= Homotrimer
A \b[homotrimer] is a <protein trimer> containing three copies of the same <protein subunit> type. “Homo” specifies composition; “trimer” specifies a total of three subunits. A <homotrimer> need not have perfectly identical instantaneous conformations or ligand occupancies at all three copies.
<Polynucleotide phosphorylase> provides a functional example. Its three repeated subunits form a ring around the <polynucleotide phosphorylase central channel>. <Cryo-electron microscopy> of the mycobacterial enzyme resolved that ring and RNA near a catalytic site, connecting an oligomeric structure with an RNA-processing mechanism.<Structure and mechanism of Mycobacterium smegmatis polynucleotide phosphorylase by Mihaela-Carmen Unciuleac and others>[Abstract and Figure1, homotrimeric core; RNA-bound reconstructions]{ref}
For an original conservation example, let $A$ be an unassembled chain and $A_3$ the assembled <homotrimer>, with no other forms present:
$$
[A]_{\mathrm{total}}=[A]+3[A_3].
$$
A total chain concentration of $6\,\mathrm{\mu M}$ can yield at most $2\,\mathrm{\mu M}$ trimers. If half the chains remain unassembled, only $1\,\mathrm{\mu M}$ trimers form. Reporting concentration in chains or in complexes therefore changes the numerical value even for the same preparation.
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