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a glycoconjugate (sugar/something else combo) where sugars get the starring role and protein serves to kinda hold them all together. These sugars are GlycosAminoGlycans – large polysaccharides with a lot of modifications. These modifications include lots of amino sugars (hence the A in GAG) but they also often include things like oxidation to carboxylate or the addition of sulfate groups. GAGs can have different patterns of modifications that serve as a sort of code that is “read” by proteins in the extracellular matrix (ECM) (the molecularly-rich environment in between cells). How it works is that the different modifications make for different binding sites that complement different proteins’ structures. So then those proteins can bind, which can be useful in several ways. Sometimes, binding may get the proteins to shape-shift (undergo a conformational change) that activates or inactivates it. Other times, binding might serve to just keep the protein there. And, since GAGs can be really long, they can bind multiple proteins and serve as “hubs” or “scaffolds” at which that multi-protein reactions can take place.⠀

In addition to those more specialized functions, proteoglycans have “generic” ones. Modifications like carboxylation & sulfation add negative charge. And when there’s negative charge, you know metal cations (positively-charged molecules) are gonna wanna come play! Sodium ions (Na⁺) drop in to serve as counter ions, and they bring water with them. So these proteoglycans soak up a bunch of water, gel-like. Different ones (which include heparan sulfate, chondroitin sulfate & keratin sulfate) have different gel-ness-es & thus are best suited for different things – from tense tendons & strong nails, to the shock absorbers between your joints, & the mucus in your nose! Yup, “mucus” is a proteoglycan consisting of a mucin protein covered with sugars! There are actually dozens of mucin proteins and the proteins involved in mucin & in all the other proteoglycans vary a lot – which they can do since it’s the sugars starring here!⠀