FIGURE 2-33 The electron-carrying coenzymes NAD⁺ and FAD. (a) NAD⁺ (nicotinamide adenine dinucleotide) is reduced to NADH by the addition of two electrons and one proton simultaneously. In many biological redox reactions, a pair of hydrogen atoms (two protons and two electrons) is removed from a molecule. In some cases, one of the protons and both electrons are transferred to NAD⁺; the other proton is released into solution. (b) FAD (flavin adenine dinucleotide) is reduced to FADH₂ by the addition of two electrons and two protons, as occurs when succinate is converted to fumarate (see Figure 2-32). In this two-step reaction, addition of one electron together with one proton first generates a short-lived semiquinone intermediate (not shown), which then accepts a second electron and proton.