Protocell

Szathmary E, Santos M, Fernando C (2005) Evolutionary Potential and Requirements for Minimal Protocells. 259 167-211... 

A second historical model for protocells is provided by the microspheres (Fox, 1980 Nakashima, 1987 Lehninger, 1975). These are formed when hot saturated proteinoid solutions are allowed to cool (see Sect. 5.4.2). In recent years, the microspheres were also consigned to the limbo of unimportant scientific models. Perhaps there will come a time when coacervates or microspheres (in their original or in modified forms) find their way back into the scientific discussion. 

After more than 20 years, Walde et al. (1994) returned in a way to coacervate experiments, although using other methods. Walde (from the Luisi group) repeated nucleotide polymerisation of ADP to give polyadenylic acid, catalysed by polynucleotide phosphorylase (PNPase). But instead of Oparin s coacervates, the Zurich group used micelles and self-forming vesicles. They were able to demonstrate that enzyme-catalysed reactions can take place in these molecular structures, which can thus serve as protocell models. Two different supramolecular systems were used ... 

The favourable properties which mark out vesicles as protocell models were confirmed by computer simulation (Pohorill and Wilson, 1995). These researchers studied the molecular dynamics of simple membrane/water boundary layers the bilayer surface fluctuated in time and space. The model membrane consisted of glycerine-1-monooleate defects were present which allowed ion transport to occur, whereby negative ions passed through the bilayer more easily than positive ions. The membrane-water boundary layer should be particularly suited to reactions which are accelerated by heterogeneous catalysis. Thus, the authors believe that these vesicles fulfil almost all the conditions required for the first protocells on earth ... 

The formation of relatively stable vesicles did not require the presence of pure compounds mixtures of components could also have done the job. However, whether the concentrations of the compounds isolated from the Murchison meteorite would have been sufficient for the formation of prebiotic protocells or vesicles is unclear, even if concentration effects are assumed. Sequences in which the technical Fischer-Tropsch synthesis is the role model have been proposed as possible sources of amphiphilic building blocks. 

Attempts have recently been made to link the RNA world with the lipid world. Two groups involved in RNA and ribozyme research joined up with an expert on membrane biophysics (Szostak et al., 2001). They developed a model for the formation of the first protocells which takes into account both the most recent experimental results on replication systems and the self-organisation processes of amphiphilic substances to give supramolecular structures. 

Szostak et al. worked on the basis of a simple cellular system which can replicate itself autonomously and which is subject to Darwinian evolution. This simple protocell consists of an RNA replicase, which replicates in a self-replicating vesicle. If this system can take up small molecules from its environment (a type of feeding ), i.e., precursors which are required for membrane construction and RNA synthesis, the protocells will grow and divide. The result should be the formation of improved replicases. Improved chances of survival are only likely if a sequence, coded by RNA, leads to better growth or replication of membrane components, e.g., by means of a ribozyme which catalyses the synthesis of amphiphilic lipids (Figs. 10.8 and 10.9). We can expect further important advances in the near future from this combination ( RNA + lipid world ). 

Fig. 10.9 Possible reaction pathway for the formation of a cell. The important precursors are an RNA replicase and a self-replicating vesicle. The combination of these two in a protocell leads to a rapid, evolutionary optimisation of the replicase. The cellular structure is completed if an RNA-coded molecular species, for example, a lipid-synthesised ribozyme, is added to the system (Szostak et al., 2001)... 

D. W. Deamer and J. P. Dworkin have reported in detail on the contribution of chemistry and physics to the formation of the first primitive membranes during the emergence of precursors to life the authors discussion ranges from sources of amphiphilic compounds, growth processes in protocells, self-organisation mechanisms in mixtures of prebiotic organic compounds (e.g., from extracts of the Murchison meteorite) all the way to model systems for primitive cells (Deamer and Dworkin, 2005). 

In recent years, the transitions from nonliving to living matter have been the subject of three seminars, bringing together theoreticians and experimentalists in the Los Alamos National Laboratory, in the Santa Fe Institute and in Dortmund. The biogenesis problem was expanded to the question, how can simple life forms be synthesised in the laboratory Artificial cells (sometimes called protocells) could be quite different from the cell types known today, or from primeval cells they might, for example, be orders of magnitude smaller than a bacterium. The seminars posed three questions for further work ... 

Local flows inside confined (protocell) volumes pumps... 

Life started as a single compartment, a protocell or cell, of selected reduced chemicals and a restricted number of internal ions using external energy, probably from mineral sources (see Chapter 1 to 4). 

A representation of all of the processes can be seen in Figure 9.2 and we must now turn attention to the formation of molecular aggregates on a larger scale encapsulation and all of the problems associated with maintaining the environment within the encapsulated protocell, pointing towards metabolism and information propagation. 

All of these make the formation of the first protocell rather critical. There is also the general problem of membrane transport and how molecules might form within the protocell. The simplest postulated mechanism for the initial inclusion of molecules in protcells is that of encapsulation. 

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 isolation of the chemicals within the protocell causes a specific problem. Once the reactions have reached the extent of equilibrium allowed, limited by composition, all chemistry stops until the protocell dries out or brings in new molecules. The drying mechanism could certainly replenish the metabolism, requiring the protocell to dry out in a localised region on a substrate and re-form. The protocell would gain some new molecules and lose some old molecules in the process. The cell metabolism chemistry will only continue away from the surface and in a fully independent way when the membrane transport problem is solved. 

Once the molecules have been captured inside the protocell, the concept of a concentration gradient and semipermeable membrane becomes important. Near a negatively charged mineral surface, such as silica, the surface concentration... 

The first equation simply states the balance in chemical potentials inside and outside of the cell. The expression for the chemical potential inside the protocell separates into a term involving the mole fraction and the chemical potential associated with the pressure difference. The work done by the cell in opposing the pressure change, assuming that the cell remains at constant volume, is given below, where the change in pressure is from p to p + tv. 

The mole fraction of water is much larger than that of sucrose and practically for all cell components, so van t Hoffs equation is easily justified for estimates of cellular osmotic pressure. Given R = 0.08314 bar L 1 mol-1 and a protocell temperature of 298 K, calculate the osmotic pressure. 

The osmotic pressure is more than twice the atmospheric pressure and represents a significant challenge for the forming protocell. 

Consider a concentration gradient for NaCl with an internal protocell concentration of 150 mM and an external concentration of order 1 mM, as might be found in a freshwater environment. There is a membrane potential of —200 mV. 

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