Reducing Equivalents

Reducing equivalents are a way of describing transferable reducing power in oxidation-reduction reactions. They represent electron equivalents that can be donated to another chemical species, causing that species to be reduced. In biological reactions, these electrons may be transferred directly or together with hydrogen, for example as hydrogen atoms or hydride ions .¹

Cells commonly transfer reducing equivalents using specialized electron carriers. Important examples include NADH, NADPH, and reduced flavin cofactors. These carriers become reduced by accepting electrons during one reaction and can later be oxidized by donating reducing equivalents in another. Thus, reducing equivalents can move between metabolic reactions without free electrons having to exist independently in the aqueous environment of the cell .^1,2

Different carriers tend to supply reducing equivalents to different processes. Reducing equivalents generated in NADH during fuel oxidation commonly contribute to the mitochondrial respiratory chain, where their transfer ultimately supports oxidative phosphorylation. NADPH instead commonly supplies reducing power for biosynthesis and for maintaining antioxidant systems such as glutathione and thioredoxin. Reduced flavin cofactors, which are often enzyme-bound, also transfer electrons in metabolic pathways and respiratory electron transport .²

The term “reducing equivalent” is therefore a biochemical bookkeeping concept rather than the name of a single molecule. It refers to transferable reducing capacity, while NADH, NADPH, reduced flavins, and other reduced carriers are chemical forms through which that capacity can be transferred .^1,2