Should We Teach FAD(H2) Is an Electron Carrier or a Cocatalyst, and Why Does It Matter?

Document Type

Article

Publication Date

3-10-2026

Publication Title

Journal of Chemical Education

Abstract

Students of biochemistry are routinely taught that there are two primary electron carriers in catabolism: nicotinamide adenine dinucleotide (NAD+/NADH) and flavin adenine dinucleotide (FAD/FADH2). In fact, the latter is not at all a “carrier” in the sense of a diffusible coenzyme that “carries” electrons. Rather, it is a tightly bound cofactor that catalyzes intraenzyme electron transfers. Yet we persist in presenting FADH2 as an intermediate in aerobic respiration, and overuse of this approximation, coupled with ambiguous diagrams and descriptions, can lead to propagation of serious misconceptions about respiration. I discuss the appeal and limitations of the narrative that FAD(H2) is an electron carrier, contrast it with an alternate conception that disposes of FADH2 “yields” entirely, and suggest a hybrid conceptualization of aerobic respiration that emphasizes the various multienzyme pathways that supply the electron transport chain via FAD(H2). This latter model preserves the notion of FADH2 “yields” and the simplicity of two classes of electrons, while minimizing the risk of misconceptions associated with treating FAD(H2) as a reactant or product. © 2026 American Chemical Society and Division of Chemical Education, Inc.

Volume

103

Issue

3

First Page

1131

Last Page

1134

DOI

10.1021/acs.jchemed.5c01069

ISSN

00219584

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