= Homodimer
A \b[homodimer] is a <protein complex> containing two copies of the same <protein subunit> type, conventionally written $A_2$. A <heterodimer> instead contains two different types, $AB$. Identical subunit type does not require the copies to occupy identical conformations or environments in every instant of the complex's motion.
Assembly is part of <protein quaternary structure>, rather than a claim that the two chains have become one longer <polypeptide>. If each subunit contains 100 <amino acid residues>, the <homodimer> contains 200 residues across two chains. Association alone need not add a <peptide bond> between them.
For reversible association $2A\rightleftharpoons A_2$, a concentration-form <dissociation constant> is
$$
K_d=\frac{[A]^2}{[A_2]},\qquad C_{\mathrm{total}}=[A]+2[A_2].
$$
The second equation counts subunit equivalents. For $[A]=2\,\mathrm{\mu M}$ and $[A_2]=1\,\mathrm{\mu M}$, $C_{\mathrm{total}}=4\,\mathrm{\mu M}$: half the subunit copies reside in dimers, even though dimers comprise only one third of the separate molecular particles. The written $K_d$ uses the usual macroscopic concentration convention.
<HIV-1 protease> supplies an experimentally important example. Recombinant enzyme had reported molecular masses of approximately 11,000 under denaturing conditions and 22,000 under nondenaturing conditions; chemical crosslinking provided additional evidence consistent with an active two-subunit enzyme. Comparing several measurements was more informative than assigning a <homodimer> solely from an apparent mass in one assay.<Human immunodeficiency virus 1 protease expressed in Escherichia coli behaves as a dimeric aspartic protease by T. D. Meek and others>[Abstract: molecular-mass comparison, chemical crosslinking and protease activity.]{ref}
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