THE STRUCTURE OF PORPHYRIN a, 

 CRYPTOPORPHYRIN a AND CHLORIN a^ 



By R. Lemberg, P. Clezy and J. Barrett 

 Institute of Medical Research, Royal North Shore Hospital, Sydney 



This paper deals mainly with the chemical structures of the prosthetic groups 

 of the cytochromes of type a. Cytochromes a, a^ and a^ have as their pros- 

 thetic group the iron complex of a formyl-porphyrin (porphyrin a, also called 

 cytoporphyrin by Warburg). The presence in heart muscle of a second formyl- 

 porphyrin, cryptoporphyrin a, suggests that another so far unknown 

 haemoprotein of type a is present in heart muscle in small amounts. The 

 prosthetic group of cytochrome a^ belongs to a different class, being the iron 

 complex of a chlorin (dihydroporphyrin) without formyl side chains. 



PORPHYRIN A 



Evidence for the Nature of the Side Chains: The Formyl Group 



The presence of a formyl side chain in the prosthetic group of the Atmungs- 

 ferment was already postulated in the classical work of Warburg. This 

 assumption, based on the similarity of the photochemical absorption spectrum 

 with the spectra of chlorocruorohaem (also called spirographs haem) com- 

 pounds has been amply confirmed by all later workers. The aldehyde group 

 reacts reversibly with methanolic hydrochloric acid forming a methyl acetal, 

 with hydroxylamine to form an oxime, with hydrazine to form a hydrazone, 

 and with sodium bisulphite. The rapid rate of the reaction of the haemo- 

 chrome with hydroxylamine at room temperature (Lemberg and Falk, 1951), 

 the reaction with bisulphite (Lemberg, cf. Parker, 1959) and the dehydration 

 of the oxime to the nitrile (Dannenberg and Kiese, 1952) prove that the 

 carbonyl side chain is not ketonic. The presence of a ring-ketone group 

 similar to that present in the isocyclic ring of chlorophyll, e.g. in phaeopor- 

 phyrin ctj, is excluded by the failure of phaeohaemochromes to react rapidly 

 with hydroxylamine, by the reversibility of methyl acetal formation of 

 porphyrin a (the conversion of phaeoporphyrins into chloroporphyrins being 

 irreversible), and by the infra-red spectrum of porphyrin a (Lemberg and 

 Willis, unpublished) which does not show the band at 1695 cm~^ characteristic 

 of the ring-ketone group. Recently, three more reactions of the formyl 

 group in porphyrins have been studied in our laboratory, its oxidation by 



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