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

The ADP addition containsDuring the initial stimulated interval, the plotted oxygen concentration falls from about $240$ to $207\mu \mathrm{M}$. The chamber volume after this addition is $1.900+0.050+0.050+0.010=2.010\mathrm{mL}$, givingThe conventional phosphorylation-to-oxidation ratio counts ATP formed per oxygen atom reduced, so its denominator is twice the number of oxygen molecules:This assumes the added ADP is phosphorylated and uses the displayed total oxygen loss over the pulse; it is an apparent experimental ratio. A separate correction for background respiration or proton leak would change the denominator and must be specified explicitly. Dividing by oxygen molecules without the factor of two would instead give an ATP:${\mathrm{O}}_2$ ratio near $3$.

Succinate is the most plausible substrate. Its oxidation by succinate dehydrogenase, or respiratory complex II, supplies electrons to the ubiquinone pool, bypassing respiratory complex I. Electron passage through respiratory complex III and cytochrome c oxidase supports a lower phosphorylation yield than entry through NADH at respiratory complex I, consistent with an apparent phosphorylation-to-oxidation ratio around $1.5$. The later involvement of a mitochondrial dicarboxylate carrier is additional evidence because succinate is one of its transported substrates. The ratio alone is not a chemically unique identification: other electron-entry routes and altered coupling can also produce a similar yield. Confirm by comparing defined substrates and sensitivity to a succinate dehydrogenase inhibitor.

Hexokinase consumes exported ATP in the reactionThe adenine nucleotide translocator imports regenerated ADP, permitting ATP synthase to use the proton-motive force. Increased proton return reduces the opposing gradient and allows faster activity of the electron transport chain and oxygen consumption: this is respiratory control. The first hexokinase additions therefore increase demand and respiration. By about the fourth addition, demand no longer limits the measured flux: the respiratory and transport system has reached its available capacity, so more hexokinase cannot increase it. This does not imply that hexokinase ceases functioning, and its zero incremental control at maximal demand does not make it dispensable for maintaining that demand.

At low ATP demand, the proton-motive force is high and much of the remaining oxygen consumption supports proton leak. Increasing leak conductance then creates a substantial extra demand for proton pumping and strongly changes respiratory flux. At high phosphorylation demand, proton return through ATP synthase is large, the proton-motive force is lower, and leak contributes a much smaller part of total proton return. Control of oxygen consumption shifts toward substrate supply and the respiratory machinery, so the leak flux control coefficient becomes small. The plot's value near zero does not mean that the physical leak current vanishes; the separate maximal-rate table likewise gives a small nonzero coefficient, $0.04$.

The mitochondrial dicarboxylate carrier is an inner-membrane transport protein that exchanges suitable dicarboxylates, including succinate and malate, with counter-substrates such as phosphate. In this experiment, it supplies external succinate to the matrix side of respiratory complex II; the inner mitochondrial membrane does not simply allow unrestricted diffusion of the charged substrate. Without adequate uptake, substrate delivery would restrict respiration even if the respiratory enzymes remained intact. Its essentiality is substrate- and preparation-dependent: it is not the obligatory entry route for every possible respiratory fuel.

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.

Important unlisted candidates include ATP synthase, the mitochondrial phosphate carrier, and succinate dehydrogenase itself, which supplies electrons from succinate to ubiquinone. Availability or transfer of ubiquinone and cytochrome c, and associated transport or metabolite constraints, may also need explicit treatment depending on how the pathway is grouped. ATP synthesis, phosphate supply and substrate oxidation have not all been individually represented. Respiratory complex I is absent from the table but is not required for the direct forward route from succinate into the ubiquinone pool; its omission alone does not account for missing control of that route.

Titrate the omitted activities using small perturbations around the same steady-state condition, measuring oxygen consumption and calculating local slopes rather than assigning control from complete inhibition. For example, suitable ATP synthase, succinate dehydrogenase and carrier perturbations could test their contributions, with independent checks of inhibitor selectivity and activity loss. Measure ATP production alongside oxygen consumption, as well as mitochondrial membrane potential, to separate a change in phosphorylation efficiency from a change in electron flux. Check chamber oxygen availability, mitochondrial integrity, substrate abundance and assay reproducibility.

Repeat the control measurements across ADP demands and with defined alternative substrates. Carefully titrated FCCP can test respiratory capacity when proton-gradient backpressure is relieved, but the uncoupled state is a different operating condition and does not itself reveal the coupled-state coefficients. Fit the measured local sensitivities with uncertainty and test whether the expanded set satisfies the flux summation theorem. This distinguishes omitted control from technical inconsistency and shows whether the distribution changes with demand.