ETS pathway types

high-resolution terminology - matching measurements at high-resolution

ETS pathway types

Description

Substrate types in mitochondrial SUIT protocols are types of reduced substrates feeding electrons into the electron transfer system (ETS) at different levels of mitochondrial pathways. Distinction of substrate types on the basis of four mt-pathway types provides the rationale for defining categories of SUIT protocols.

Substrates of type 4 feed electrons into dehydrogenases and enzyme systems upstream of the type 3 pathway level. Electron transfer from type 4 substrates (N and F) converges at the NADH-junction and FADH₂-junction (N- and F-junction). Representative type N substrates are pyruvate, glutamate and malate, and also citrate, oxoglutarate. The corresponding dehydrogenases (PDH, GDH, MDH and mtME; IDH, OgDH) generate NADH, the substrate of Complex I (CI). Type F substrates are fatty acids involved in β-oxidation, generating (enzyme-bound) FADH₂, the substrate of electron transferring flavoprotein (CETF). Succinate does not belong to the type 4 substrates, since FADH₂ is the product of CII, whereas FADH₂ is the substrate of CETF. Fatty acid oxidation (FAO) not only depends on electron transfer through the F-junction (which is typically rate-limiting) but simultaneously generates NADH and thus depends on N-junction throughput. Hence FAO can be inhibited completely by inhibition of Complex I (CI). In addition and independent of this source of NADH, the type N substrate malate is required as a co-substrate for FAO in mt-preparations, since accumulation of AcetylCo inhibits FAO in the absence of malate. Malate is oxidized in a reaction catalyzed by malate dehydrogenase to oxaloacetate (yielding NADH), which then stimulates the entry of AcetylCo into the TCA cycle catalyzed by citrate synthase.

Substrates of type 3 (e.g. succinate, glycerophosphate) feed electrons into respiratory complexes directly upstream of the Q-junction. Succinate is the substrate of succinate dehydrogenase (SDH, CII) localized on the inner side of the inner mt-membrane. Glycerophosphate is the substrate of glycerophosphate dehydrogenase complex (CGpDH) localized on the outer face of the inner mt-membrane. Choline is the type 3 substrate of choline dehydrogenase.

Substrates of type 2 feed electrons into Complex III (CIII) with further electron transfer downstream of the Q-junction.

Substrates of type 1 may be artificial electron donors (e.g. TMPD, Tm) essentially bypassing the ETS and feeding electrons directly into the terminal electron acceptor, cytochrome c oxidase (CIV) or alternative oxidases (single enzymatic step).

Abbreviation: n.a.

Reference: Gnaiger 2014 MitoPathways

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MitoPedia topics: Substrate and metabolite

Contributed by Gnaiger E 2016-02-01, edited 2016-02-10

Substrate types on different pathway levels

Substrate types on the pathway level of converging NADH- and FADH₂-linked dehydrogenases, including the TCA cycle and beta-oxidation:

N: NADH-linked substrates ('CI-linked')

F: FADH₂-linked substrates (FAO)

Substrate types on the pathway level of electron transfer complexes converging at the Q-junction:

S: Succinate ('CII-linked')

Gp: Glycerophosphate ('CGpDH-linked')

Substrate types on the single step level of cytochrome c oxidase (CIV), the terminal step in the aerobic electron transfer system:

Tm: Artificial electron transfer susbstrate TMPD (Tm) maintained in a reduced state by ascorbate (As) and reducing cytochrome c as the substrate of CIV.