NADH dehydrogenase: The free proton plus the hydride ion car- ried by NADH are next transferred to NADH dehydrogenase , a protein complex (Complex I) embedded in the inner mitochondrial membrane. Complex I has a tightly bound molecule of flavin mono nucleotide (FMN, a coenzyme structurally related to FAD, see Figure 28.15, p. 380) that accepts the two hydrogen atoms (2e – + 2H + ), becoming FMNH 2 . NADH dehydrogenase also con- tains iron atoms paired with sulfur atoms to make iron–sulfur cen- ters (Figure 6.9). These are necessary for the transfer of the hydrogen atoms to the next member of the chain, coenzyme Q (ubiquinone)