Tan, Change; Stadler, Rob. The Stairway To Life:
In all living systems, homochirality is produced and maintained by enzymes, which are themselves composed of homochiral amino acids that were specified through homochiral DNA and produced via homochiral messenger RNA, homochiral ribosomal RNA, and homochiral transfer RNA. No one has ever found a plausible abiotic explanation for how life could have become exclusively homochiral.
A. G. CAIRNS-SMITH Seven clues to the origin of life, page 40:
There are CONVENTIONS in the universal system, features that could easily have been otherwise. The exact choice of the amino acid alphabet, and the set of assignments of amino acid letters to nucleic acid words - the genetic code - are examples. A particularly clear case is in the universal choice of only 'left-handed' amino acids for making proteins, when, as far as one can see, 'right-handed' ones would have been just as good. Let me clarify this.
Molecules that are at all complex are usually not superposable on their mirror images. There is nothing particularly strange about this: it is true of most objects. Your right hand, for example, is a left hand in the mirror. It is only rather symmetrical objects that do not have 'right-handed' and 'left-handed' versions. When two or more objects have to be fitted together in some way their 'handedness' begins to matter. If it is a left hand it must go with a left glove. If a nut has a right-hand screw, then so must its bolt. In the same sort of way the socket on an enzyme will generally be fussy about the 'handedness' of a molecule that is to fit it. If the socket is 'left-handed' then only the 'left-handed' molecule will do. So there has to be this kind of discrimination in biochemistry, as in human engineering, when 'right-handed' and 'left-handed' objects are being dealt with. And it is perhaps not surprising that the amino acids for proteins should have a uniform 'handedness'. There could be a good reason for that, as there is good reason to stick to only one
'handedness' for nuts and bolts. But whether, in such cases, to choose left or right, that is pure convention. It could be decided by the toss of a coin.
My comment: Conventions depend on decisions. Decisions are made by minds.
A. G. CAIRNS-SMITH: It is one of the most singular features of the unity of biochemistry that this mere convention is universal. Where did such agreement come from? You see non-biological processes do not as a rule show any bias one way or the other, and it has proved particularly difficult to see any realistic way in which any of the constituents of a 'probiotic soup' would have had predominantly 'left-handed' or right-handed' molecules. It is thus particularly difficult to see this feature as having been imposed by initial conditions.
A. G. CAIRNS-SMITH genetic takeover, page53
It is commonly believed that proteins of a sort, or nucleic acids of a sort (or both) would have been necessary for the making of those first systems that could evolve under natural selection and so take off from the launching platform provided by prevital chemical processes. We have already come to a major difficulty here: Much of the point of protein and the whole point of nucleic acid would seem to be lost unless these molecules have appropriate secondary/tertiary structures; and that is only possible with chirally defined units. As we saw, the ‘abiotic‘ way of circumventing this problem (by prevital resolution of enantiomers) seems hopelessly inadequate, and ‘biotic’ mechanisms depend on efficient machinery already in action.
Homochirality Originates from the Handedness of Helices 2020 November 20
Homochirality is a common feature of amino acids and carbohydrates, and its origin is still unknown.
On the possible origin of protein homochirality, structure, and biochemical function December 26, 2019
How L-chiral proteins emerged from demi-chiral mixtures is unknown.The lack of understanding of the origins of the breaking of demi-chirality found in the molecules of life on Earth is a long-standing problem, and models to date either focused on the RNA world hypothesis, which does not explain how RNA became chiral, or the use of chiral templates (e.g., chiral crystal surfaces). The alternative view due to Dyson conjectures that metabolism, likely from proteins, came first, followed by replication. But how did the ultimately homochiral proteins responsible for metabolism emerge from the short peptides that formed spontaneously and probably contained a mixture of D and L amino acids? The foldamer hypothesis suggests that such oligomers acted as templates to catalyze the synthesis of likely demi-chiral proteins. Other mechanisms such as molecular mutualism or the spontaneous peptide formation from aminonitriles might have been operative. By whatever means, we assume that, somehow, proteins, whose lengths range from 50 to 300 residues, were generated.
Principles of chemical geometry underlying chiral selectivity in RNA minihelix aminoacylation 30 November 2018
The origin of homochirality in L-amino acid in proteins is one of the mysteries of the evolution of life. Experimental studies show that a non-enzymatic aminoacylation reaction of an RNA minihelix has a preference for L-amino acid over D-amino acid.
The origin of biological homochirality along with the origin of life January 8, 2020
How homochirality concerning biopolymers (DNA/RNA/proteins) could have originally occurred (i.e., arisen from a non-life chemical world, which tended to be chirality-symmetric) is a long-standing scientific puzzle.
Earthly life has the remarkable property that virtually all proteins are constructed only from left-handed amino acids, whereas the nucleic acids RNA and DNA utilize only righthanded sugars in their structures. Terrestrial organisms cannot utilize right-handed proteins (with a few exceptions) or lefthanded sugars in their biochemical processes; they would starve to death if such wrong-handed materials were the sole food source. Yet, abiotically produced amino acids such as those in meteorites and the Miller–Urey experiments are a roughly equal mixture of left-handed (l) and right-handed (d) molecules (there is one meteorite in which there is a modest excess of left-handed amino acids). Furthermore, chemical production of polymers such as proteins or nucleotides does not prefer a particular handedness when the starting molecules are a mixture of l- and d-enantiomers. 1
Chirality selection on some rocks cannot be cumulated enough through some mysterious filtration process to justify the homochirality in a living organism, not even with highly selective lab procedure.
Dr. Stanley L. Miller, University of California San Diego
The original study raised many questions. What about the even balance of L and D (left and right oriented) amino acids seen in your experiment, unlike the preponderance of L seen in nature? How have you dealt with that question?
All of these pre-biotic experiments yield a racemic mixture, that is, equal amounts of D and L forms of the compounds. Indeed, if you're results are not racemic, you immediately suspect contamination. The question is how did one form get selected. In my opinion, the selection comes close to or slightly after the origin of life. There is no way in my opinion that you are going to sort out the D and L amino acids in separate pools. My opinion or working hypothesis is that the first replicated molecule had effectively no asymmetric carbon. 2
Is an abiologic origin of chirality as is found in (2R)-2,3-dihydroxypropanal (D-glyceraldehyde), and also in amino acids, sugars, etc., possible? 3
The origin of the homochirality of amino acids is still an unsolved issue. There must have been a definite process to ensure that the sequence-based mechanism functioned in the RNA world. Future experiments will provide insights regarding the basis using which this mystery can be solved. 4
Enantiomers are molecules that are mirror-images of each other. Today, amino acids and sugars exist in only one enantiomeric form in most biological systems on earth. This homochirality remains one of the greatest unsolved mysteries to scientists. 5
However, the question of the origin of biological homochirality remains as yet unanswered. 6
left and right-handed molecules of a compound will form in equal amounts (a racemic mixture) when we synthesize them in the laboratory in the absence of some type of directing template.
Several mechanisms have been proposed for elucidating the origins of the chirality of organic compounds, such as circularly polarized light (CPL) (3) and quartz (4); however, a suitable amplification process for chirality is required to reach single-handedness of biological compounds (biological homochirality)
Mirror Image Catalysis Chiral molecules
When alanine is produced in a laboratory under normal conditions, a mixture is obtained, half of which is (S)-alanine and the other (R)-alanine. The synthesis is symmetrical in the sense that it produces equal amounts of both enantiomers.
Now suppose that we want to make a protein that involves 100 amino acids (this would be a short protein – most are at least three times as long). Amino acids exist in two chiral forms that are mirror images of each other, called L and D forms. These two forms appear in equal numbers in prebiotic simulation experiments, so that the probability of getting one or other of the forms is roughly 1/2. However, the great majority of the proteins found in nature contain only the L-form. The probability of getting 100 amino acids of L-form is, therefore, (1/2)100, which is about 1 chance in 10^30.
Nature is chiral
One may well think that both forms of chiral molecules ought to be equally common in nature, the reactions should be symmetrical. But when we study the molecules of the cells in close-up, it is evident that nature mainly uses one of the two enantiomers. That is why we have – and this applies to all living material, both vegetable and animal – amino acids, and therefore peptides, enzymes and other proteins, only of one of the mirror image forms. Carbohydrates and nucleic acids like DNA and RNA are other examples.
Thus the enzymes in our cells are chiral, as are other receptors that play an important part in cell machinery. This means that they prefer to bind to one of the enantiomers. In other words, the receptors are extremely selective; only one of the enantiomers fits the receptor's site like a key that fits a lock. (This metaphor comes from another Nobel Laureate in Chemistry, Emil Fischer, who was awarded the Prize in 1902.)
Since the two enantiomers of a chiral molecule often have totally different effects on cells, it is important to be able to produce each of the two forms pure.
Life only uses the left handed variety. All 20 amino acids that are used in life all exhibit this characteristic except glycine. The left handed and right handed isomers react
chemically the same, and are virtually impossible to separate. Moreover, when death occurs, the left handed isomers over time spontaneous reverse into the right handed
variety, a process called racemization. proceedings from a conference where papers were being presented trying to find a way to synthesize pure left handed amino acids, and all met with failure.
One of the greatest challenges of modern science is to understand the origin of the homochirality of life: why are most essential biological building blocks present in only one handedness, such as L-amino acids and D-sugars? Efforts towards understanding this phenomenon ultimately have to rely on the effect of a chiral, external influence that, at some point during the evolution, has driven certain systems towards specific chiralities. Several possible chiral fields have been suggested, with scenarios based on circularly polarized light in photochemistry, the electroweak interaction, vortex motion and external electric and magnetic fields. However, to date only few of these scenarios have been experimentally demonstrated.
The question of how proteins can pick out only the left-handed ones from among all amino acids, and how not even a single right-handed amino acid gets involved in the life process, is a problem that still baffles evolutionists. Such a specific and conscious selection constitutes one of the greatest impasses facing the theory of evolution.
Enantioselection of supramolecular chirality by external fields
Naturally occurring biological molecules are made of homochiral building blocks. Proteins are composed of L-amino acids (and not D-amino acids);
nucleic acids such as DNA have D-ribose sugars (and not L-ribose sugars). It is not clear why nature selected a particular chirality. Selection could have
occurred by chance or as a consequence of basic physical chemistry.
Explanation of the homochirality of amino acids in the biosphere is one of the most important mysteries in the origin of life
life uses only left-handed amino acids in the construction of proteins
Proteins cannot assemble unless all the chiral amino acids (20 out of the 21 bioactive amino acids are chiral) are either 100 percent left-handed or 100 percent right-handed. Likewise, DNA and RNA molecules cannot assemble unless all pentose sugars are 100 percent left-handed or right-handed. All organisms on Earth manifest only left-handed chiral amino acids and right-handed pentose sugars.
In all living systems the building blocks of the DNA and RNA exist exclusively in the right-handed form, while the amino acids in virtually all proteins in living systems, with very rare exception, occur only in the left-handed form.
The dilemma for materialists is that all "spark and soup-like" experiments produce a mixture of 50% left (levo) and 50% right-handed (dextro) products. Such a mixture of dextro and levo amino acids is called a "racemic mixture." Unfortunately, such mixtures are completely useless for the spontaneous generation of life.
Complex molecules such as DNA and proteins are built by adding one building block at a time onto an ever-growing chain. In a "primordial soup" made up of equal proportions of right and left-handed building blocks, there is an equal probability at each step of adding either a right or left-handed building block. Consequently, it is a mathematical absurdity to propose that only right-handed nucleotides would be added time after time without a single left-handed one being added to a growing DNA molecule. Sooner or later an incorrect, left-handed nucleotide will be added. The same goes for proteins. Every time another amino acid is added to the growing chain of amino acids the chances are virtually certain that both right and left-handed amino acids will be added.
With unguided or undirected chemistry, a primordial ooze consisting of right and left-handed building blocks can only result in the production of DNA and proteins composed of a mixture of right and left-handed building blocks.
This dilemma has enormous implications for the materialistic scenario.30 For a living cell to function properly, it is absolutely necessary for it to contain the correct three-dimensional structure in its DNA and proteins.
This correct three-dimensional structure is in turn dependent upon proteins built from a pure mixture of left-handed amino acids and DNA built from right-handed nucleotides. Consequently, if even one nucleotide or amino acid with the incorrect "handedness" is inserted into a DNA or protein molecule, the three-dimensional structure will be annihilated and it will cease to function normally.
Last edited by Otangelo on Sat Feb 27, 2021 10:08 am; edited 34 times in total