You have probably heard of keratin before–it is the protein that makes up your hair and nails. It also makes up hooves, antlers, horns, and feathers. It is classified as a filamentous protein (because it forms filaments).
Keratin filaments look a bit like rope and consist of several individual keratin proteins. The type of keratin in hair, alpha-keratin, adopts a structure called an alpha-helix along most of its length. (The type in nails and horns, beta-keratin, adopts a different structure called a beta-sheet. This post will focus on alpha-keratins).
Two keratins twist together and make a bit more complex structure called a coiled-coil.
Two coiled-coils stick together and make an even more complex structure called a protofilament.
Four protofilaments twist together and make a filament.
Remember that all proteins are made of protein pieces called amino acids. Keratin has a very high amount of cysteine amino acids, which allows it to be durable. Cysteines are unique because they can bind to each other very strongly, a lot stronger than any other kind of amino acid, to form bridges.
The more of these bridges there are (known in biochemistry as “disulfide bridges”), the more durable and less flexible the keratin structure is. For example, the keratin in a ram’s horns has a lot more bridges than your hair does!
The curliness of your hair also comes from the arrangement of these bridges. You’ve probably noticed that your hair is not 100% permanently stuck in the same exact shape all the time–that is because the bridges can re-arrange too. The heat from a flat iron breaks bridges. Similarly, “perms” provide curls by applying chemicals that break bridges and re-form them in the desired arrangement.
The abundance of cysteine in keratin makes it extremely insoluble in water. This is good for our hair and nails… can you imagine if your hair just dissolved every time you took a shower or walked in the rain?? Thank you, cysteine!
Check out this video for another explanation of disulfide bridges and curls: What Makes Hair Curly | CURLY HAIR SCIENCE SERIES Pt.2
Garrett, R. H., & Grisham, C. M. (2010). Biochemistry. Belmont, CA: Brooks/Cole, Cengage Learning.