= Solution
\b[A: downward when <ligand> is present; B: downward; C: upward], over the intended <protein> and <ligand> <concentration> range. <Ligand> should favour its bound Y state, and the ligand-bound subunits should favour tetramer assembly. Without <ligand>, the unassembled state should predominate, rather than forming a stable ligand-free tetramer.
These preferences are thermodynamically coupled. The <ligand> must bind preferentially to states participating in the assembled structure strongly enough to compensate for the cost of shifting the unliganded conformational and assembly equilibria. At extremely low <protein> <concentration>, association entropy can still favour monomers; “ligand-dependent” does not mean concentration-independent assembly. This is a <thermodynamic linkage> argument, rather than three independently adjustable arrows.
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