= Heterotrimer
A \b[heterotrimer] is a <protein trimer> containing more than one type of <protein subunit>. It may have composition $A_2B$ or $ABC$, for example; the total number of subunits is three in either case. This distinguishes it from a <homotrimer>, whose three subunits have the same type.
A <heterotrimeric G protein> illustrates the $ABC$ arrangement, with distinct $\alpha$, $\beta$ and $\gamma$ subunits. A structural inhibitor study crystallized an engineered $\alpha\beta\gamma$ complex and separately measured <nucleotide> binding and release. Such experiments distinguish the composition of the assembled complex from the chemical activity of one of its components.<Structural basis for the specific inhibition of heterotrimeric Gq protein by a small molecule by Akiyuki Nishimura and others>[Materials and Methods, protein preparation, crystallization and nucleotide assays]{ref}
For an original assembly bound, start with $3\,\mathrm{\mu M}$ of $A$, $4\,\mathrm{\mu M}$ of $B$ and $1\,\mathrm{\mu M}$ of $C$, allowing only $ABC$ complexes. At most $1\,\mathrm{\mu M}$ complexes can form, leaving at least $2\,\mathrm{\mu M}$ of $A$ and $3\,\mathrm{\mu M}$ of $B$ outside them. Adding more $A$ alone cannot overcome the shortage of $C$. Actual assembly may be lower because of incomplete binding or competing complexes.
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