Metabolism primitive

Dyson s model has been the subject of careful criticism as well as well-meaning agreement. Shneior Lifson (1997) found fault in particular with Dyson s assumption that metabolism (and other properties) could have developed without natural selection. In his third assumption, Dyson postulates that There is no Darwinian selection. Evolution of a molecule population occurs via genetic drift (Dyson, 1999). Lifson (1997) points out that, while Dyson stresses the role of primitive metabolism, its adaptability, error tolerance etc., he himself considers that such properties can only evolve via natural selection. 

There are many potential molecules and possible routes to the synthesis of biomolecules that might form the basis of a primitive metabolism but thus far we have not addressed the question of information propagation or Darwinian evolution. Information storage must be contained within a sequence, such as words in a sentence or the base sequences within the genetic code, and that requires a polymerisation reaction, which is preferably autocatalytic to reproduce the information accurately. Peptides and nucleotides have this property, although the condensation reaction joining them together needs to be activated. 

The encapsulation results in a chance collection of molecules that then form an autocatalytic cycle and a primitive metabolism but intrinsically only an isolated system of chemical reactions. There is no requirement for the reactions to reach equilibrium because they are no longer under standard conditions and the extent of reaction, f, will be composition limited (Section 8.2). Suddenly, a protocell looks promising but the encapsulation process poses lots of questions. How many molecules are required to form an organism How big does the micelle or liposome have to be How are molecules transported from outside to inside Can the system replicate Consider a simple spherical protocell of diameter 100 nm with an enclosed volume of a mere 125 fL. There is room within the cell for something like 5 billion molecules, assuming that they all have a density similar to that of water. This is a surprisingly small number and is a reasonable first guess for the number of molecules within a bacterium. 

The basic working idea of compartmentaUsm is that these primitive shells have encapsulated some simple peptide catalysts together with other molecules, and that a primitive protocell metabolism may have started in this way. However, the question of how the primitive metabolism really started is still unanswered - and in particular how that particular metabolism could have started, that led the way to... 

In addition, as we will learn in the next two chapters, there is still something important missing. In fact, these views refer mostly to the world of low-molecular-weight compounds, namely the bricks for making the house. However, you can have all the low molecular weight compounds in the world, made by hydrothermal vents or by pyrite or by clay or by primitive metabolisms - and you do not make life with that. To make the house, you need at least the macromolecular specific sequences of enzymes and nucleic acids. This leads us nicely into the next two chapters. 

The search for RNAs with new catalytic functions has been aided by the development of a method that rapidly searches pools of random polymers of RNA and extracts those with particular activities SELEX is nothing less than accelerated evolution in a test tube (Box 26-3). It has been used to generate RNA molecules that bind to amino acids, organic dyes, nucleotides, cyano-cobalamin, and other molecules. Researchers have isolated ribozymes that catalyze ester and amide bond formation, Sn2 reactions, metallation of (addition of metal ions to) porphyrins, and carbon-carbon bond formation. The evolution of enzymatic cofactors with nucleotide handles that facilitate their binding to ribozymes might have further expanded the repertoire of chemical processes available to primitive metabolic systems. 

Semiconductors are considered to be catalytic particles that contributed to the development of primitive metabolism. According to Wahtershauser life could have developed on the surface of iron sulphide minerals (eg mackinawite or pyrrhotite [FeS] or pyrite [FeS2]) [8], The chemoautotrophy theory is based on the reaction between iron sulphide and hydrogen sulphide, which acts as a reducing agent, whereas iron sulphide provides adsorption sites for substrates and acts as a catalyst (equation 10.1) ... 

The first scientific theories on the origin of life were proposed by Alexander Oparin in 1924 and by J.B.S. Haldane in 1929. Oparin discovered that a solution of proteins can spontaneously produce microscopic aggregates - which he called coacervates - that are capable of a weak metabolism, and proposed that the first cells came into being by the evolution of primitive metabolic coacervates. Haldane, on the other hand, was highly impressed by the replication properties of viruses, and attributed the origin of life to the evolution of viruslike molecular replicators. 

A primitive metabolic system had to have a certain initial complexity to start with it cannot contain fewer than 10 000 monomers for its molecules, and the monomers must be of at least ten different types (which means that amino acids are in but nucleic acids are out). 

This tells us that chemical evolution was really different from postchemical evolution. In the course of chemical evolution, the jump of primitive metabolic systems from chaos to order was only a question of statistical probability and energy conditions. During postchemical evolution, instead, a new type of antichaos appeared, an order that was based on conventional rules of correspondence between two independent molecular worlds, and it was from these first natural conventions that the genetic code finally emerged. 

Glycolysis is a process that results in the conversion of a molecule of glucose into two molecules of pyruvate. It is a primitive metabolic pathway since it operates in even the simplest cells and does not require oxygen. The pathway of glycolysis performs five main functions in the cell ... 

Another possibihty is that the first membranes arose from phospholipid-like molecules produced by primitive metabolism. Phosphate, with or without additional components, could have provided the hydrophilic heads of the molecules. As to their hydrophobic tails, the French chemist Guy Ourisson has built a strong case in favor of polyisoprenoid molecules, the building blocks of ether lipids, as opposed to long-chain fatty acids, the main constituents of ester lipids (see Ourisson and Nakatani, 1994 Chapter 19, this volume). In nature, isoprenoid chains arise from isopentenyl pyrophosphate, a relatively simple precursor. 

Figure 9. The hypothetical "hydrophobic start" in the origin of life. The hydrophobic, spontaneously formed vesicles can undergo self-reproduction if they bind the corresponding precursor they can scavenge hydrophobic peptides and condense them into longer chain once a hydrophobic condensing agent is also present and they can also bind water-soluble peptide catalyst (or any other potential hydrophobic catalyst) and induce an enzyme-like turnover. The catalyst can eventually be internalized, thus giving rise to a protocellular structure capable of a primitive metabolism.

The FeS world is a still incomplete model for building up a primitive metabolism and other pathways to enlarge the repertoire of biogenic compounds. Although limited because of the missing enantiomeric amino acids, any FeS scenario is open-ended... 

Ribozymes could have played a key role in the early stages of evolution of life on earth, bridging the gap between a primitive metabolism and a crude translational apparatus where the RNA acted as both an informational molecule and a catalyst. 

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