Major metabolic pathways and their inadequacy for origin of life proposals
According to geneticist Michael Denton, the break between the nonliving and the living world ‘represents the most dramatic and fundamental of all the discontinuities of nature.
And John Lennox writes in his book has science buried God ?
It is hard for us to get any kind of picture of the seething, dizzyingly complex activity that occurs inside a living cell, which contains within its lipid membrane maybe 100 million proteins of 20,000 different types and yet the whole cell is so tiny that a couple of hundred could be placed on the dot in this letter ‘i’.
The meaning of the genetic code is also virtually identical in all cells. The size, structure and component design of the protein synthetic machinery is practically the same in all cells. In terms of their basic biochemical design, therefore, no living system can be thought of as being primitive or ancestral with respect to any other system, nor is there the slightest empirical hint of an evolutionary sequence among all the incredibly diverse cells on earth.’
This view is supported by Nobel Prize-winner Jacques Monod, whom Denton cites. ‘We have no idea what the structure of a primitive cell might have been. The simplest living system known to us, the bacterial cell… in its overall chemical plan is the same as that of all other living beings. It employs the same genetic code and the same mechanism of translation as do, for example, human cells. Thus the simplest cells available to us for study have nothing “primitive" about them… no vestiges of truly primitive structures are discernible.’ Thus the cells themselves exhibit a similar kind of ‘stasis’ to that referred to in the previous chapter in connection with the fossil record.
Its interesting to try to figure out what that supposed last universal common ancestor ( LUCA ) was, in order to understand what kind of biochemical mechanisms, metabolism, enzymes, co-factors, proteins and genome information would have to be explained, and its origin.
From a biochemist’s perspective, life at the cellular level can be defined as a network of integrated and carefully regulated metabolic pathways, each contributing to the sum of activities that a cell must carry out. Cellular metabolism is a complex process involving about a thousand chemical reactions catalyzed by globular proteins, enzymes.
In the scientific paper: The Enzymatic and Metabolic Capabilities of Early Life, the author states that several independent studies have used comparative bioinformatics methods to identify taxonomically broad features of genomic sequence data, protein structure data, and metabolic pathway data in order to predict physiological features that were present in early, ancestral life forms. We survey modern metabolic pathways to identify those that maintain the highest frequency of metaconsensus enzymes. Using the full set of modern reactions catalyzed by these metaconsensus enzyme functions, we reconstruct a representative metabolic network that may reflect the core metabolism of early life forms.
Their research revealed the mind blowing complexity of Luca, and its metabolic pathways:
According to another research paper : Evolution of the first metabolic cycles, There are two alternatives concerning the origin of life: the origin may be either heterotrophic or autotrophic. The paper : Analysis of the Intermediary Metabolism of a Reductive Chemoautotroph gives a idea of the complexity of it:
No wonder, do the authors of the paper: How Life Began: The Emergence of Sparse Metabolic Networks , openly admit that: " The process by which the network of extant metabolism emerged is one of the major puzzles in the origin of life field." Another paper admits that " An open question for scientists is when and how cellular metabolism, the network of chemical reactions necessary to produce nucleic acids, amino acids and lipids, the building blocks of life, appeared on the scene." The pathways for synthesis of most of the twenty amino acids used in proteins and the four nucleotides used in RNA are identical or nearly identical in Archaea, bacteria and eukaryotes, suggesting that these pathways were inherited from the LUCA. metabolic network. Thus, it appears that that the LUCA had the ability to synthesize the critical building blocks of life and did not rely on exogenous sources of these compounds. This supposition is supported by bioinformatic reconstructions of the genome of the LUCA. Biosynthetic pathways in extant organisms clearly resemble those in the LUCA. In the scientific paper : In The Ancient Ocean, Did Metabolism Precede The Origin Of Life?
the author writes :
The observed chemical reactions occurred in the absence of enzymes but were made possible by the chemical molecules found in the Archean sea. Finding a series of reactions that resembles the “core of cellular metabolism” suggests that metabolism predates the origin of life. This implies that, at least initially, metabolism may not have been shaped by evolution but by molecules like RNA formed through the chemical conditions that prevailed in the earliest oceans.
Whether and how the first enzymes adopted the metal-catalyzed reactions described by the scientists remain to be established.
Its easily observable the hudge gap between the just so, almost helpless explanation attempts of the origin and arise of essential metabolic pathways, and their complexity observed even in the simplest cells.
This made the leading Origin of Life researcher Leslie Orgel say following:
The Implausibility of Metabolic Cycles on the Prebiotic Earth
Leslie E Orgel†
Almost all proposals of hypothetical metabolic cycles have recognized that each of the steps involved must occur rapidly enough for the cycle to be useful in the time available for its operation. It is always assumed that this condition is met, but in no case have persuasive supporting arguments been presented. Why should one believe that an ensemble of minerals that are capable of catalyzing each of the many steps of the reverse citric acid cycle was present anywhere on the primitive Earth, or that the cycle mysteriously organized itself topographically on a metal sulfide surface? The lack of a supporting background in chemistry is even more evident in proposals that metabolic cycles can evolve to “life-like" complexity. The most serious challenge to proponents of metabolic cycle theories—the problems presented by the lack of specificity of most nonenzymatic catalysts—has, in general, not been appreciated. If it has, it has been ignored. Theories of the origin of life based on metabolic cycles cannot be justified by the inadequacy of competing theories: they must stand on their own.