Principles of Biochemistry Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Electron Transport, Oxidative Phosphorylation, and the Regulation of ATP Synthesis
NADH dehydrogenase accepts electrons from NADH

At the next stage of electron transfer (Fig. 17-5), a pair of reducing equivalents is transferred from NADH to NADH dehydrogenase located in The inner mitochondrial membrane. In this reaction, the tightly bound prosthetic group of NADH dehydrogenase is reduced (Fig. 17-8). The Role of the prosthetic group is played by flavin mononucleotide (FMN), which contains a molecule of vitamin B₂, or riboflavin (Section 10.5). NADH dehydrogenase belongs to the Class of flavin-dependent dehydrogenases, or Flavoproteins. As a result of The transfer of two reducing equivalents from NADH to NADH dehydrogenase (designated here as E-FMN), the FMN prosthetic group of the enzyme is reduced to FMNH₂:

Fig. 17-7. The collector function of NAD and ubiquinone (Q). NAD collects reducing equivalents from many NAD-dependent substrates, as well as from NADPH. Ubiquinone collects reducing equivalents from NADH dehydrogenase and from various substrates acted upon by other flavin-dependent dehydrogenases. Pairs of reducing equivalents supplied by most flavin-dependent dehydrogenases bypass the first phosphorylation site, and therefore only two ATP molecules are generated from their energy.

In addition to the flavin nucleotide prosthetic group, the NADH dehydrogenase molecule contains several non-heme iron atoms. These atoms are assembled into clusters in which they are combined with an equal number of acid-labile sulfur atoms. Such groups are called iron-sulfur centers (Fig. 17-9). Recall that iron-sulfur centers are also present in the succinate dehydrogenase molecule (Section 16.5,e). In [Fe(II)-Fe(III)] cycles associated with changes in valence, the iron atoms of the iron-sulfur centers transfer reducing equivalents from the reduced prosthetic group of NADH dehydrogenase (FMNH₂) to the next carrier in the Respiratory Chain, ubiquinone. Thus, this complex consisting of NADH dehydrogenase and iron-sulfur Proteins (referred to as NADH:ubiquinone oxidoreductase) incorporates Two Types of electron-transfer structures: FMN and several iron-sulfur centers, which apparently function sequentially.

Fig. 17-8. Transfer of reducing equivalents from NADH to flavin mononucleotide (FMN), the prosthetic group of NADH dehydrogenase. R denotes the five-carbon phosphorylated side chain.

Fig. 17-9. Postulated arrangement of iron atoms (red circles) and sulfur atoms (gray circles) in iron-sulfur centers. The number of iron atoms and acid-labile sulfur atoms in these centers is always equal, but some centers contain only two iron atoms, while others contain four.

An iron-sulfur center containing four iron atoms is shown here. The peripheral sulfur atoms belong to four Cysteine residues in the polypeptide chain of the enzyme.