Some Reflections on Prebiotic Porphyrins and Their Complexes with Other Primordial Compounds

SOME REFLECTIONS ON PREBIOTIC PORPHYRINS AND THEIR COMPLEXES WITH OTHER PRIMORDIAL COMPOUNDS Z. J. PETRYKA and C. J. WATSON* Extensive evidence has now been presented of the prebiotic genesis of organic compounds of potential biological utility [I]. The appropriate combination of certain of these might have provided simple photosynthetic or other systems essential for primeval life. Except for viruses (whose status in respect to the definition of life may be debatable) and certain prokaryotes, all forms of life require porphyrins for chlorophylls and/or hemes, in addition to amino and nucleic acids and proteins of various types. Under simulated geochemical conditions of primordial type [2] (i.e., an atmosphere of methane, ammonia, and water) the formation of formaldehyde and pyrrols has been noted [3-7]. In this environment , porphyrins have also been observed to form upon irradiation, heating, or electrical discharge [8-10]. Under similar conditions, the formation of other primeval compounds essential to the origin of life has been observed, including amino acids [11, 12], di- [13, 14] and polypeptides [15], as well as polyamino acid complexes with adenylate [16]. When amino acids are formed under such conditions, glycine is dominant in the mixture; this is of special interest, since, with succinate, glycine is essential for the biosynthesis of the porphyrins via 8-aminolevulinic acid and porphobilinogen. However, long before there was knowledge of porphyrin biosynthesis, as first provided in the now classical studies of Shemin and co-workers [17], Fischer and Fink [18] observed that acetylacetic ester and glycine condensed to yield a carboxypyrrol, which they considered as a possible model for porphyrin biosynthesis. * From the University of Minnesota Medical Research Unit, Northwestern Hospital, Chicago Avenue at 27th Street, Minneapolis, Minnesota 55407. Aided by a grant from the American Cancer Society. Perspectives in Biology and Medicine · Spring 1972 I 443 More recently, Calvin [19] has shown decisively that, under appropriate conditions, acetic acid and succinic acid are formed sequentially on irradiation of CO., with molecular hydrogen. Thus, it may be, as Calvin suggests, that porphyrin biosynthesis recapitulates prebiotic synthesis. We will return in a moment to certain aspects of this question. The observations by Hanada and Fox [20] and by Ponnamperuma [21] on the formation of ATP and nucleotides, respectively, in a simulated primeval atmosphere supplement the foregoing information and indicate that the most important constituents of living matter can arise in prebiotic fashion. The recent observation by PaechtHorowitz et al. [16], that certain prebiotic clays provide a suitable surface for polymerization of amino acids and combination with adenylate, confirms an earlier suggestion of Bernai [22], at the same time permitting insight into one of the most significant facets of the problem of life's origin. Porphyrins are believed to represent the first organic compounds absorbing the energy of visible light [23], although open-chain tetrapyrryl pigments, such as the phycocyanin or phycoerythrin of the blue or red algae might be permitted to compete for this honor. Evidence that they are derived from heme [24, 25] would nevertheless appear to give primacy to the porphyrins. Fossil blue-green algae have been found in rock at least 1.5 billion years old [19]. The phycocyanin- and chlorophyll-type compounds which may be presumed responsible for their color were in all likelihood biosynthetic in origin, since the algae, even at that early period, though simple, were in some measure organized living forms. In contrast to the porphyrins, there is no evidence to suggest a prebiotic formation of the open-chain tetrapyrryl compounds. Furthermore, algal pigments are of the IXa configuration, thus indicating a probable origin from heme(s) [24, 25] or chlorophyll. Hodgson [10] noted that the porphyrins generated in his discharge system (methane-ammonia-water atmosphere) exhibited a fairly ready solubility in water, possibly due to carboxylic side chains. This observation at least suggests the presence of (polar) beta substituents on the porphine ring, as in the porphyrins of living organisms. If, as mentioned in the foregoing, porphyrin biosynthesis recapitulates prebiotic synthesis, the presence of the hydrophilic uroporphyrin(s) might be anticipated. Actually, prebiotic porphyrins or porphyrins formed under simulated prebiotic conditions, such as those of Hodg444 I 7- J. Petryka and C. J. Watson · Prebiotic Porphyrins son, have...