Substrate control ratios, SCR, are flux control ratios, FCR, at a constant mitochondrial coupling-control state. Whereas there are only three well-defined coupling-control states of mitochondrial respiration, L, P, E (LEAK, OXPHOS, Electron transfer pathway), numerous Electron-transfer-pathway states are possible.
Careful selection of the reference state, Jref, is required, for which some guidelines may be provided without the possibility to formulate general rules. FCR are best defined by taking Jref as the maximum flux (e.g. NSE), such that flux in various other respiratory states, Ji, is smaller or equal to Jref. However, this is not generally possible with SCR. For instance, the N/S substrate control ratio (at constant coupling-control state) may be larger or smaller than 1.0, depending on the mitochondrial source and various mitochondrial injuries. The S-pathway control state may be selected preferentially as Jref, if mitochondria with variable N-linked injuries are studied. In contrast, the reference state, Z, is strictly defined for flux control factors.
MitoPedia concepts: Respiratory control ratio
MitoPedia methods: Respirometry