 Homotrimer

A 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.^[ref]

For an original conservation example, let $A$ be an unassembled chain and $A_3$ the assembled homotrimer, with no other forms present:A total chain concentration of $6\mu \mathrm{M}$ can yield at most $2\mu \mathrm{M}$ trimers. If half the chains remain unassembled, only $1\mu \mathrm{M}$ trimers form. Reporting concentration in chains or in complexes therefore changes the numerical value even for the same preparation.