In more than 95% sand by weight.

In this lesson, you’ll explore the definition and purpose of proteoglycans, specialized molecules found around cells and in the joints of our bodies. Learn about their structure and functions, then take a quick quiz to test your understanding.

Proteoglycans: Definition ; Location

Ever touched a slug or a frog? Slimy! That sliminess is also found protecting different organs of your body, like in your gut and your throat.

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Some of the components of that slime include proteoglycans, or PGs. PGs are protein molecules found in the space between our cells, a physiological component known as the extracellular matrix (ECM).Cells are like a flexible pile of pebbles. If they were merely surrounded by air, they’d be easily injured. ECM is like bubblegum in that it keeps our cells together and mobile as a group.

It also helps to protect them from outside forces and chemicals that would tear them apart. ECM also acts as a filter for other molecules, like chemicals and proteins. It also blocks and delays chemical signals or proteins.In addition to the ECM, PGs are found anchored to the surface of cells or traversing the cell membrane, where they are involved in cellular signaling. We also find PGs in the joints, cartilage, and even the cornea, where they aid in wound healing and embryonic development.

Chemical Composition

The word proteoglycan helps to provide us with clues to the molecule’s chemical composition. For example, proteo refers to protein, while glycan means sugar or a group of sugars. As such, a proteoglycan is a long polysaccharide (sugar) chain covalently attached to a protein and produced in the endoplasmic reticulum (ER), a specialized organelle.PGs are more than 95% carbohydrate (sugar) by weight.

But what does that mean? Well, imagine picking up a handful of sand and pebbles. If you had mostly sand and one or two pebbles, then your handful would be more than 95% sand by weight. By comparison, if you picked up more pebbles and only a pinch of sand, your handful would be more than 95% pebbles by weight. Thus the weight of PGs is mostly made up of sugar.

Chemical Structure

Have you ever seen one of those little gel capsules crammed with a sponge that magically inflates when you put it in water? Well, the 3-D structure of a PG is rather like those sponge-filled capsules: They fill up lots of space but are not very dense, and they often love water.

The chains of sugar in a PG extend from its main core protein, kind of like the strands of your hair when you rub a balloon on them. Because PGs often have lots and lots of chains of sugar chains, they are frequently very big and heavy molecules. All PGs have a particular sugar chain known as glycosaminoglycan (GAG). A GAG is a sequence of two sugars, where one of the sugars is a sugar acid, and the other contains an amine group. The sugars often have various numbers of sulfurs attached in diverse places. A GAG chain is where the two sugars constantly repeat.There are different types of sugar acid and sugars with amine groups involved, so there are different types of GAGs.

The sulfate groups attached to the GAG sugars increase the negative charge. The negative charge attracts lots of positive charges, like sodium (Na+), which invites water to come along as well. GAGs, and by association PGs, are hydrophilic, or water loving.

Also, GAGs and sugar chains are rigid – that is, they don’t bend easily. Their rigidity and water-loving property also protects the cells from outside forces that would harm them.

Lesson Summary

Proteoglycans (PGs) are composed of a core protein with long chains of sugars covalently attached.

They must be at least 95% sugar by weight and have at least one glycosaminoglycan (GAG) attached to the protein. Although not very dense, PGs are often large and heavy molecules. They’re also hydrophilic, or water loving.PGs are most often found in the extracellular matrix (ECM), but can also be anchored to or traverse the cell membrane. The stiffness and hydrophilic nature of the sugar chains help the PGs protect cells from outside forces.

PGs are important aids in wound healing, embryonic development, and cellular signaling; they also act as filters for proteins and chemicals. In addition to the ECM, proteoglycans can be found in the protective mucosa of the gut and cornea, as well as joints and cartilage.

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