Constraint-Conditioned Closure: A Unified Framework for Pathway Realization and Failure Across Chemical and Biological Systems

Chemical reactions and biological signaling operate within high-dimensional spaces of possible pathways, only a subset of which are realized under specific conditions. In catalytic systems, this manifests as context-dependent selectivity; in biological systems, as variability in functional response and therapeutic resistance. We introduce a unified framework in which pathway realization is governed by constraint-conditioned closure, defined as the ability of a pathway to stabilize as a coherent, self-sustaining trajectory under environmental constraints. Pathways are drawn from a space of possibilities, filtered by accessibility and closure viability. We show that catalytic systems can be understood as static closure selection problems, while biological systems represent dynamic closure stability problems, in which initially accessible pathways may fail over time due to feedback, competition, and regulatory adaptation. This distinction explains why identical pathway structures yield predictable selectivity in chemistry but variable outcomes and resistance in biology. The framework unifies catalytic selectivity and biological resistance as manifestations of the same structural process.