Hemeproteins

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Contents

Catalase

Cytochromes

Complex III of Electron Transport Chain includes Cytochrome c1 and cytochrome b subunits

Cytochrome b

Cytochrome b5

Cytochrome b5 (CB) functions as an electron transport carrier for several membrane-bound oxygenases. CB is heme-containing protein. The microsomal and mitochondrial CB are membrane-bound while bacterial and other animal tissue CB are soluble. Cytochrome b562 is the the b-type cytochrome from E. coli.[1] Rat heme-containing cytochrome b5 (PDB entry 1b5m[2]) is shown.

Cytochrome bc1 complex

Cytochrome bc1 (Cbc1) functions as the central pump which transfers protons across the cell membrane. The protons are used to power the rotation of ATP synthase. Cbc1 binds ubiquinol which carries hydrogen atoms. Cbc1 separates the protons and the electrons. The protons are released in the inner side of the membrane for use by ATP synthase and the electrons are transferred to cytochrome c or to the outer side of the membrane. Plants use cytochrome b6f in the same manner binding plastoquinol as a hydrogen carrier. Stigmatellin inhibits the Cbc1 electron transfer by binding to its quinone oxidation site. Antimycin inhibits Cbc1 by binding to its quinone reduction site.[3]

More details in Complex_III_of_Electron_Transport_Chain.

Structural highlights

Cbc1 is a dimeric protein composed of 11 proteins and cofactors which include heme-carrying proteins like cytochrome b (Cb) and cytochrome c1 (Cc1) and iron-sulfur cluster proteins like Rieske Fe-S protein (RISP). The iron containing moieties are heme, heme C (where vinyl side chain of heme are replaced by thioether) and Fe2S2. [4]

Cytochrome c

Structural and kinetic studies of imidazole binding to two members of the cytochrome c6 family reveal an important role for a conserved heme pocket residue[5]

      Cytochrome c6 is a member of the class I family of c-type cytochromes with a distinctive α-helical fold and a methionine and histidine residue serving as axial heme iron ligands. They function in the photosynthetic electron transport chain of cyanobacteria where they shuttle an electron from the cytochrome b6f complex to photosystem I. Structures of numerous cytochrome c6 proteins have been determined and all have the methionine ligand coordinating to the iron. In the present work we have solved the structure of the Q51V site-directed variant of Phormidium laminosum cytochrome c6. This project is part of a study that is aimed at gaining insight into protein factors which modulate the heme mid-point redox potential in the cytochrome c6 family. The Q51V variant has been shown to tune over 100 mV of heme redox potential, which for a single heme pocket mutation is very significant and has consequences for function.

      The Q51V structure confirms that the Val replacing the Gln has the same side-chain orientation in the heme pocket as found in other cytochrome c6 proteins, that naturally have a Val at this position. The significance of this structure is that the axial heme iron methionine is dissociated and an exogenous ligand present in the crystallisation solution, imidazole, is now bound to the heme iron. Two other structures of imidazole cyt c-adducts have been reported, but neither appear to undergo the large structural changes seen in the Q51V structure. Both protein and heme structural changes are observed, with the later centered on a 180 degree rotation around the CA atom of the two heme propionate groups accompanied by the upward movement of an alpha helix and the displacement of two loop regions.

      Protein (un)folding studies on cytochrome c have revealed that (un)folding involves structural units called 'foldons'. The regions in the Q51V imidazole-adduct where structural changes occur map well to the two foldons predicted to unfold first in cytochrome c. Thus imidazole triggers the release of the methionine ligand in the Q51V variant, leading to the formation of an early unfolding intermediate that is stabilised by imidazole binding to the vacant heme iron coordination position, enabling it to be captured in the crystalline form.

Structural model of the [Fe]-hydrogenase/cytochrome C553 complex combining NMR and soft-docking[6]

The complex shows the specific interaction of the hydrogenase (light blue) with the cytochrome (pink), revealing the path of electron transport from the active site metal cluster, through three iron-sulfur clusters, and ending in the cytochrome heme (colored red). Two cysteine amino acids at the interface, CYS 38 in the hydrogenase and CYS10 in the cytochrome, are thought to provide the electron transfer pathway between the two proteins (these scenes were created by Jaime Prilusky, David S. Goodsell, and Eran Hodis).

Conformational control of the binding of diatomic gases to cytochrome c’ [7]

The cytochromes c′ (CYTcp) are found in denitrifying, methanotrophic and photosynthetic bacteria. These proteins are able to form stable adducts with CO and NO but not with O2. The binding of NO to CYTcp currently provides the best structural model for the NO activation mechanism of soluble guanylate cyclase. Ligand binding in CYTcps has been shown to be highly dependent on residues in both the proximal and distal heme pockets. Group 1 CYTcps typically have a phenylalanine residue positioned close to the distal face of heme, while for group 2, this residue is typically leucine. We have structurally, spectroscopically and kinetically characterised the CYTcp from Shewanella frigidimarina (SFCP), a protein that has a distal phenylalanine residue and a lysine in the proximal pocket in place of the more common arginine (monomer A is colored in red, monomer B in green, and heme group in yellow). Each monomer of the SFCP dimer folds as a 4-alpha-helical bundle in a similar manner to CYTcps previously characterised.

  • Heme group and its environment in as-isolated SFCP.
  • Proximal NO complex of SFCP (monomer A).
  • Click here to see the difference between these structures.

SFCP exhibits biphasic binding kinetics for both NO and CO as a result of the high level of steric hindrance from the aromatic side chain of residue Phe 16. The binding of distal ligands is thus controlled by the conformation of the phenylalanine ring.

  • A superposition of the heme environments of SFCP (in green;4ulv), RCCP (R. capsulatus; in magenta; 1cpq), RSCP (R. sphaeroides; in red; 1gqa) and RGCP (R. gelatinosus; in cya); 2j8w).
  • Click here to see morph of this scene.

Only a proximal 5-coordinate NO adduct, confirmed by structural data, is observed with no detectable hexacoordinate distal NO adduct.

Rhodothermus marinus cytochrome c

Structure

'Figure 1. The heme group of monoheme cytochrome ''c'' purified from ''Rhodothermus marinus''

All members in the C-type cytochrome superfamily contain a heme prosthetic group that is covalently attached to the protein via two thioether bonds to cysteine residues. Most cytochromes c occur in a where the histidine residue is one of the two axial ligands of the heme iron.[8] In monoheme cytochromes c, the other axial position may be left vacant or be occupied by histidine or methionine residues; however, it can sometimes be occupied by cysteine or lysine residues.[8]. In Rmcytc, XX represents a threonine (Thr46) and an alanine residue (Ala47) that help form the loop 2 structure.

The typical monoheme cyt c fold is formed by helices A, C, and E. Rmcytc contains seven α-helices that are folded around the heme, all connected by random coils.[8] The heme group is axially coordinated by His49 and Met100, and the disulfide linkages exist at Cys45 and Cys48. The heme group in Rmcytc is almost completely shielded from solvent due to it being in a mostly hydrophobic pocket. This pocket is formed in part by the seven helices surrounding the ring, but also by two structures that are uncommon in other cytochromes c. First, a 21 amino acid extension of the N-terminal exists, forming α-helix A' and loop 1, which wraps around the back of the polypeptide.[8] An extension resembling such has only been seen in Thermus thermophilus; however, the extension occurs at the C-terminus rather than the N-terminus.[9] A second rarity is that of helix B', inserted between helix D and loop 3, that shields the bottom part of the heme from any solvent.[8] In cytochrome c2 as well as mitochondrial cyt c, a similar yet shorter helix was found, though this helix was present at a different place in the primary sequence. Also, instead of helix B', T. thermophilus contains a two-stranded β-sheet.[8] One final note is the number of methionine residues that Rmcytc contains. In general, cyt c contains about two methionines whereas Rmcytc contains seven, located on the left of the heme.[8]

As determined by X-ray crystallography, the Rmcytc structure was found to contain a sulfate ion coordinated to Glu122 via hydrogen bonding to the protonated carboxylate oxygen. In the protein complex, this ion has been seen to mediate crystal contact between neighbouring protein molecules.[8]

The observation of these structural motifs in other C-type cytochromes can support the divergent evolution of cytochromes c.[8] These motifs are present in a number of different bacteria and are seen in similar regions of the secondary structure; however, they exist in the primary sequence in places distinct to the phylum. For example, monoheme cytochromes c in the rest of the Bacteroidetes phylum have an N-terminus extension that is highly conserved to that of Rmcytc, and the regions in the primary structure that correspond to these secondary motifs are not observed in other bacterial phyla.[8] Also, due to these motifs being absent from other phyla, the Bacteroidetes monoheme cyt c has been said to form a new subfamily of cyt c.

Function

Monoheme cytochromes c are involved in electron transport chains in both prokaryotes and eukaryotic mitochondria.[8] They mediate the transfer of electrons mainly from the bc1 complexes or their analogs to heme-copper oxygen reductases (HCOs) in the electron transport chain of oxidative phosphorylation. Heme c containing domains are often found fused to other protein domains such as these HCOs, including the caa3 oxygen reductases[8][10]; these enzymes are membrane-bound and catalyze the reduction of O2 to water.[11] In addition to being involved in oxidative phosphorylation, monoheme cyt c has also been seen to participate in the electron transport chain of photosynthesis.[8] Cytochrome c has also been determined to be a major signalling molecule in the apoptotic pathways.

Electron transport chain

In the electron transport chain (ETC), cyt c shuttles electrons between the respiratory complexes III and IV; complex III is the cytochrome bc1 complex and IV is cyt c oxidase. Initially, the heme iron in cyt c is in the reduced, Fe3+ state; this allows for the uptake of one electron, oxidizing the iron to the Fe2+ state.[12] The ETC in eukaryotes is quite simple compared to that of prokaryotes.


In prokaryotic systems, electrons can enter the ETC at a number of places and multiple donors can be in play; however, the underlying transport system remains the same. Electrons are ultimately transferred from donor to various redox complexes including the bc1 complex and cytochrome c, and finally to a terminal electron acceptor such as molecular oxygen in eukaryotes.[12]

The cytochrome oxidase reaction accounts for nearly 90% of all oxygen uptake in most cells.[12] Due to the large role of cytochromes within the ETC, it would be highly detrimental to the cell if any inhibitors were to be present in the organism. Cyanide and azide bind tightly to the cytochrome oxidase complex, halting electron transport and reducing the overall ATP production.[12]

Apoptosis

In all organisms, cells undergo apoptosis, or programmed cell death, by which there is an extrinsic and an intrinsic pathway. The extrinsic pathway involves an immune response by killer lymphocytes, and once the lymphocyte has been bound to the target cell, an apoptotic cascade occurs.[12] The intrinsic pathway includes cyt c, present in the intermembrane space of mitochondria. In this pathway, the presence of an apoptotic stimulus causes cyt c to be released into the cytosol. Cytochrome c in the cytosol now can be recognized and bound to various apoptotic factors, activating them and forming the apoptosome. The apoptosome recruits caspases, which are activated and result in a caspase cascade to proceed with apoptosis.[12]

Cytochrome c is required for the intrinsic apoptotic process to function properly. Such as with the electron transport chain, a mutation affecting cyt c or other structures in apoptosis could cause either an increase or a decrease in the rate of apoptosis.

Cytochrome c 7

The crystal structure of heme d1 biosynthesis-associated small c-type cytochrome NirC reveals mixed oligomeric states in crystallo

For details on decaheme cyt see MtrF

Cytochrome f

Cytochrome f (Cytf) is the largest subunit of the cytochrome b6f complex. This complex transfers electrons from plastocyanin to the two reaction center complexes of oxygenic photosynthetic membranes.[13]

The cytochrome b6f complex contains 4 subunits: Cytf, Cytb6, Rieske iron-sulfur protein and subunit IV. Cytf has an internal chain of water molecules conserved in all its 3D structures. The water chain is assumed to be a proton wire.

Heme binding site in Turnip cytochrome f

Covalent binding of heme to 2 Cys residues

Fe coordination site (PDB entry 1ctm[14]).

Cytochrome P450

Cytochrome P450 (P450) catalyzes the oxidation of organic substances like lipids. The P450 contains a heme cofactor. The protein is numbered by its gene.[15].

  • P450cam, also known as camphor 5-monooxygenase, catalyzes the hydroxylation of camphor.
  • P450nor is a Nitric Oxide Reductase P450.
  • P450epoK converts epothilone D to epothilone B.
  • P450cin monooxygenates 1,8-cineole which is similar to camphor.
  • Bifunctional P450/NADPH P450 reductase (P450 BM3) is a fatty acid monooxygenase.
  • P450 3A4 see CYP3A4.
  • P450 19 family see Aromatase.

Additional details on cytochrome P450 interactions with drugs see

The heme moiety is stabilized by several side chains. The heme iron is pentacoordinated with Cys as one ligand.[16]

Flavocytochrome

Cytochrome c Nitrite Reductase

Shewanella oneidensis Cytochrome c Nitrite Reductase

Laue Crystal Structure of Shewanella oneidensis Cytochrome c Nitrite Reductase from a High-yield Expression System[17] Cytochrome c nitrite reductase (ccNIR) is a central enzyme of the nitrogen cycle. It binds nitrite, and reduces it by transferring 6 electrons to form ammonia. This ammonia can then be utilized to synthesize nitrogen containing molecules such as amino acids or nucleic acids. However, ccNiR’s primary role is to help extract energy from the reduction; ammonia is simply a potentially useful byproduct. In general, heterotrophic organisms feed on electron-rich substances such as sugars or fatty acids. During the metabolism of these substances large numbers of electrons are produced. Many organisms use oxygen as the final acceptor of these electrons, in which case water is formed. However, some organisms can use alternative electron acceptors such as nitrite, which is where ccNiR comes in. The ccNiR described here is produced by the Shewanella oneidensis bacterium, which is remarkable in its own right due to the large number of electron acceptors that it can utilize. Shewanella is a facultative anaerobe, which means that it will use oxygen if available, but in the absence of oxygen can get rid of its electrons by dumping them on a wide range of alternate acceptors, of which nitrite is only one example. To handle the electron flow Shewanella uses a large number of promiscuous c-heme containing electron transfer proteins. Indeed, Shewanella is exceptionally adept at producing c-heme proteins under fast-growth conditions, which many bacteria commonly used for large-scale laboratory gene expression, such as E. coli, are incapable of unless they are first extensively reprogrammed genetically. Since Shewanella can be easily grown in the lab, and can naturally and easily produce c-hemes, it is an ideal host for generating large quantities of c-heme proteins such as ccNiR. The 2012 paper by Youngblut et al. [17] describes a genetically modified Shewanella strain that can produce 20 – 40 times more ccNiR per liter of culture than the wild type bacterium. The ccNir so produced can be purified easily and in large amounts. This result is important because c-heme proteins have historically proved difficult to over-express in traditional vectors such as E. coli. With large quantities of Shewanella ccNIR available, Youngblut et al [17] were able to obtain the crystal structure (3ubr) and do a variety of experiments. The ccNIR consists of two equal subunits (colored in darkmagenta and in green) with five c-hemes each. In the oxidized ccNIR all central heme irons are Fe3+. They can be subsequently reduced to Fe2+ either by reducing agents or electrochemically. An important conclusion of the paper is that electrons added to ccNiR are likely delocalized over several hemes, rather than localized on individual hemes. The hemes 3-5 (colored in yellow) and the hemes-2 (colored in seagreen) are six coordinate and used for electron transport only, whereas the two hemes-1 (colored in magenta) are the active sites. Electrons are believed to enter via the hemes-2, but can move between subunits. Though the physiological significance of this result is not yet known, it is possible that delocalizing the electrons keeps the active site redox-potential sufficiently high until enough electrons are accumulated that the reaction with nitrite can take place. That is, CcNIR acts like a capacitor that can store electrons until they are needed. The X-ray structure of the ccNIR reveals the architecture of this capacitor. To solve the structure a non-standard method, the Laue method, was used. This became necessary since attempts to collect a high resolution data set with monochromatic X-ray radiation were not successful. At room temperature the ccNIR crystals are susceptible to radiation damage. Freezing damaged the crystals because a suitable cryoprotectant could not be found. Single pulsed Laue crystallography with 100 ps highly intense polychromatic X-ray pulses provided a solution. A dataset was collected in a few minutes. The crystals were cooled slightly to 0 °C but not frozen. Crystal settings spanned a range of 180 °C and the crystals were orthorhombic. Therefore, a Laue dataset with very high multiplicity and good quality in terms of resolution and Rmerge could be collected. The structure of this ccNIR was then solved by molecular replacement using the E. coli ccNIR as a template. An overlay of the S. oneidensis hemes within one monomer with the corresponding E. coli hemes reveals significant similarity. S. oneidensis hemes 3-5, hemes-2, and hemes-1 are colored in yellow, seagreen, and magenta, respectively, whereas their corresponding E. coli hemes are in similar, but darker colors. The overall structure of S. oneidensis ccNiR also is similar to that of E. coli ccNiR, except in the region where the enzyme interacts with its physiological electron donor (CymA in the case of S. oneidensis ccNiR, NrfB in the case of the E. coli protein) near heme 2. Subunits of S. oneidensis ccNiR colored in darkmagenta and in green; subunits of E. coli ccNiR colored in hot pink and in deep-sky-blue. Protein film voltammetry (PFV) experiments performed on S. oneidensis ccNiR films in the absence of substrate produced a broad envelope of reversible signals that span approximately 450 mV. At high pH values the envelope appears as a single peak, whereas at pH values below 7 the envelope appears to be composed of two large overlapping peaks. At pH values below 6 and at 0 °C, the envelope of signal can be better resolved and more than two peaks can be observed. This resulting envelope of signal can be deconvoluted as the sum of five one-electron peaks, each corresponding to one of the five hemes in a ccNiR monomer (see image below).

PFV of S. oneidensis ccNiR (a) Typical signal on a graphite electrode. (b) Baselinesubtracted non-turnover voltammogram
PFV of S. oneidensis ccNiR (a) Typical signal on a graphite electrode. (b) Baselinesubtracted non-turnover voltammogram

The Ca2+ ion within conserved site is coordinated in bidentate fashion by Glu205, and in monodentate fashion by the Tyr206 and Lys254 backbone carbonyls, and the Gln256 side-chain carbonyl. In the S. oneidensis structure only one water molecule is assigned to the Ca2+ ion in subunit B. In subunit A the difference electron density that represents this water molecule is very close to the noise level, and it is difficult to identify even one water molecule there. The carbonyl side chain of Asp242 and the hydroxyl of Tyr235 come near to the open calcium coordination sites, but are not within bonding distance. Instead they interact with the water molecule that is weakly coordinated to the Ca2+ ion. The ccNiR calcium ions appear to play a vital role in organizing the active site (as was mentioned above hemes-1 are the active sites).

Cytochrome c nitrite reductase

Cytochrome c oxidase

Cytochrome c oxidase (CcO) is a transmembrane protein complex found in bacteria and mitochondria. CcO is the last enzyme in the electron transport chain. [18]

Disease

CcO deficiency is a genetic condition which affects muscle weakness, muscle tone upto severe brain dysfunction in severe cases.

Structural highlights

Mammalian CcO is composed of 13 subunits, 2 hemes, two copper centers, and cytochrome a and a3. The fully oxidized form of CcO active site shows the heme, Cu+2 ion and an O2 molecule. Second heme binding site. [19] Bacterial CcO is composed of 2 subunits.

Cytochrome c peroxidase

Cytochrome c peroxidase (CcP) is a protoporphyrin-containing enzyme which reduces hydrogen peroxide to water.[20]

Structural highlights

The O2 molecule in the active site coordinates with the heme (PDB entry 1dcc).[21] Water molecules are shown as red spheres.

Cytochrome P450 hydroxylase

Cytochrome P450 hydroxylase (CPH) acts in the in-chain hydroxylation of lauric acid which is required for the development of the male organ in higher plants[22]. CPH hydroxylyzes the anti-cholesterol natural product herbosidiene[23].

  • Lauric acid binding site.
  • Heme group binding site.
  • Fe coordination site.
  • Distances between Lauric acid oxygens and heme group Fe in Cytochrome P450 hydroxylase from Steptomyces avermitilis (PDB code 5cwe).

Heme oxygenase

Hemoglobin

Myoglobin

Ascorbate peroxidase

Function

Ascorbate peroxidase (APX) catalyzes the conversion of ascorbate and hydrogen peroxide to dehydroascorbate and water thus detoxifying hydrogen peroxide. APX is involved in the glutathione-ascorbate cycle. APX is accumulated in plants in response to heat and drought stress. APX contains a heme group.

Relevance

APX is part of plants antioxidant defense. APX converts H2O2 to water using ascorbate as an electron donor. APX is used in electron microscopy as a genetic tag which can be stained independently of light activation.

Structural highlights

The heme-containing active site of APX contains a His residue (H163 in soybean) which coordinates with the heme and confers stability to the Fe state in the heme. [24]

Influence of the presence of the heme cofactor on the JK-loop structure in indoleamine-2,3-dioxygenase-1

Drag the structure with the mouse to rotate

References

  1. ↑ Schenkman JB, Jansson I. The many roles of cytochrome b5. Pharmacol Ther. 2003 Feb;97(2):139-52. PMID:12559387
  2. ↑ Rodriguez-Maranon MJ, Qiu F, Stark RE, White SP, Zhang X, Foundling SI, Rodriguez V, Schilling CL 3rd, Bunce RA, Rivera M. 13C NMR spectroscopic and X-ray crystallographic study of the role played by mitochondrial cytochrome b5 heme propionates in the electrostatic binding to cytochrome c. Biochemistry. 1996 Dec 17;35(50):16378-90. PMID:8973214 doi:10.1021/bi961895o
  3. ↑ Crofts AR. The cytochrome bc1 complex: function in the context of structure. Annu Rev Physiol. 2004;66:689-733. PMID:14977419 doi:http://dx.doi.org/10.1146/annurev.physiol.66.032102.150251
  4. ↑ Berry EA, Huang LS, Saechao LK, Pon NG, Valkova-Valchanova M, Daldal F. X-Ray Structure of Rhodobacter Capsulatus Cytochrome bc (1): Comparison with its Mitochondrial and Chloroplast Counterparts. Photosynth Res. 2004;81(3):251-75. PMID:16034531 doi:http://dx.doi.org/10.1023/B:PRES.0000036888.18223.0e
  5. ↑ Rajagopal BS, Wilson MT, Bendall DS, Howe CJ, Worrall JA. Structural and kinetic studies of imidazole binding to two members of the cytochrome c (6) family reveal an important role for a conserved heme pocket residue. J Biol Inorg Chem. 2011 Jan 26. PMID:21267610 doi:10.1007/s00775-011-0758-y
  6. ↑ Morelli X, Czjzek M, Hatchikian CE, Bornet O, Fontecilla-Camps JC, Palma NP, Moura JJ, Guerlesquin F. Structural model of the Fe-hydrogenase/cytochrome c553 complex combining transverse relaxation-optimized spectroscopy experiments and soft docking calculations. J Biol Chem. 2000 Jul 28;275(30):23204-10. PMID:10748163 doi:10.1074/jbc.M909835199
  7. ↑ Manole A, Kekilli D, Svistunenko DA, Wilson MT, Dobbin PS, Hough MA. Conformational control of the binding of diatomic gases to cytochrome c'. J Biol Inorg Chem. 2015 Mar 20. PMID:25792378 doi:http://dx.doi.org/10.1007/s00775-015-1253-7
  8. ↑ 8.00 8.01 8.02 8.03 8.04 8.05 8.06 8.07 8.08 8.09 8.10 8.11 8.12 Stelter M, Melo AM, Pereira MM, Gomes CM, Hreggvidsson GO, Hjorleifsdottir S, Saraiva LM, Teixeira M, Archer M. A Novel Type of Monoheme Cytochrome c: Biochemical and Structural Characterization at 1.23 A Resolution of Rhodothermus marinus Cytochrome c. Biochemistry. 2008 Oct 15. PMID:18855424 doi:10.1021/bi800999g
  9. ↑ Than ME, Hof P, Huber R, Bourenkov GP, Bartunik HD, Buse G, Soulimane T. Thermus thermophilus cytochrome-c552: A new highly thermostable cytochrome-c structure obtained by MAD phasing. J Mol Biol. 1997 Aug 29;271(4):629-44. PMID:9281430 doi:10.1006/jmbi.1997.1181
  10. ↑ Soares CM, Baptista AM, Pereira MM, Teixeira M. Investigation of protonatable residues in Rhodothermus marinus caa3 haem-copper oxygen reductase: comparison with Paracoccus denitrificans aa3 haem-copper oxygen reductase. J Biol Inorg Chem. 2004 Mar;9(2):124-34. Epub 2003 Dec 23. PMID:14691678 doi:10.1007/s00775-003-0509-9
  11. ↑ Pereira MM, Santana M, Teixeira M. A novel scenario for the evolution of haem-copper oxygen reductases. Biochim Biophys Acta. 2001 Jun 1;1505(2-3):185-208. PMID:11334784
  12. ↑ 12.0 12.1 12.2 12.3 12.4 12.5 Karp, Gerald (2008). Cell and Molecular Biology (5th edition). Hoboken, NJ: John Wiley & Sons. ISBN 978-0470042175.
  13. ↑ Prince RC, George GN. Cytochrome f revealed. Trends Biochem Sci. 1995 Jun;20(6):217-8. PMID:7631417
  14. ↑ Martinez SE, Huang D, Szczepaniak A, Cramer WA, Smith JL. Crystal structure of chloroplast cytochrome f reveals a novel cytochrome fold and unexpected heme ligation. Structure. 1994 Feb 15;2(2):95-105. PMID:8081747
  15. ↑ Danielson PB. The cytochrome P450 superfamily: biochemistry, evolution and drug metabolism in humans. Curr Drug Metab. 2002 Dec;3(6):561-97. PMID:12369887
  16. ↑ Williams PA, Cosme J, Ward A, Angove HC, Matak Vinkovic D, Jhoti H. Crystal structure of human cytochrome P450 2C9 with bound warfarin. Nature. 2003 Jul 24;424(6947):464-8. Epub 2003 Jul 13. PMID:12861225 doi:http://dx.doi.org/10.1038/nature01862
  17. ↑ 17.0 17.1 17.2 Youngblut M, Judd ET, Srajer V, Sayyed B, Goelzer T, Elliott SJ, Schmidt M, Pacheco AA. Laue crystal structure of Shewanella oneidensis cytochrome c nitrite reductase from a high-yield expression system. J Biol Inorg Chem. 2012 Mar 2. PMID:22382353 doi:10.1007/s00775-012-0885-0
  18. ↑ Tsukihara T, Aoyama H, Yamashita E, Tomizaki T, Yamaguchi H, Shinzawa-Itoh K, Nakashima R, Yaono R, Yoshikawa S. Structures of metal sites of oxidized bovine heart cytochrome c oxidase at 2.8 A. Science. 1995 Aug 25;269(5227):1069-74. PMID:7652554
  19. ↑ Yoshikawa S, Shinzawa-Itoh K, Nakashima R, Yaono R, Yamashita E, Inoue N, Yao M, Fei MJ, Libeu CP, Mizushima T, Yamaguchi H, Tomizaki T, Tsukihara T. Redox-coupled crystal structural changes in bovine heart cytochrome c oxidase. Science. 1998 Jun 12;280(5370):1723-9. PMID:9624044
  20. ↑ Atack JM, Kelly DJ. Structure, mechanism and physiological roles of bacterial cytochrome c peroxidases. Adv Microb Physiol. 2007;52:73-106. PMID:17027371 doi:http://dx.doi.org/10.1016/S0065-2911(06)52002-8
  21. ↑ Miller MA, Shaw A, Kraut J. 2.2 A structure of oxy-peroxidase as a model for the transient enzyme: peroxide complex. Nat Struct Biol. 1994 Aug;1(8):524-31. PMID:7664080
  22. ↑ Yang X, Wu D, Shi J, He Y, Pinot F, Grausem B, Yin C, Zhu L, Chen M, Luo Z, Liang W, Zhang D. Rice CYP703A3, a cytochrome P450 hydroxylase, is essential for development of anther cuticle and pollen exine. J Integr Plant Biol. 2014 Oct;56(10):979-94. doi: 10.1111/jipb.12212. Epub 2014, Jul 15. PMID:24798002 doi:http://dx.doi.org/10.1111/jipb.12212
  23. ↑ Yu D, Xu F, Shao L, Zhan J. A specific cytochrome P450 hydroxylase in herboxidiene biosynthesis. Bioorg Med Chem Lett. 2014 Sep 15;24(18):4511-4514. doi:, 10.1016/j.bmcl.2014.07.078. Epub 2014 Aug 6. PMID:25139567 doi:http://dx.doi.org/10.1016/j.bmcl.2014.07.078
  24. ↑ Sharp KH, Mewies M, Moody PC, Raven EL. Crystal structure of the ascorbate peroxidase-ascorbate complex. Nat Struct Biol. 2003 Apr;10(4):303-7. PMID:12640445 doi:http://dx.doi.org/10.1038/nsb913

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