Past exam of the biochemistry and Molecular Biology course of the University of Cambridge / 2025 / Paper 2 / Section B / 12 / f / Solution

The reported coefficients add toFor a complete set of independently scaled activities controlling a steady-state flux, the flux summation theorem gives ${\sum }_i{C}_i^J=1$, where $C_i^J=\partial \ln J/\partial \ln E_i$. One way to see this is to scale all reaction capacities by the same factor: steady-state concentrations can remain the same while all fluxes scale by that factor, so the summed logarithmic sensitivities equal one. The listed processes therefore leave a net coefficient of about $0.14$ unaccounted for under the model assumptions. Measurement uncertainty, imperfect inhibitor specificity or incompatible measurements could also contribute to the discrepancy. Control is distributed; neither the sum nor the largest individual coefficient identifies a single universal rate-limiting step.