= Protein trimer
A \b[protein trimer] is an assembly of three <protein subunits>. A <homotrimer> contains three copies of the same subunit type; a <heterotrimer> contains different subunit types. Subunit count describes the assembly, while the number of different sequences describes its composition.
For an original mass example, three identical chains of molar mass $M$ form an ideal complex of molar mass $3M$, ignoring modifications and bound ligands. Chains of $25\,\mathrm{kDa}$ would therefore give $75\,\mathrm{kDa}$. A heterotrimer made from chains of 20, 25 and 30 kDa has the same total mass. Measuring total mass alone cannot distinguish those two stipulated compositions.
<Polynucleotide phosphorylase> illustrates why trimerization can matter mechanistically. Its three-subunit ring forms a <polynucleotide phosphorylase central channel> and positions catalytic regions around the assembly. The original structural study determined that arrangement by <X-ray crystallography>.<A duplicated fold is the structural basis for polynucleotide phosphorylase catalytic activity, processivity, and regulation by M. F. Symmons and others>[Abstract, trimer assembly and accessory RNA-binding domains]{ref}
Three subunits need not always form a ring or an exactly symmetric structure. Different interfaces, bound ligands and conformational changes can distinguish the subunits even when their sequences match. An observed trimer is therefore a starting point for explaining function, not a complete description of every molecular interaction.
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