The conventional model: flexible enzymes are versatile enzymes. An active site that can rearrange accommodates more substrates. Rigidity means specificity; flexibility means promiscuity. The metaphor is a lock — a stiff lock fits one key, a loose lock fits many.

A systematic study of 147 esterases tested against 96 esters found the opposite. The most promiscuous esterases — those that process the widest range of substrates — are significantly more rigid, more thermostable, and have higher specific activity than their specialist counterparts. The generalists are stiffer, not looser.

The mechanism the authors propose inverts the role of flexibility entirely. In specific esterases, flexibility serves as conformational proofreading — the active site samples multiple conformations, and only one conformation processes the correct substrate. The flexibility is not permissive; it is selective. The enzyme uses its loose structure to reject wrong substrates by failing to achieve the catalytic conformation when they bind. The flexibility is the specificity mechanism.

The promiscuous esterases, by contrast, have a rigid active site that doesn’t proofread. Any substrate that fits geometrically gets processed, because the enzyme’s structure doesn’t interrogate the binding event. The rigidity is not a constraint — it’s an absence of discrimination.

The through-claim: flexibility and rigidity are not intrinsic properties that map directly onto versatility and specificity. They are mechanisms, and the same structural feature can serve opposite functions depending on the system’s architecture. Flexibility is the specificity device; rigidity is the generality device. The assumption that structural freedom equals behavioral freedom inverts the actual relationship. The lock that fits many keys is not the one with the loosest tumbler — it is the one that doesn’t check.