Molecule of the Day

This is one of those boring-seeming ones: cysteine. However, biochemistry really wouldn’t work without it. Cysteine is the sulfur analogue of the amino acid serine. I say “sulfur analogue” because oxygen and sulfur are in the same group (column) of the periodic table. They are in group 6B, also known as the “chalcogens”, from the greek for “ore formers.” (Oxygen and sulfur, as well as, to a lesser extent, selenium and tellurium being ubiquitous in ores of metals). Many oxygen compounds have a sulfur analogue. In many ways members of the same group are very similar to each other, because their electrons are configured in analagous ways.

Oxygen can form a structure called a peroxide. R-OH (R representing something else attached to the oxygen) is an alcohol that has a peroxide brother in R-O-O-R. Peroxides are potent oxidizers (witness hydrogen peroxide and benzoyl peroxide’s use as disinfectants in low concentrations). At very high concentrations, they can be initiators for explosives (or explosives themselves!).

The sulfur analogue of a peroxide is a disulfide. This is a dimer of R-SH (a “thiol”), or R-S-S-R. This is an oxidized compound as well, but R-S-S-R is a much milder oxidant than R-O-O-H. Mild is the name of the game in biochemistry, since you only have cells, enzymes, blood, etc. to hold these things in. Peroxides are a bit much outside of our glass and plastic vessels. So it’s fortunate we have something mild and easily reversed, like disulfide bonds. Cysteine is what forms just about every disulfide bond in your body. The disulfide oxidized cysteine dimer is called cystine:

This reaction provides a profoundly unique and versatile biochemical toolbox. It is often used structurally in protein folding. Certain enzymes (which are just long chains of tens to hundreds of amino acids) use these disulfide bonds as sort of structural bolts. In a more macroscopic use of disulfide bonds, they are responsible for the structure of your hair. Curly hair has a certain structure because its disulfide bonds hold it that way; likewise for straight hair.

See where this is going? If you use a reducing agent, you can break up the disulfide bonds. If you shape the hair how you want it, then re-oxidize it, you can “permanently” change the shape of your hair (hence the “permanent wave” or “perm”:

Note that the oxidized cystine is reduced by the thioglycolate (the reducing agent), making an oxidized dimer of thioglycolate. All oxidations work this way: something else is reduced. Most perms use some related sulfur compound. The vast majority of volatile sulfur compounds stink, and this is why most of them smell so terrible.

Interestingly, since your hair contains so much cysteine, it is a cheap source of cysteine as an article of commerce. Hair contains on the order of 10% cysteine. And here you are letting Supercuts hold onto your hair like a sucker.

See you tomorrow.