 Solution

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.