Cytochrome b

Cytochrome b (encoded in eukaryotes by the mitochondrial gene MT-CYB or cob) is an integral polytopic membrane hemoprotein that forms the catalytic core of Complex III (ubiquinol–cytochrome c oxidoreductase, or the cytochrome bc₁ complex, EC 1.10.2.2) in the inner mitochondrial membrane of eukaryotes and the plasma membrane of aerobic alpha-proteobacteria. Inside the homodimeric Complex III assembly (~480 kDa, 11 subunits per monomer in mammals), cytochrome b is the only subunit encoded by the mitochondrial genome (mtDNA); the remaining 10 subunits—including cytochrome c₁ (CYC1) and the Rieske [2Fe-2S] iron-sulfur protein (UQCRFS1)—are encoded in the nucleus and imported from the cytosol.

Functionally, cytochrome b coordinates two non-covalently bound b-type protoheme IX prosthetic groups of unequal midpoint redox potential—low-potential heme b_L (b₅₆₆) near the intermembrane space (P-side) and high-potential heme b_H (b₅₆₂) near the mitochondrial matrix (N-side)—alongside two distinct quinone-binding pockets (Q_o and Q_i) to execute Peter Mitchell's protonmotive Q-cycle (1975): bifurcating the two electrons of ubiquinol (QH₂) along divergent pathways so that 4 protons (H⁺) are translocated across the inner mitochondrial membrane for every 2 electrons transferred to cytochrome c. In parallel, because MT-CYB combines deep purifying structural constraint across its heme-ligating helices with high synonymous substitution rates at third codon positions, it serves alongside Cytochrome c Oxidase Subunit I (COX1) as one of the two primary mitochondrial barcode markers for vertebrate molecular systematics and phylogeography.

High-resolution X-ray crystal structures (Xia et al., 1997; Iwata et al., 1998) demonstrate that mitochondrial cytochrome b (~379–380 amino acids, ~42.5 kDa in vertebrates) folds into a compact bundle of eight transmembrane α-helices (Helices A through H) connected by intermembrane-space (P-side) and matrix (N-side) loops, including the critical cd1/cd2 and ef surface helices containing the invariant PEWY motif (residues 270–273) that roofs the Q_o active site:

Note: A homologous protein, cytochrome b₆ (encoded by plastid petB alongside Subunit IV petD), forms the catalytic core of the cytochrome b₆f complex in cyanobacterial and chloroplast thylakoid membranes, executing an analogous Q-cycle between plastoquinol (PQH₂) and plastocyanin.

Because coenzyme Q (ubiquinol, QH₂) is a two-electron, two-proton carrier inside the lipid bilayer whereas cytochrome c is a water-soluble one-electron carrier in the intermembrane space, Complex III solves this stoichiometry mismatch while doubling its proton-pumping efficiency via the two-turnover Q-cycle:

1. First Turnover at the Q_o Site (Bifurcated Oxidation of the First QH₂): A molecule of ubiquinol (QH₂) from the inner-membrane Q-pool docks into the P-side Q_o pocket, forming a hydrogen-bonded bridge between His161 of the mobile Rieske [2Fe-2S] domain and Glu272 of the cytochrome b PEWY motif.

• First Electron (High-Potential Branch): One electron is transferred to the Rieske [2Fe-2S] cluster (E_m ≈ +280 mV), releasing 2 protons (2H⁺_P) into the intermembrane space. The reduced Rieske headgroup swings ~16 Å across a flexible hinge toward cytochrome c₁ (E_m ≈ +240 mV), which passes the electron to soluble cytochrome c (E_m ≈ +254 mV).
• Second Electron (Low-Potential Recycling Branch): The removal of the first electron leaves an unstable, low-potential ubisemiquinone radical (Q^•−) inside Q_o, which immediately transfers its second electron to heme b_L (E_m ≈ −50 mV) → heme b_H (E_m ≈ +50 mV) across the hydrophobic core of cytochrome b. At the matrix-facing Q_i pocket, heme b_H reduces a bound oxidized ubiquinone (Q) to a stable ubisemiquinone (Q_i^•−).

1. Second Turnover at the Q_o Site (Completing Q_i Reduction): A second QH₂ molecule binds at the Q_o site and undergoes the same bifurcated oxidation: releasing another 2 protons (2H⁺_P) into the intermembrane space, reducing a second molecule of cytochrome c via the Rieske/Cyt c₁ branch, and sending a second electron through heme b_L → heme b_H to the Q_i pocket. There, Q_i^•− accepts the second electron and takes up 2 protons from the mitochondrial matrix (2H⁺_N) to regenerate one molecule of QH₂:

Thus, by recycling one of the two oxidized ubiquinol molecules at the Q_i site, cytochrome b couples the net oxidation of 1 QH₂ to the release of 4 protons into the intermembrane space (contributing 4 charges to the protonmotive force Δ*p* because 2 electrons move inward from P to N via b_L → b_H while 2 matrix protons are consumed at Q_i).

Since Thomas Kocher, Allan Wilson, and colleagues (1989) introduced universal vertebrate PCR primers (L14841 / H15149) targeting MT-CYB, the ~1,140 bp mitochondrial cytochrome b gene has served as one of the most widely sequenced markers in mammalian, avian, amphibian, and ichthyological phylogenetics, wildlife forensics, and species delimitation (Johns & Avise, 1998):

• Optimal Taxonomic Resolution Window (Intraspecific Phylogeography → Genus → Family): Across the eight transmembrane helices of MT-CYB, first and second codon positions experience intense purifying selection (d_N/d_S = ω ≈ 0.02–0.05; the four axial histidine ligands His83, His97, His182, His196 and the PEWY motif are invariant across Bilateria), whereas third codon positions substitute rapidly (transition/transversion ratio κ ≈ 8–15 in mammals), yielding an interspecific-to-intraspecific Kimura 2-Parameter (K2P) barcoding gap of 5×–18× across congeneric vertebrates (typical mammalian intraspecific divergence < 2% vs. sister-species divergence 4%–11%, supporting the Bradley & Baker 2001 genetic species concept).
• Empirical Failure Boundaries and Rescue Protocols:

1. Deep Transition Saturation (>30–50 Ma): Above the family/order level, third-codon transitions in MT-CYB reach mutational saturation, compressing apparent branch lengths and triggering long-branch attraction unless recoded into amino acids or Dayhoff-6 physicochemical groups.
2. Nuclear Mitochondrial Pseudogenes (NUMTs): Nuclear insertions of MT-CYB that have escaped mitochondrial purifying selection (ω → 0.85–1.00, accumulating frameshifts or vertebrate-vs-nuclear TGA Trp/Stop codon mismatches) can co-amplify with short universal primers and masquerade as cryptic sister species unless filtered via full-length reading-frame translation, ω < 0.10 gates, and long-range mtDNA amplification.

How does the antimalarial drug Atovaquone kill Plasmodium falciparum by targeting Cytochrome b?

Atovaquone (a hydroxynaphthoquinone structural analog of ubiquinone, combined clinically with proguanil as Malarone) selectively binds to the Q_o pocket of Plasmodium falciparum mitochondrial cytochrome b, blocking ubiquinol oxidation and collapsing the parasite's inner mitochondrial membrane potential (ΔΨ_m). Whereas blood-stage P. falciparum parasites generate nearly all their ATP via cytosolic glycolysis rather than mitochondrial oxidative phosphorylation, they depend on Complex III to re-oxidize ubiquinol produced by mitochondrial dihydroorotate dehydrogenase (DHODH)—the rate-limiting fourth enzyme of de novo pyrimidine biosynthesis (which erythrocytes lack and parasites cannot salvage). Because human cytochrome b differs from Plasmodium cytochrome b at key residues lining the Q_o pocket (whereas single point mutations at P. falciparum Tyr268Ser/Asn/Cys within the PEWY region confer high-level atovaquone resistance), atovaquone selectively starves the malaria parasite of pyrimidines for DNA replication with minimal toxicity to human mitochondria.

Why does Antimycin A increase mitochondrial superoxide (O₂^•−) production from Complex III?

Antimycin A (a Streptomyces macrolactone antibiotic) binds with nanomolar affinity to the matrix-facing Q_i pocket of cytochrome b, blocking electron transfer from heme b_H to ubiquinone. When Q_i is blocked while ubiquinol (QH₂) continues to arrive at the Q_o site, both heme b_H and heme b_L become fully reduced ("backed up"). Consequently, when the next QH₂ molecule transfers its first electron to the Rieske [2Fe-2S] cluster at Q_o, the resulting ubisemiquinone radical (Q_o^•−) has nowhere to send its second electron because heme b_L is already reduced; this prolongs the lifetime of Q_o^•− inside the P-side pocket, allowing it to react directly with dissolved molecular oxygen (O₂) to generate superoxide radicals (O₂^•−) into both the intermembrane space and matrix. Conversely, Q_o-site proximal inhibitors such as stigmatellin and myxothiazol prevent QH₂ from entering Q_o or transferring its first electron to the Rieske cluster, thereby suppressing Complex III superoxide generation.

1. Mitchell, P. (1975). "Protonmotive redox mechanism of the cytochrome b–c₁ complex in the respiratory chain: Protonmotive ubiquinone cycle." FEBS Letters, 56(1), 1–6. https://doi.org/10.1016/0014-5793(75)80098-680098-6)
2. Xia, D., Yu, C. A., Kim, H., et al. (1997). "Crystal structure of the cytochrome bc₁ complex from bovine heart mitochondria." Science, 277(5322), 60–66. https://doi.org/10.1126/science.277.5322.60
3. Kocher, T. D., Thomas, W. K., Meyer, A., et al. (1989). "Dynamics of mitochondrial DNA evolution in animals: Amplification and sequencing with conserved primers." Proceedings of the National Academy of Sciences, 86(16), 6196–6200. https://doi.org/10.1073/pnas.86.16.6196
4. Johns, G. C., & Avise, J. C. (1998). "A comparative summary of genetic distances in the vertebrates from the mitochondrial cytochrome b gene." Molecular Biology and Evolution, 15(11), 1481–1490. https://doi.org/10.1093/oxfordjournals.molbev.a025875