Last week I taught Allison about collagen, which is approximately 7000% more cool than you think it is. There are 28 (!) types of the protein in our bodies, with 5 types being the most common. Although each type is interesting and fun, I mostly care about the first one because it’s what’s found in skin (and therefore parchment).
No matter where you’re getting your collagen from, however, it’s got some basic features: it’s composed of 3 chains of amino acids that form a left-handed triple helix, with 3 of these triple helices going on to form ANOTHER triple helix. These tropocollagen building blocks all bundle together and become stabilized by lysine and hydroxylysine cross-linking, leaving a tangled mess of collagen fibrils that are stronger than steel. The extent of cross-linking in collagen actually changes depending on the age of the animal, with younger animals having less links (making their skin more flexible) and older animals having more links (making their skin tough and brittle – aging, yay!!).
Beyond these basic structural features, looking closely at the amino acids that make up collagen helices reveals that it’s quite the rebellious protein. The most common motifs found are Glycine-Proline-X and Glycine-X-Hydroxyproline, with “X” representing any amino acid that is not Glycine, Proline, or Hydroxyproline. If you haven’t taken biochemistry then you probably don’t know that this is wildly unusual, but it is – real deal.
Glycine, Proline, and Proline’s variants are notorious in the amino acid world for their dramatic steric properties: Glycine’s R group is literally just a Hydrogen atom, making it small, super bendy, and perfect for causing unwieldy kinks. Proline, on their other hand, is unique in that its R group twists around and forms a ring with an amino group that’s usually unburdened. This conformation locks the amino acid up, making it kind of a sore thumb. Basically, these amino acids can cause issues in protein structure/folding and so it’s wild to see that they make up most of the amino acid content in collagen.
We don’t know exactly why these motifs work in collagen, but it seems like they cause size/shape/charge patterns that result in immense stability. Collagen is so stable, in fact, that it’s been successfully isolated from the bones of 60+ million year old dinosaurs. These stabilizing motifs also appear to be essential for the health of the animal, as substituting a Glycine with something else has been connected with many diseases.
Now that we’ve got a decent handle on collagen, the goal for 5th week is to learn about the chemistry of PVC erasers and to think about how these erasers interact with collagen fibers. Allison and I will also continue to establish contact with UW Oshkosh in order use their MALDI-TOF, but it’s looking like this summer is going to require some quality time with vacuum tubes before the MS is up and running. Woo!