I found this question interesting, so that, despite my general ignorance in this area, I have attempted to address the question of the evolution of tetrapyrroles rather than answer it. Additional contributions are welcome as comments. I apologize for the large graphic, which I feel is indispensable to my answer, however I have included a summary above it, so readers can decide whether they wish to read further.
- There are four main types of tetrapyrrole associated with proteins
- They share a common pathway from 5-aminolevulinic acid to uroporphyrinogen II
- The unresolved question of which subsequent pathway evolved first is discussed in relation to their antiquity and complexity.
- The possible evolutionary relevance of the two different pathways for the synthesis of 5-aminolevulinic acid — found in different organisms — is also discussed.
Tetrapyrroles in proteins — Diversity and Chemical Rationale
Although the cyclic tetrapyrrole structure looks complex, it appears to be very ancient, presumably because of the greater chemical versatility of its complexes with metal ions in proteins, compared with simpler proteins that contain metal ions. The ring provides a precise four-co-ordination which can be held in position to provide two additional co-ordination sites (at right angles to the plane of the ring) which can interact with protein side-chains or other molecules (such as oxygen). The combination of ion, substitution of the tetrapyrrole and interaction with the protein allows fine-tuning of the redox properties of the ion.
The diagram below (adapted from Zappa et al.) shows the four main types of tetrapyrrole in proteins and how their biosynthesis is related. The two mentioned in the question are actually both derivatives of protoporphyrin IX: chlorins, such as chlorophyll — with its role in photosynthesis — and haem proteins — with the examples of the cytochromes of the electron transport chain being more widespread than that of haemoglobin in eukaryotes. In addition, however, are the corrins — most notably in Vitamin B12 with a role in methyl transfer — and sirohaem — which occurs in certain sulphite and nitrite reductases.
Which tetrapyrrole is oldest in evolutionary terms?
How can we address the question of which tetrapyrrole was first to arise? This question is, in fact, which synthetic pathway — which series of enzymes — was first to arise. In my review of the literature I (like the poster) find this an area which angels are fearful of, but, being foolhardy, I suggest two criteria. The first is what we consider the relative ages of the functions the proteins subserve, and the second the relative complexity of the pathways needed to synthesize them.
The first criterion is subject to dispute. Unfortunately the easy comparison between haem as an oxygen carrier in eukaryotes and chlorophylls as ancient prokaryotic photosynthetic proteins is complicated by the more ancient role of haem in cytochromes. One might argue that cytochromes are a feature of aerobic metabolism that followed the emergence of oxygen generation by photosynthesis, but unfortunately the electron transport chain exists in anaerobic organisms, where the terminal electron acceptor may be nitrate, nitrite, ferric iron, sulphate or carbon dioxide. Given this, and the greater complexity of the pathway from protoporphyrin IX to bacteriochlorophyll a compared with that to protohaem (10 steps compared to one), one might argue that chlorins emerged after haems.
But what about the other tetrapyrroles? One approach to relative ages is to consider the biochemical processes that might have been present early after the emergences of life. One such analysis of this was made by Weiss et al. in a paper published in Nature Microbiology (2016) 1, 1–8, entitled “The physiology and habitat of the last universal common ancestor”. They drew the conclusion that the Last Universal Common Ancestor (LUCA) was an anaerobic autotroph that existed in a hydrothermal setting and obtained energy using a Wood–Ljungdahl pathway with hydrogen as the electron donor and carbon dioxide as the electron acceptor. The two types of tetrapyrroles that they postulate would have been present in the LUCA are corrin (as cobalamin for methyl group transfer) and sirohaem for sulphur metabolism (sulphite reductase) in the hydrothermal environment. No mention of porphyrins! Given the complexity of the 16 step pathway to corrin, compared to the one-step pathway to sirohaem, one might suggest that the latter evolved first.
Synthesis of 5-aminolevulinic acid — evolutionary implications?
5-aminolevulinic acid (frequently referred to by its old name of δ-aminolevulinic acid) is the precursor of uroporphyrinogen II, the common tetrapyrrole precursor. However there exist two pathways by which this can be formed, of which, as I understand it, only one is present in any organism. One of these (at least to my thinking) could have evolutionary implications. These pathways are shown below in a figure adapted from a paper on prokaryotic haem biosynthesis by H. Panek and M.R. O’Brian (2002).
The intriguing aspect of this is that one of the pathways (the C5 pathway) involves reduction of glutamyl-tRNA, whereas the other (the Shemin pathway) is a conventional synthase reaction with substrates glycine and succinyl-CoA. Arguments supporting the RNA world hypothesis include catalytic RNA, and the presence of nucleosides in molecules such as NAD where there complexity belies the simplicity of their function. So, putting my head over the parapet again (to vary my metaphors), I would suggest that the use of glutamyl-tRNA for the synthesis of this ancient and important intermediate might be another fossil of the RNA world. (Perhaps not what the poster had in mind in posing his question.)