Section 4

Contents of this section

• 4. Cytochromes
  • 4.1 Introduction and definitions
  • 4.2. Cytochrome Groups
  • 4.3 Variability in Cytochrome Groups
  • 4.4 List of Cytochromes
    • 4.4.1 Cytochrome a group
    • 4.4.2 Cytochrome b group
    • 4.4.3 Cytochrome c group
    • 4.4.4 Cytochrome d group
• References for this section

4.1 Introduction and definitions

Hemeproteins (see Note 1) that transfer electrons belong to the family of the cytochromes. The name 'cytochrome' was introduced by Keilin (ref 3) in 1925 to describe a group of intracellular hemeproteins that undergo oxidation-reduction and, upon reduction, exhibit intense absorption bands between 510 and 615 nm. As currently used, the name appears to include all intracellular hemeproteins with the exception of hemoglobin, myoglobin, the peroxidases, catalase, tryptophan 2,3-dioxygenase, heme-thiolate proteins (P-450) and the nitrite and sulfite reductases. Consequently, proteins of markedly different function are found in this family. (see Note 2)

Note 1 The American spelling 'heme', as opposed to 'haem', is used here although both are acceptable and widely used.

Note 2 Thus a number of enzymes are also referred to as cytochromes. These include cytochrome-c oxidase (EC 1.9.3.1), L-lactate dehydrogenase (cytochrome) (yeast cytochrome b₂, EC 1.1.2.3) and cytochrome P-450 (EC 1.14.14.1).

Since the publication of the 1978 report (ref 7), knowledge of the primary and tertiary structure, and of the physical properties, of the cytochromes has continued to accumulate at a substantial rate. It has become apparent that the cytochromes represent such a diverse family of compounds that no simple, complete and self-consistent basis for classification currently exists, although comprehensive suggestions have been made for a system of classification within the c-family of proteins from both sequence (ref 8) and crystallographic data (ref 9). Consequently, as was the conclusion in earlier recommendations, 'it is premature to do more than to monitor and, hopefully, to coordinate present practices in classification and nomenclature of cytochromes', to present guidelines for the future reporting of the properties of newly discovered proteins, and to revise aspects of earlier recommendations in the light of more recent knowledge. The present recommendations follow the same lines as the 1978 report (ref 7).

The term 'heme' is usually understood as any tetrapyrrolic chelate of iron (ref 10). The terms 'ferroheme' and 'ferriheme' (see Note 3) still refer to the Fe(II) and Fe(III) oxidation states in heme; however, the Fe(IV) oxidation level of heme iron is found as a catalytic intermediate in some systems. A hemochrome is defined as a low-spin compound of heme in which fifth and sixth coordination places are occupied by strong field ligands regardless of the oxidation state of the iron. (see Note 4) Finally, the terms 'hemoprotein' or, preferably, 'hemeprotein' refer to a protein containing a heme as a prosthetic group.

Note 3 Ferriheme is sometimes referred to as 'hematin', a usage still sanctioned by tradition. However, this term should not be used for the compound crystallized as the chloride or other salt; such compounds are customarily termed 'hemin'.

Note 4 The traditional definition that the fifth and sixth coordination positions are occupied by nitrogen atoms was obviously too restrictive as borne out by the case of cytochrome c, wherein one of these sites is occupied by the sulfur atom of methionine.

4.2. Cytochrome Groups

Four major groups of cytochromes are currently recognized:

Cytochromes a. Cytochromes in which the heme prosthetic group is heme a, i.e. the iron chelate of cytoporphyrin IX (ref 10,11).

Cytochromes b. Cytochromes with protoheme [the iron chelate of protoporphyrin IX, see (ref 10)] as prosthetic group but which lack a covalent bond between the porphyrin and the protein.

Cytochromes c. Cytochromes with covalent thioether linkages between either or both of the vinyl side chains of protoheme side chains and the protein.

Cytochromes d. Cytochromes with a tetrapyrrolic chelate of iron as prosthetic group in which the degree of conjugation of double bonds is less than in porphyrin, e.g. dihydroporphyrin [chlorin; heme d (ref 12)], tetrahydroporphyrin [isobacteriochlorins; heme d₁ (ref 13), siroheme (ref 14)]. Heme d has also been known as heme a₂.

[Comment. It was originally suggested that the heme component of cytochromes a could be called heme a. However, the hemes present in groups b-d were not classified. Heme b is acceptable for noncovalently bound protoheme. However, the various alternative forms for hemes 'c' and 'd' preclude a simple classification.]

Use of the small unprimed italicized letter implies that the heme prosthetic group is in a hemochrome linkage. To indicate that in both the oxidized and reduced forms the heme prosthetic group is not in a hemochrome linkage, a primed lower-case italicized letter, e.g., c', should be used.

In the case of a cytochrome having two or more differing heme groups attached to a specific protein, each different heme may be indicated, e.g. in Pseudomonas cytochrome oxidase (cd₁, EC 1.9.3.2). In the case of a cytochrome having two or more of the same heme groups attached to a specific protein but in different environments, so that one or more is in a hemochrome linkage and one or more in a non-hemochrome linkage, both types of linkage should be indicated by using both the unprimed and primed lower-case italicized letter, appropriate for the heme in question. As an example, cytochrome-c oxidase (EC 1.9.3.1) is considered to contain both a heme a in hemochrome-type linkage (called 'cytochrome a') and a heme a in a non-hemochrome-type linkage (called 'cytochrome a₃'). By the suggested convention, this cytochrome should be called 'cytochrome aa', although the current usage of 'cytochrome aa₃' is unlikely to be abandoned. The recommended notation should be adopted for future cases. In general, the name of a cytochrome will not necessarily indicate the number of identical heme centres per molecule of protein.

The main practical tests to be adopted as criteria in determining the group to which a cytochrome belongs should be (a) the position of the a band of the pyridine Fe(II) hemochrome and (b) the ether solubility of the hemin after treatment of the cytochrome with acidified acetone, or acidified methylethylketone, as shown in Table 1.

For the moment it is recommended that for groups a, b and d the position of the a band of the pyridine Fe(II) hemochrome should be determined after the acidified acetone-cleaved hemin has been extracted into ether and then extracted from the ether by dilute sodium hydroxide. (see Note)

Note In addition, a number of chemical reactions that can be carried out with small amounts can also be used for the establishment of the group (the original literature should be consulted for details). Oxime formation of the isolated hemin with hydroxylamine combined with conversion of the oxime into a nitrile, and a number of spectroscopically observable reactions of the hematin or porphyrin moiety with sodium hydrogen sulfate or with dimedone, prove the presence of a heme with a formyl side-chain (group a). The reduction of an unsaturated side-chain with hydrazine-HI, or Pt-H₂, together with the criterion of a-band position shows the presence of protoheme (group b). The splitting of the thioether bond with silver or Hg(II) sulfate is a test for group c. Group d diagnosed from the presence of four bands between 500-700 nm in the optical spectrum of the reduced heme; the assignment should be verified by preparation of the pyridine hemochrome (Table 1). (It is desirable that the result should be verified with authentic samples of the pertinent heme.)

Table I: Practical criteria for determining the cytochrome group

* Requires elaborate precautions to prevent decomposition to an insoluble product

a. At the first mention in a publication, the name should be expanded to the systematic name that will include the source in parentheses, e.g. cytochrome b₁ (Bacterium X). Since the need has arisen to refer to different b and c type cytochromes present in the same cell or in the same organelle of a given cell type, it will often be necessary to add distinguishing characteristics, such as 'microsomal', 'tetraheme', 'high potential', etc.

b. The names of the already well-established cytochromes with consecutive subscript numbering, listed below, are retained. All cytochromes not fitting in this category should be given a name based upon the a-band wavelength (nm) and written thus: cytochrome c-554.

c. The a-band wavelength used should be determined at room temperature, not liquid-nitrogen temperature, and should, if possible, be obtained from absolute absorption spectra of the purified protein under carefully defined conditions. Since an error of 1 nm in assigning the position of the band alters the name, care should be taken to standardize the spectrophotometer with standard lines, e.g. with those given by Nd(III). Failing this, the absorption maximum can be determined by calibration against horse heart mitochondrial cytochrome c using 550.25 nm for the wavelength maximum when dissolved in phosphate buffer, pH 7. Note. The value of 550.25 nm stems from early work and may need revision; a value redetermined carefully is 499.9 nm (G. Palmer, personal communication)

In application of the naming procedures based on location of the a-wavelength maximum, any asymmetry or splitting of the a-peak should be noted, e.g., cytochrome c-555 (550) indicates a minor peak or shoulder (in parenthesis) at 550 nm. It is urged that assignments should not be made solely on the location of the a-band maximum, and reviewers and editors of reports dealing with discovery and description of cytochromes should insist that, whenever possible, the above procedures, based on chemical characterization, be followed. However, these rules need elaboration whenever solubilization of cytochromes is not possible without denaturation, as in some membrane-bound proteins. The rules given are applied readily only to whole cell systems that have a simple composition, e.g. c-type proteins only. Problems arise when bound cytochromes c occur together with frequently encountered b-type complexes whose a-band maxima overlap those of the c-type proteins. In such cases, attempts should be made to separate adequately the membrane from the soluble fractions in a whole-cell system. Electron-paramagnetic resonance spectroscopy (EPR, also called electron spin resonance, ESR) is of considerable value in resolving complex heme-protein systems which include mixture of high- and low- spin components. Use of redox buffers combined with spectroscopy can also be used occasionally to characterize cytochrome components but redox potentials measured at ambient temperature should not be applied to spectral moieties determined at the low temperature associated with EPR spectroscopy.

Purification of soluble proteins to homogeneity after separation from membrane-bound components should be effected by the necessary chromatographic, gel-filtration and electrophoretic procedures, together with conventional salting-out techniques and, if feasible, crystallization, before assay of physico-chemical properties. Cytochromes should not be assigned on the basis of spectral data obtained with whole cells or other complex systems, unless it can be unequivocally demonstrated that the spectral features arise from a single species.

4.3 Variability in Cytochrome Groups

The variations in spectroscopic and functional character of cytochromes, found especially as a result of work with prokaryotic systems, has led to an undesirable proliferation of subscripts to distinguish subgroups of uncertain character. It appears that this unfortunate tendency has been halted, but adequate characterization of subgroups remains a problem. Presumably, the primary sequence will eventually provide a rational basis for proper assignments to subgroups and some attempts along these lines have been made for the c family (ref 8), but the suggestion that a phylogenetic basis for such classification based on sequence similarities may exist is still premature. However, sequence similarity can be invoked to reclassify a number of prokaryotic cytochromes c, as is indicated in the revised list of cytochromes given below.

Certain variant cytochromes remain difficult to classify. Cytochromes 'o', a type of protoheme oxidase found in prokaryotes, and helicorubin, should be listed as b-type cytochromes.

Likewise, cytochrome 'P-450', which originally received its name in a casual manner (location of Soret peak of the reduced CO compound) not in conformity with any of the recommendations given herewith or previously, has been characterized as a class of proteins with activity as a monooxygenase involved in hydroxylation associated with electron tranfer. Its chemical nature as a b-type cytochrome with atypical non-nitrogenous ligands is well established in a number of systems, notably that of the pseudomonads that effect hydroxylation of terpenes.

This group of monooxygenases known as cytochrome P-450 enzymes could, however, easily be included in the cytochrome b class, since they are enzymes in which protoheme is non-covalently attached to the protein. On the other hand, since the characteristic mode of action of these enzymes is not electron transfer (some P-450-enzymes probably do not even involve the reversible Fe(II)/Fe(III) equilibrium), but rather oxygen atom transfer, the name 'cytochrome' did not seem appropriate. Based on the fact that a thiolate ligand at the heme is responsible for the unusual spectral and catalytic properties of these hemoproteins the name 'heme-thiolate proteins' is now recommended.

The characteristic feature of the heme-thiolate prosthetic group is its activating power on an oxygen species as verified in e.g. monooxygenases (such as EC 1.14.14.1), prostacyclin synthase (EC 5.3.99.4), thromboxane synthase (EC 5.3.99.5), leukotriene-B4 20-monooxygenase (EC 1.14.13.30) and probably chloride peroxidase (EC 1.11.1.10). However, it is noted that this example appears to have a similar ligand environment; consequently the ascription heme-thiolate is not exclusive to the P-450 class of monooxygenases.

4.4 List of Cytochromes

4.4.1 Cytochrome a group

Cytochrome aa3 is probably identical with one of the subunits of cytochrome c oxidase of eukaryotes and some prokaryotes. The protein complex contains two hemes a, a low-spin component called cytochrome a and a high-spin component called cytochrome a3. The a-band of the reduced cytochromes is at about 605 nm, the (Soret) at around 445 nm. The reduced cytochrome a3 combines with CO with a shift of the a-band to 590 nm and the g-band to approximately 430 nm; it also combines with cyanide with a shift of the a-band to 590 nm with little effect on the g-band. The reduced form is autoxidizable. In the presence of cyanide, one-half of the heme content is autoxidizable. The enzyme complex also contains two copper atoms (see section 6.5) cytochrome aa3 is membrane bound and catalyses the oxidation of mitochondrial cytochrome c and some related bacterial proteins by O₂.

Cytochrome a₁ has been isolated from Nitrobacter agilis. It has an absorption maximum at 587 nm and is not autoxidizable. Autoxidizable proteins with absorption maxima have been reported for Acetobacter pasteurianum and Escherichia coli. However, no heme a has been detected in Escherichia coli and thus the existence of this cytochrome in these latter organisms must be considered speculative until such time as a pure protein has been isolated and shown to contain heme a.

4.4.2 Cytochrome b group

Cytochrome b is present in mitochondria of eukaryotes and in chloroplasts. It is an integral membrane protein which contains two hemes in a protein of molecular mass of about 40 kDa. The two heme centres can be distinguished by the position of the a-band in the optical spectrum of the reduced protein, the location of the low-field g-value in the EPR spectrum of the oxidized protein and by differences in redox potential. A strongly related protein, cytochrome b-563, also known as cytochrome b₆, functions in the bc complex present between photosystems I and II of green plant photosynthesis. Related bc complexes can be found in some photosynthetic bacteria. Cytochrome b-559 is present between the centre for water cleavage and photosystem II of green plants. White blood cells contain a cytochrome b-562.

Cytochrome b₁ has been detected in Escherichia coli by the presence of an absorption maximum at 560 nm. However, it seems that this band reflects contributions from several different cytochromes of slightly differing optical properties and redox potentials. Thus the existence of this group should be considered doubtful.

Cytochrome b₂ is present in yeast. It contains one mole each of heme and FMN as prosthetic groups and acts as an L-lactate dehydrogenase (cytochrome) (EC 1.1.2.3). It is also called flavocytochrome b. The cytochrome is part of the membrane-bound NADPH-oxidoreductase, which reduces O₂ to H₂O₂ and O₂^.-.

Cytochrome b₃ is present in microsomal material from non-photosynthetic plant tissues. Its characteristic absorption band is at 559 nm. This category is not sufficiently defined to warrant further use.

Cytochrome b₅ is present in animal microsomes and the cytoplasm of the erythrocyte. It is reduced by NADH in the presence of cytochrome b₅ reductase (EC 1.6.2.2) or NADH ferrihemoglobin reductase (EC: see Note). It contains 1 heme per polypeptide of 137 residues; the heme iron has bis-histidine coordination and is low-spin. An X-ray structure is available for the proteolytically degraded form from microsomes; the erythrocyte protein is analogous to this short form. Its characteristic band is the a-band at 557 nm with a shoulder at about 560 nm. It functions in electron transfer associated with desaturation and elongation of higher fatty acids, hydroxylation (detoxification) and maintaining hemoglobin in the ferrous state.

Note Not yet listed in Enzyme Nomenclature (ref 1).

Cytochrome b₈ is a monomeric, monoheme protein, typified by cytochrome b-562 from Escherichia coli. It contains 1 heme per 12 kDa with iron coordination provided by methionine and histidine. The tertiary structure is similar to that of cytochrome c'. The mid-point potential is 190 mV (pH 7) and is pH dependent.

Cytochrome b' was formerly known as cytochrome o. It is a prokaryotic terminal oxidase with protoheme as prosthetic group. (It has also been reported in some protozoans.) The reduced CO compound has absorption peaks at 557-567 nm (a) and 532-537 nm (b). Its absorption spectrum indicates it is a high-spin heme protein. It exists in two forms: the soluble (s) form appears to transport oxygen, the membrane (m) form is a terminal oxidase. The term cytochrome o is discouraged, but for continuity it can be indicated as 'cytochrome b' (formerly cytochrome o). (Some autoxidizable low-spin cytochromes o have been reported).

4.4.3 Cytochrome c group (see ref 15)

Cytochrome c is present in eukaryotic mitochondria where it functions as the substrate for the terminal oxidase (EC 1.9.3.1) in oxidative phosphorylation. It is a soluble, low-spin, monohemeprotein with 103-112 residues. Its midpoint redox potential over most of the physiological pH range is about 250 mV. In its reduced form, the a-band maximum is at 550 nm, the b-band at 520 nm and the Soret peak at 415 nm. Both vinyl groups are present in thioether bonds and the heme iron is coordinated by histidine and methionine. It is the prototypic c-type cytochrome.

Cytochrome c₁ is the 30 kDa membrane-bound c-type protein of mitochondria with a-band maximum at 553 nm in the reduced form. On solubilization with detergents this amaximum remains unchanged. It is a low-spin monoheme protein with the same axial ligands as cytochrome c. The mid-point potential is about 270 mV. It functions as electron donor to cytochrome c in the mitochondrial and bacterial respiratory chain. The related protein present in the bc complex of green plants is also called cytochrome f; it has histidine and lysine residues as axial ligands and a midpoint potential of about 360 mV. Cytochrome f transfers electrons to the copper protein, plastocyanin.

Cytochrome c₂ is a small, soluble, low-spin cytochrome with the same tertiary folding and binding of heme as in mitochondrial cytochrome c, but with limited ability to replace it as a substrate for cytochrome-c oxidase (EC 1.9.3.1). It is a monoheme protein with midpoint redox potential usually some 100 mV higher than that of mitochondrial c, and with much different pH redox profile. In Rhodopseudomonas viridis (and probably Chromatium vinosum) it functions to transport electrons from the bc complex to the membrane bound c-type cytochrome which functions as electron donor to the bacterial photosystem. In Rhodobacter sphaeroides it reacts directly to reduce the photooxidized special pair of bacteriochlorophyll.

Cytochrome c₃ is a low-potential (much less than 0 mV), low-spin cytochrome with the same thioether binding to heme as in mitochondrial c, but with very different tertiary structure and no sequence similarity to the eukaryotic proteins. The monomeric form is about 13 kDa; it can exist as a dimer. Each iron atom has bis-histidine coordination. It exists in triheme and tetraheme forms as a monomer with the same 68-115 residues and is found in the strictly anaerobic sulfate- and sulfur- reducing bacteria where it participates in sulfate respiration coupled to phosphorylation. It exhibits no reactivity with mitochondrial reductase or oxidase. The triheme form was formerly called cytochrome c₇. Cytochrome c-552 from Escherichia coli may be related.

Cytochrome c₄ is a diheme, high-potential protein with about 190 residues found in Azotobacter vinelandii and some Pseudomonas species. The heme attachment and tertiary structure resemble eukaryotic cytochrome.

Cytochrome c₅ is a low-spin dimeric monoheme protein containing a single heme in each of two subunits, usually with 80-90 residues per monomer exhibiting the same thioether binding and extraplanar ligands as in mitochondrial cytochrome c. It is unreactive with cytochrome c reductase and oxidase. Its midpoint redox potential is somewhat higher than that of mitochondrial c and its a-peak is red-shifted to about 554-555 nm. It may be proteolytically modified as isolated. It appears to be present also in the strictly aerobic nitrogen-fixing Azotobacter.

Cytochrome c₆ is a soluble monoheme, monomeric low-spin cytochrome which in algae has a molecular mass of about 10 kDa and the same binding and extraplanar ligands as in mitochondrial cytochrome c. Its midpoint redox potential is usually about 100 mV higher than that of the mitochondrial protein. Its reduced a-peak is asymmetric and red-shifted to about 552-554 nm. It functions like cytochrome c₂ in that it mediates electron transfer at the high potential terminus of the photophosphorylation chain in chloroplasts and algae. It is unreactive with mitochondrial cytochrome c reductase or oxidase but reactive with Pseudomonas cytochrome cd₁. It is the functional equivalent to photocyanin. It should not be confused with the membrane-bound cytochrome c-553 (of the bc complex).

'Pseudomonas' cytochrome c-551 is another prokaryotic, monomeric monoheme cytochrome with 80-90 residues, unreactive with mitochondrial cytochrome c reductase or oxidase, although possessing similar midpoint redox potential, the same extraplanar ligands and heme thioether binding. It apparently functions in nitrite and nitrate reduction in pseudomonads, but it is also found in other bacteria. It is functionally analogous to cytochrome c.

Bacterial photosystem cytochrome c is a subunit of the photosynthetic reaction centre of the purple non-sulfur bacterium Rhodopseudomonas viridis. The subunit contains 4 heme-c centres in a polypeptide of about 330 residues and mediates electron transfer from cytochrome c₂ to the photooxidized bacteriochlorophylls b. The cytochrome exhibits a-band maxima at 552 and 558 nm. Two of the hemes have a low potential (about 0 mV); the other two have a high potential (about 300 mV). It is structurally unrelated to cytochromes c₂ and c₃. Similar proteins appear to be present in Thiocapsulata pfenigii, Chromatium vinosum and Rhodopseudomonas gelatinosa.

Cytochrome c-555 from green bacteria is a low-spin monoheme cytochrome of 80-90 residues found exclusively in the green photosynthetic bacteria (Chlorobium) with intermediate midpoint redox potential (about 150 mV) and variable reactivities with mitochondrial cytochrome c reductase and oxidase and Pseudomonas cytochrome cd₁. Like other prokaryotic cytochromes c mentioned above, it possesses the characteristic thioether heme binding, the same extraplanar ligands, and a tertiary structure distinctly related to mitochondrial cytochrome c. Its reduced a-peak is asymmetric and red-shifted (to about 555 nm) as in cytochrome c₆.

Cytochrome c' is a high-spin variant cytochrome c (originally known as 'RHP') with heme binding through side-chain thioether linkage as in mitochondrial cytochrome c but with unrelated tertiary structure. It is unreactive with the mitochondrial c reductase or oxidase. It occurs usually as a dimer, with monomer molecular masses of about 14 kDa, and has a midpoint redox potential at pH 7 close to zero. The heme is 5-coordinated with a single histidine axial ligand but exhibits a low-spin hemochrome EPR spectrum at pH values higher than 12. It is found in purple photosynthetic bacteria as well as in nitrate-reducing pseudomonads. When soluble, it reacts only with NO and CO in its reduced form and with NO in its oxidized form. It is unreactive with the usual ligands (CN^-, F^-, N₃^-, etc.) for high-spin heme proteins. The reduced heme exhibits a broad absorption band centred between 540 and 560 nm. It is the most widely distributed bacterial cytochrome known.

Various other forms are as yet insufficiently characterized to be placed in sub-groups. An example is flavocytochrome c, found in purple sulfur photosynthetic bacteria.

4.4.4 Cytochrome d group

Cytochrome d was earlier known as cytochrome/heme a₂. It is present in many aerobic bacteria, especially when grown with a limited oxygen supply. Typical species include Escherichia coli and Aerobacter aerogenes. In protein complexes, it typically gives an absorption band at about 636 nm (oxidized) or 628 nm (reduced). It is found associated with other prosthetic groups in multi-subunit complexes. It is difficult to detect as a Fe(II) pyridine alkaline hemochrome, because of limited stability under these conditions. If it is extracted from the protein complex and placed in ether containing 1 to 5% HCl, it gives a distinctive band at 603 nm (oxidized).

Cytochrome bd functions as a terminal oxidase in Photobacterium phosphoreum and Escherichia coli.

Cytochrome cd₁ is found in a common dissimilatory nitrite reductase present in many denitrifying bacteria. Typical species include Pseudomonas aeruginosa, Paracoccus denitrificans, and Thiobacillus denitrificans. Heme d is non-covalently associated while the heme c is covalently bound to polypeptide. The band maximum of the a-peak in the Fe(II) state is broad and very sensitive to pH and the reductant used, appearing at 625-630 nm with dithionite reduction, and 655 nm with ascorbate reduction. The d₁ chromophore is typically found to auto-reduce under a carbon monoxide atmosphere. The Fe(II) alkaline pyridine hemochrome band maximum is at 618 nm. The heme is very hydrophilic and is readily water soluble at neutral pH.

References for this section

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6. International Union of Biochemistry (1973) Enzyme Nomenclature, Recommendations 1972, Elsevier, Amsterdam.

7. International Union of Biochemistry (1979) Enzyme Nomenclature, Recommendations 1978, Academic Press, New York.

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15. Moore, G. & Pettigrew, F.(1987) Cytochromes c, Springer-Verlag, Berlin.