Spin Haem Proteins

Magnetic circular dicliroism (MCD) is independent of, and thus complementary to, the natural CD associated with chirality of nuclear stmcture or solvation. Closely related to the Zeeman effect, MCD is most often associated with orbital and spin degeneracies in cliromophores. Chemical applications are thus typically found in systems where a chromophore of high symmetry is present metal complexes, poriihyrins and other aromatics, and haem proteins are... 

An immediate working hypothesis from these results is that the heme-iron in P-450 and P-420 is bound in an unusual chemical form which gives rise to the rather curious low-spin signals — they are different from those in most other haem proteins, Table 3. Moreover these signals... 

All haem proteins that utilise ferryl intermediates can react with peroxide. These proteins are high-spin in their ferric state with an easily displaced ligand (usually water) at the sixth coordination position. The majority of non-enzymatic electron-transfer haems are low-spin with two strongly bound amino-... 

Leaving aside myoglobin and haemoglobin we turn to the large number of haem-proteins for which there is little or no structural information. We shall consider those which are most like myoglobin first — i. e. mainly high-spin — and then turn to the low-spin proteins. Their high-spin and low-spin Fe(III) haem spectra should assist in the discussion of chemical structure. 

Catalase clearly remains high-spin to a more marked degree than any other haem-protein, for example the cyanide is about 50% high-spin, and this observation too is in keeping with a weak ligand field group in the fifth coordination position. 

These cytochromes contain haem a which differs from the haem of other haem-proteins in that it has an unsaturated substituent, —CHO. In accord with theoretical expectation such a substituent shifts all the absorption bands to lower energy, and increases the intensity of the a/J (especially the a) bands relative to the Soret band. Thus in this series both Fe(II) and Fe(III) haem a complexes have well-pronounced a-bands. The introduction of an aldehyde substituent is also likely to stabilise low-spin as opposed to high-spin states. Thus it is not surprising that magnetic susceptibility data on the cytochromes a show that neither the Fe(II) nor the Fe(III) forms are more than 75% high-spin (133). 

Earlier applications of ESR to the study of lipid free radicals and autoxidation were reviewed by Chapman (1965). More recently. Brown and Wiithrich (1977) have shown an elegant example of a spin label study of lipid oxidation catalysed by haem proteins, and Bascetta etal. (1983, 1984) have used ESR to study the abstraction of hydrogen from saturated, olefinic and acetylenic fatty acids and esters. 

Cyt c is associated with the outer surface of the inner mitochondrial membrane. Phospholipids induce conformational changes in the protein and, in certain instances, the haem can convert to the high spin (S = 5/2) form, indicative of a weakening of the ligand field caused by displacement of the sixth ligand (Met-80). This has been associated with the detection of lipid radicals by direct EPR (at 11 K).65 Indeed, peroxidase-type activity is also evident in the reaction of cyt c with lipid hydroperoxides, as studied by spin trapping in conjunction with HPLC and MS.66... 

At the present time we have no certain knowledge of the state of the heme in these 450 nm species. We do not know if there are heme aggregates although they are unlikely. It is therefore reasonable to look at systems where the haem is aggregated as well as those where it is not in order to see how the absorption spectra can be mimic-ed. It seems reasonable to assume that the iron is low-spin in the carbon monoxide, isocyanide, and nitric oxide complexes as no high-spin iron complexes of this type are known. In the high-spin or low-spin state it may be that the thiol is weakly bound, if at all, for Fe(II) heme in models or in hemoglobin does not bind to thiols. In an attempt to understand these spectra we shall use a semi-empirical approach based on the theoretical discussion in the previous article (52) and elaborated in what follows immediately. Only Fe(II) complexes will be analysed as the Fe(III) proteins have been previously examined (52). 

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