Pop quiz: You are a virus with under 10,000 nucleotides in your genome. To make the capsid enclosing your genome, you need to make 250 hexamers of a particular protein. How do you do it?
Give up?
You grab a cellular metabolite with a mass under 1,000 Daltons to bind the 6 monomers together. The metabolite occurs at fairly substantial concentrations (for a metabolite) of 10 – 40 microMolar.
What is the metabolite?
Give up?
It has nearly perfect 6 fold symmetry.
Still give up?
[ Nature vol. 560 pp. 509 – 512 ’18 ] https://www.nature.com/articles/s41586-018-0396-4 says that it’s inositol hexakisphosphate (IP6) — nomenclature explained at the end. http://www.refinebiochem.com/pages/InositolHexaphosphate.html
Although IP6 looks like a sugar (with 6 CHOH groups forming a 6 membered ring), it is not a typical one because it is not an acetal (no oxygen in the ring). All 6 hydroxyls of IP6 are phosphorylated. They bind to two lysines on a short (21 amino acids) alpha helix found in the protein (Gag which has 500 amino acids). That’s how IP6 binds the 6 Gag proteins together. The paper has great pictures.
It is likely that IP6 is use by other cellular proteins to form hexamers (but the paper doesn’t discuss this).
IP6 is quite symmetric, and 5 of the 6 phosphorylated hydroxyls can be equatorial, so this is likely the energetically favored conformation, given the bulk (and mass) of the phosphate group.
I think that the AIDS virus definitely has more chemical smarts than we do. Humility is definitely in order.
Nomenclature note: We’re all used to ATP (Adenosine TriPhosphate) and ADP (Adenosine DiPhosphate) — here all 3 or 2 phosphates form a chain. Each of the 6 hydroxyls of inositol can be singly phosphorylated, leading to inositol bis, tris, tetrakis, pentakis, hexakis phosphates. Phosphate chains can form on them as well, so IP7 and IP8 are known (heptakis?, Octakis??)
Chemiotics II 