid with N-Acetyl Mannosamine (Figure-2). N-Acetyl Neuraminic acid (NANA), also called Sialic acid, is a nine carbon derivative and is an important component of glycoproteins and gangliosides (lipids). Neuraminidase is the enzyme which removes NANA from its binding with other compounds.

Amino sugar acids

Figure-2- Structures of Muramic acid and Sialic acid.

As regards other options:

A. Glycolipids are compound lipids that contain carbohydrate and lipid. Cerebrosides and gangliosides are examples of glycolipids.

B. Amino sugar- explained above.

D. Lipoproteins are conjugated proteins, that contain lipid as a prosthetic group.

E. Glycoproteins- are also conjugated proteins that contain carbohydrate as a prosthetic group.

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