Brown 2000 Biochem Educ

Brown S (2000) Does succinate oxidation yield FADH2 or ubiquinol? Biochem Educ 28:52-4.

» PMID: 10717459 Open Access

Brown Simon (2000) Biochem Educ

Abstract: Most textbooks still show the oxidation of succinate in the tricarboxylic acid (TCA) cycle resulting in the reduction of FADH₂. Such a presentation does not reflect the reaction catalysed by the enzyme in vivo or in vitro, does not simplify the treatment of the reaction, and is unnecessarily misleading and confusing.

• Bioblast editor: Gnaiger E

Selected quotes

In discussing the TCA cycle, most authors choose to present the reaction as the oxidation of succinate and the reduction of FAD, but in presenting the mitochondrial electron transfer chain most authors show the oxidation of succinate coupled to the reduction of ubiquinone. This duplication, which is probably an historical artefact, is unnecessary, confusing to students and misleading.

Succinate:ubiquinone oxidoreductase (SQR) catalyses the oxidation of succinate to fumarate and the reductionof ubiquinone (UQ) to ubiquinol (UQH₂), ..

In many textbooks the reducing equivalents produced during the oxidation of pyruvate by the TCA cycle are usually described as 4 NADH + 1 FADH₂, the latter being the product of the reaction catalysed by SDH/SQR, which is frequently written something like succinate + FAD <-> fumarate + FADH₂ [9-18]. This presentation implies that FAD is the final electron acceptor, rather than being a covalently bound prosthetic group within the enzyme (although this is emphasised by some authors who replace FAD and FADH₂ with enzyme-FAD and enzyme-FADH₂, respectively). Rather, the reduction of FAD is simply the first step in the reaction catalysed by SQR [8].

The overall reaction catalysed by SQR is succinate + UQ <-> fumarate + UQH₂ ..

Labels:

Enzyme: Complex II;succinate dehydrogenase