Origin of Life: chemical cycles (Introduction)

by David Turell , Thursday, December 27, 2012, 14:57 (4350 days ago) @ David Turell

Inorganic chemical cycles are rare. Organic molecular cycles are common. But how do you get from inorganic Earth to organic chemistry cycles to start life? No one knows. Reason for cycles: they continuously work. Life is continuity. Leslie Orgel, one of the leading OOL researchers concluded below: (I particularly like his if pigs could fly comment about most of the OOL proposals!)-The demonstration of the existence of a complex, nonenzymatic metabolic cycle, such as the reverse citric acid, would be a major step in research on the origin of life, while demonstration of an evolving family of such cycles would transform the subject. In view of the importance of the topic, it is essential to subject metabolist proposals to the same kind of detailed examination and criticism that has rightly been applied to genetic theories [29,30]. In the case of these latter theories, an appraisal of their plausibility can be based on a substantial body of experimental work. In the case of the former, because little experimental work has been attempted, appraisal must be based on chemical plausibility.
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 [8], or that the cycle mysteriously organized itself topographically on a metal sulfide surface [6]? 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.
The situation with respect to chemical cycles unrelated to those involved in contemporary metabolism is different. At least one well-established autocatalytic cycle, the core of the formose reaction, is understood reasonably well [1,18] and, as discussed previously, there is experimental support for the existence of one or two other simple cycles [2,3]. This suggests that there may be more cycles to be discovered, and they could be relevant to the origin of life. The recognition of sequences of plausible reactions that could close a cycle is an essential first step toward the discovery of new cycles, but experimental proof that such cycles are stable against the challenge of side reactions is even more important.
Proposals involving complex metabolisms that are stable even in the absence of informational polymers usually are linked to the context of hydrothermal synthesis in the deep sea vents or some equivalent environment. Such linkage, however, need not be an essential feature of these theories [31]. A metabolist theory based on the self-organization of the Calvin cycle, for example, would be a logical possibility, although not necessarily an attractive one. Conversely, a theory in which metal sulfide...catalyzed reactions provided some or all of the organic molecules needed for the formation of a primitive genetic system would have many attractive features. A number of prebiotic syntheses catalyzed by transition metal sulfides under hydrothermal conditions have already been reported [16,17], and this is now an active area of research. It is important to realize that recognition of the possible importance of prebiotic syntheses that could occur hydrothermally does not necessitate a belief in their ability to self-organize.
The prebiotic syntheses that have been investigated experimentally almost always lead to the formation of complex mixtures. Proposed polymer replication schemes are unlikely to succeed except with reasonably pure input monomers. No solution of the origin-of-life problem will be possible until the gap between the two kinds of chemistry is closed. Simplification of product mixtures through the self-organization of organic reaction sequences, whether cyclic or not, would help enormously, as would the discovery of very simple replicating polymers. However, solutions offered by supporters of geneticist or metabolist scenarios that are dependent on "if pigs could fly" hypothetical chemistry are unlikely to help. _
http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0060018