Proteinoid cells

ABSTRACT Electrical phenomena in artificial cells are described. The constituent material of the cells, referred to as proteinoid or as thermal protein, have been extensively studied in the context of the origin of life, which led to the finding of excitability as one of the biofunctions. The activities found in proteinoid cells are such as to make them useful models for modern excitable cells as well as for protocells. For example, the proteinoid cells display double membrane, asymmetric permeability, membrane potentials, action potentials, and photoactivity. 

The emergence of proteinoid microspheres as scientific objects is predominantly a history of experiments. In a seemingly logical progression, some scholars have deduced that components of cells as we know them arose first. That is to say, nucleic acids, proteins, and lipids arose and then the surrounding cell and its membrane assembled. The experiments have, however, indicated another sequence in evolution. Proteinoid cells arise easily, quickly, and abundantly. This was learned only by experiment. The further incorporation of this phenomenon into evolutionary theory visualizes the extensive evolution of such protocells into modern cells. 

The only up-to-date suitable experimental model to help answer this question is the proteinoid cell assembled from thermal proteins, or proteinoids. To date, no other experimental protein-related model of the first cell has been shown to have comparable properties. 

In 1973 the first finding of excitability in a proteinoid cell was reported. These data were further expanded.Transmission electron microscope evidence of a double membrane in these cells,selective permeability, and osmotic properties as well as bilayer membranes made of proteinoidsprovide an experimental background to understanding the origin of excitability. 

The photosensitivity of proteinoids appears to explain the energy source for electrical phenomena in the proteinoid cells. 

The data showed that, with the membrane voltage held constant, the current across the membrane displays nonlinear characteristics during stimulation. These findings seem to be crucial in explanation of the phenomenon of electrical behavior of the artificial proteinoid cell, as the negative resistance is a necessary characteristic for the generation of the oscillations observed. 

Similar responses are observed in proteinoid cells (Figures 6 and 7). 

FIGURE 8, Pattern of spontaneous electrical discharges of the proteinoid cell made of copoly (lys,leu,arg,his). 

Proteinoids also have other properties, such as the formation of cell-like structures (microspheres), and they show weak or very weak activities such as decarboxylation (Rohlfing, 1967) or oxidoreduction (Dose and Zaki, 1971). 

A number of studies have been performed in the context of a theory that proteins and polynucleotides were formal in a suspension of proteinoid microspheres and the microspheres could then have evolved to contemporary cells. The experimental results and evolutionary considerations have been summarized in the textbook of Fox and Dose published in 1977 2). This review therefore deals with studies since 1977, although some description of literature before 1977 is reviewed as occasion demands. Since the evolutionary consideration of proteinoids and proteinoid micro-spheres has been discussed in much literature and many books, (e.g. 2, 3), the attention in this paper is focussed on the description of the biochemical and experimental parts of the literature. Inasmuch as protobiological activities of proteinoids in solution are carried into microspheres 2), experiments with proteinoids in solution are not excluded. 

The intrinsic properties of proteinoids suggest a model of the assembly of a self-ordered structure representing the first cell. The more complex syntheses by the microspheres themselves require, in addition to enzymes, the provision of energy donor molecules such as ATP. Both ribosomal and aribosomal synthesis of peptides... 

Proteinoids produced in laboratories can cluster together into droplets that separate, and that may protect their components from degrading influences of the surrounding environment. These droplets are like extremely simple cells, although they can not reproduce. Such droplets are called microspheres. When fats (i.e., lipids) are present, the microspheres that form are even more cell-like. If a mixture of linked amino acids called polypeptides, sugars called polysaccharides, and nucleic acids is shaken, droplets called coacervates will form. All of these kinds of droplets are called protobionts, and they may represent a stage in the genesis of cellular life. 

The microspheres (7) which form easily from proteinoids (67 Fig. 14) lend themselves to the investigation of another facet of enzyme chemistry, the study of distribution and coordination of activity in particulate systems. This kind of exploration is particularly needed for enzymes, inasmuch as their organismic setting is in close relationship in a membrane-controlled unit, the cell. The substrates which are acted upon by enzymes are also governed by rates of diffusion. The role of the cell in metabolic reactions must be manifold, and is largely not yet understood. 

The proteinoid microsphere (7) possesses a sufficient number of properties to permit its study in this context as a model for a primitive cell. In the case of the microsphere, the proteinoid material is itself catalytic. Metabolism is a functional expression of molecular structures, enzyme protein. This emphasis on polymers is clearest in the context of origins. The ways in which particulate structures, i.e., cells, contribute to and control such expression can be studied uniquely in the cell model. 

Morawetz has reviewed the effects of micellar solutions on catalysis and inhibition (p. 341, this volume). While such molecular structures are not capable of providing the versatility in compartmentalization of catalysts that is available from cells, the effects of association within solution may in some cases be significant for proteinoids. 

Of most note, spherules made from proteinoid and lecithin have been recently shown to possess electrical membrane phenomena. Membrane, action, and oscillatory potentials were next recorded in the microspheres made from proteinoid only. While many of the functions of the model protocells are much weaker than the corresponding functions in modern cells, e.g., the catalytic functions, the electrical behavior is quantitatively comparable. Consequently the units are models of modern excitable cells, as well as of excitable protocells, and are treated mainly as such in this chapter. 

It has been found that some proteinoids and cells made of them display the negative resistance of current-voltage characteristics in the first quadrant of coordinates. 

FIGURE 6. Current-induced changes of membrane polarization of the 2 2 1 proteinoid-ledthin cell. 

FIGURE 7. Membrane potential, spontaneous discharges, and oscillations of the 2 2 1 proteinoid-lecithin cell. 

Since the proteinoid materials and results highly resemble those of natural proteins, many applications would be in modeling natural proteins, cells, and membranes in which we may change at will the composition of constituent amino adds and then monitor the behavior of the polymers. Additionally, the quantum chemical calculations of molecular orbitals of such polymers may be performed and the resultant picture may be easier to... 

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