= Heterodimer
In <protein> chemistry, a \b[heterodimer] is an assembly of two different <protein subunits>, conventionally written $AB$. A <homodimer> has two copies of the same subunit type, $A_2$. Different types can have similar <protein folds> and nearly equal <molecular masses>; “different” refers to molecular identity, not simply size or shape. An $A_2B_2$ <protein complex> contains two types but four chains, so it is not itself a <heterodimer>.
The alpha/beta <tubulin> <heterodimer> is the repeating building block of a <microtubule>. The alpha and beta <protein subunits> have similar folds but distinct sequences and nucleotide environments. Their structural relationship was resolved by electron crystallography of zinc-induced <tubulin> sheets: the 1998 model reached $3.7$ angstrom resolution. This is a concrete example of a two-chain <protein quaternary structure>, rather than the <protein tertiary structure> of a single chain.<Structure of the alpha beta tubulin dimer by E. Nogales and others>[Abstract: approximately 40% alpha/beta sequence identity, closely similar folds, distinct nucleotide-binding sites and experimental method.]{ref}
Stoichiometry gives a useful upper bound. If an idealized mixture contains $3\,\mathrm{\mu mol}$ of $A$ and $2\,\mathrm{\mu mol}$ of $B$, and only $AB$ can assemble, at most $2\,\mathrm{\mu mol}$ of <heterodimer> can form; at least $1\,\mathrm{\mu mol}$ of $A$ remains outside $AB$. The actual amount also depends on binding equilibrium and experimental conditions: chain counting alone does not guarantee complete association.
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