HAEM PROTEIN CONTENT AND FUNCTION IN RELATION TO STRUCTURE 285 



We can infer consequences of such a process by analogy with many 

 observations available in the literature, even though nothing is known 

 directlv about solution chemistry of higher oxidation states of iron. 

 George and Irvine [41] have shown that metmyoglobin treated by a 

 varietv of oxidizing agents (peroxide, permanganate, chloriridate) gives a 

 product spectroscopically identical with the intermediate "complex II" 

 formed when metmvoglobin acts as a peroxidase. They have established 

 the £"|j as ~ +0-9 V. This \alue is ~ o- 1 V. more oxidizing than that for 

 the standard oxvgen electrode. Hence, the oxidizing equivalents present 

 in this complex can extract an electron from water. If it is assumed that 

 the haem chelate-protein complex acquires a similar E'^, then the E'q for 

 the semichlorinogen formed would be ~ — o-6 to —0-7 e.V., assuming 

 I • ^ e.V. as the value of J/s^ between the reducing and oxidizing com- 

 ponents. Such a strong reducing potential would be more than sufficient 

 to provide an electron transfer step to pyridine nucleotide (£",',= —0-3) 

 which could be coupled to formation of ATP from ADP and inorganic 

 phosphate ("photophosphorylation ") [42, 43]. On this basis, the 

 " photoreductase " of San Pietro and Lang [44] would have assigned as its 

 substrate the semichlorinogen as the photoreductant generated by the light 

 reaction. 



The reactions initiated by the presence of the Fe'^-haem complex 

 depend on the environment presented, h simple combination of chloro- 

 phvll and haem protein would have only the possibility of back reaction, 

 or reversal of the process shown in Fig. i . However, if an enzymic pathway 

 (such as through the photoreductase to pyridine nucleotide) is available to 

 remove the electrons from the semichlorinogen, then it can be expected 

 there will be a preferential flow of electrons to the enzyme substrate. If a 

 source of electrons is present in the haem complex, either in the protein or 

 as a simple ligand (water), then reduction of the Fe^^-haem to its original 

 state would occur with the production of a free radical. 



The evidence available from paramagnetic spin resonance studies of 

 the metmyoglobin oxidation complex, while somewhat ambiguous, appears 

 to be consistent with this postulated sequence of events. Gibson, Ingram, 

 and Nichols [45] have shown that the complex, studied by George and 

 Irvine and produced by peroxidation of metmyoglobin, exhibits an ESR 

 signal with a »- value close to that for the free electron. The precise value 

 for g is somewhat smaller than expected for a 7T-electron localized at a 

 methine bridge carbon. It is more consistent with the presence of a 

 delocalized electron in an orbital spread over the whole macrocyclic 

 structure, or of a substrate free radical, such as OH. At the same time the 

 signal at ff = 6 corresponding to the unpaired electrons at the Fe site is 

 quenched, indicating a change in the bonding at the metal ion site. George 

 and Irvine [41] have presented evidence for this change as a production 



