Carbohydrates- Glycoproteins and Proteoglycans

Know how carbohydrates affect the physical properties of proteins.

Carbohydrate chains attached to proteins are bulky and hydrophilic. Hydrophilicity may affect the solubility of the protein. Some carbohydrates also have charges, particularly the acidic ones (e.g. N-acetylmuramic acid). Adding carbohydrates may also protect proteins from digestion by proteases.

Be able to recognise a hydrogen bond.

Know what a polar group is.

I've covered hydrogen bonds and polarity before back when I was in high school blogging about intermolecular bonding, so I'm not going to go into these concepts again now.

Know what hydrophilic means.

Hydrophilic, literally "water-loving," basically means that a molecule is soluble in water.

Know why carbohydrates are hydrophilic.

Carbohydrates have lots of -OH groups which can form polar bonds with water molecules. This allows carbohydrates to dissolve in water.

Know that carbohydrates can have multiple binding sites on glycoproteins.

As mentioned before, carbohydrates can O-link to serine or threonine residues or N-link to asparagine residues. If there are multiple serine, threonine or asparagine residues, carbohydrates can have multiple binding sites on glycoproteins.

Know that proteoglycans are a major component of the extracellular matrix.

As proteoglycans are hydrophilic, proteoglycans (carbohydrates attached to a protein backbone) are common in the extracellular matrix. They can link cells to extracellular components or extracellular components to each other. They can also bind growth factors, thereby influencing cell growth.

Know that proteoglycans are a major component of collagen.

Hmm that's interesting... the summary is the only place where the word "collagen" appears in these lecture slides. Perhaps he meant cartilage? Proteoglycans are important in cartilage...

Be able to explain the role of proteoglycans in cartilage.

Cartilage, as you probably know, helps to cushion joints. It does this through its water content. Hydrophilic proteoglycans are good at holding water, and hence they are essential to the structure and function of cartilage.

Know the major proteoglycans in cartilage.

One of the main proteoglycans in cartilage is aggrecan, which consists of a protein backbone with many carbohydrate chains. In fact, many proteoglycans are heavily glycosylated, making them even better for holding water. Some of the carbohydrates here are keratan sulfate and chondroitin sulfate. Both of these are glycosaminoglycans, which I'll explain in a bit.

Be able to describe the key structural features of proteoglycans.

As mentioned before, proteoglycans are heavily glycosylated proteins. Many of the carbohydrate groups are glycosaminoglycans- don't worry, I'll get to these soon! These glycosaminoglycans, however, are not linked directly to the backbone- there is usually a short oligosaccharide between them and the backbone.

Proteoglycans can be held together by yet another backbone. For example, in cartilage, aggrecan molecules are often seen joined to a hyaluronan backbone via non-covalent bonds. Hyaluronan is also a glycosaminoglycan. Now it's time to finally get around to explaining what glycosaminoglycans are!

Know the key structural characteristics of glycosaminoglycans.

Glycosaminoglycans are just repeating chains of disaccharides. Yup, I built up the intensity just so I could tell you that.

These disaccharides do, however, have some special features. For example, one or both of the monosaccharides has a negative charge, which makes glycosaminoglycans even better at holding water. Also, one of the monosaccharides has an amino group, hence glycosaminoglycans.

Be able to describe the type of linkages between repeating disaccharides.

I'm not really sure what I'm meant to describe here, so I'm just going to link back to an earlier post, in which I wrote about glycosidic bonds between carbohydrates.

http://year11misadventures.blogspot.com.au/2015/11/carbohydrates-biologically-important.html

Be able to describe the key functional groups of glycosaminoglycans.

Once again, functional groups are something you can just learn and apply to a variety of situations. Here's one of my better posts on functional groups: http://year11misadventures.blogspot.com.au/2014/08/organic-chemistry-some-new-functional.html

Be able to distinguish glycosaminoglycans from other heteropolysaccharides and homopolysaccharides.

I'm fairly sure that I've already covered the main distinguishing features of glycosaminoglycans: at least one negative charge and at least one amino group. Done.