Carbohydrates- Biologically Important Heteropolysaccharides

Be able to describe the difference between gram-positive and gram-negative bacteria.

Bacteria can be divided into gram-positive and gram-negative depending on the composition of their cell walls. Gram-positive bacteria have a single bilayer membrane with a layer of peptidoglycans on top (I'll describe these in a bit). The carbohydrates in the peptidoglycan layer react with Gram stain, which is why they are called gram-positive. Gram-negative bacteria, on the other hand, have two bilayer membranes with a layer of peptidoglycans between them.

Be able to describe the peptidoglycan structure of gram-positive and gram-negative bacteria.

Know that the two carbohydrates are N-acetylglucosamine and N-acetylmuramic acid.

Peptidoglycans are made up of parallel chains of repeating units of N-acetylglucosamine and N-acetylmuramic acid. Each N-acetylmuramic acid molecule has a short peptide chain attached to it, consisting of L-Ala, D-Glu, L-Lys and D-Ala. The L-Lysine residues attached to one carbohydrate chain are then joined to the D-Ala residues of the next via pentaglycine crosslinks (in the case of gram-positive bacteria) or direct crosslinks (in the case of gram-negative bacteria).

Be able to recognise the key functional groups in peptidoglycans.

Recognising functional groups is pretty easy- once you learn some functional groups, you can recognise them anywhere. Some points of interest: both N-acetylglucosamine and N-acetylmuramic acid have acetyl groups attached to -NH attached to C2 (hence the "N-acetyl" parts in both of their names). N-muramic acid also has a -COOH group in its structure, hence the "acid" part.

Be able to describe the glycocalyx and the types of carbohydrates found in the glycocalyx.

The glycocalyx is the carbohydrate layer surrounding eukaryotic cells. Carbohydrates are often bound to proteins and lipids. Carbohydrates bound to proteins can either be O-linked (joined to an -OH group of a serine or threonine residue) or N-linked (joined to the -NH2 group of an asparagine residue). Proteoglycans, a special type of glycoprotein that I'll cover in my next post, are also found here.

Be able to describe how carbohydrates found in the glycocalyx interact with proteins found on the surface of other cell types, bacteria or viruses.

Carbohydrates can add some extra shape and structure, allowing other proteins to interact with them more efficiently. For example, cell-surface antigens can help cells recognise each other. Proteins that interact with carbohydrates are called lectins.

Know that most plasma proteins contain carbohydrates.

Didn't know that before, but I do now. Apparently glycosylation (adding of carbohydrates to a protein) is the most common form of post-translational modification too.

Know that O-linked glycans link through serine & threonine and N-linked glycans linked through asparagine.

Know that N-acetylglucosamine and N-acetylgalactosamine are commonly linked to the protein backbone.

I've already mentioned this before. Carbohydrates that O-link tend to O-link via their N-acetylgalactosamine or N-acetylglucosamine residues. N-links, on the other hand, are always via N-acetylglucosamine residues.