Proteinoid

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 cellular units described in this review are assembled from proteinoids, i.e., thermal copolymers of amino acids. 

The proteinoids, thermal copolyamino acids, represent a spectrum of polymers having compositions and structures like those of natural proteins and peptides. Since they are products of the synthetic laboratory, compositions not known in nature may be produced in abundance, and in sensitively graded products. These materials are referred to as artificial proteins, synthetic proteins, thermal proteins, or proteinoids. The last term is used because of the similarity to proteins. Since 1972, each semiannual 

Aleksander T. Przybylski and Sidney W. Fox Institute for Molecular and Cellular Evolution, University of Miami, 521 Anastasia Avenue, Coral Gables, Florida 33134. 

It is easily possible to include in a single polymer some proportion of each of the 20 amino adds common to modem protein, and to obtain a proteinoid that on hydrolysis gives an amino add analysis typical of proteins. 

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It is probable that other types of linking are present in the proteinoids reactive molecular fragments lead to more complex compounds such as heterocycles (Heinz et al., 1979). Under other conditions (temperatures above 458 K), pteridines and flavines can be detected. The thermal polycondensation of lysine, alanine and glycine (458 K, 5 h) gave a "chromo-protenoid the chromophores identified were flavines (a) and diazoflavines (b) (Heinz and Ried, 1984). 

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). 

S. W. Fox (from 1984, director of the Institute for Molecular and Cellular Evolution of the University of Miami) made the highly controversial suggestion that the amino acid sequences in the proteinoids are not random. Nakashima prepared a thermal polymer from glutamic acid, glycine and tyrosine the analysis showed that two tyrosine-containing tripeptides had been formed pyr-Glu-Gly-Tyr and pyr-Glu-Tyr-Gly (Nakashima et al 1977). The result was confirmed (Hartmann, 1981). A closer examination of the reaction mechanism showed that the formation of these two tripeptides under the reaction conditions used depends on three parameters ... 

Proteinoids have gone out of fashion it does, however, seem possible that this class of substance may return to influence the prebiotic discussion, perhaps in another form. 

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. 

Catabolic and Synthetic Enzyme Activities of Proteinoids and Proteinoid... 

The literature of metabolism in proteinoids and proteinoid microspheres is reviewed and criticized from a biochemical and experimental point of view. Closely related literature is also reviewed in order to understand the function of proteinoids and proteinoid microspheres. Proteinoids or proteinoid microspheres have many activities. Esterolysis, decarboxylation, animation, deamination, and oxido-reduction are catabolic enzyme activities. The formation of ATP, peptides or oligonucleotides is synthetic enzyme activities. Additional activities are hormonal and inhibitory. Selective formation of peptides is an activity of nucleoproteinoid microspheres these are a model for ribosomes. Mechanisms of peptide and oligonucleotide syntheses from amino acids and nucleotide triphosphate by proteinoid microspheres are tentatively proposed as an integrative consequence of reviewing the literature. 

Proteinoids, as a model of primitive abiotic proteins >, are formed by polymerization from protobiologically plausible micromolecules (amino acids) under presumed protobiological conditions. Proteinoids have enzyme-like activities and metabolic qualities. Proteinoid microspheres are the most suitable model for protocells since they do not consist of macromolecules extracted from contemporary organisms. 

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. 

In this context, the literature concerning closely relevant experiments and the understanding of mechanisms of metabolism in proteinoid microspheres are reviewed. 

The proteinoid microspheres (Fig. 1), as simulated protocells, form from virtually all of the known wide variety of thermal copolyamino acids 2). Microspheres are formed if the aqueous or aqueous salt solution of proteinoid is heated and the clear decanted solution is allowed to cool. This self-assembly may also be effected by chilling solutions saturated at room temperature. Sonication at room temperature can be used. 

The shape of the microspheres is usually spheroidal each population tends to be uniform. The sizes tend to fall in the range 0.5-7.0 pm in diameter and the operational factors controlling size of the mierospheres appear to be many kind of proteinoid,... 

When the pH of a suspension of microspheres of acidic proteinoid is raised by 1-2 units, diffusion of material from the interior to the exterior, fission into two particles, and the appearance of a double layer in the boundary are observed 2 Proteinoid microspheres shrink or swell on transfer to hypertonic or to hypotonic solutions respectively. Some experiments show that polysaccharides are retained under conditions in which monosaccharides diffuse out2. Some proteinoid microspheres possess the intrinsic capacity to grow by accretion, to proliferate through budding, and to form junctions 2). The morphology and other characteristics of proteinoid microspheres are altered by the inclusion of other materials such as polynucleotides, lipids or salts. 

Esterolysis, decarboxylation, amination, deamination and oxidoreduction are catabolic enzyme activities of proteinoids and proteinoid microspheres. The formation... 

Acidic proteinoids accelerate the hydrolysis of the unnatural substrate, p-nitrophenyl acetate 7,8). P-Nitrophenyl acetate has been used as a substrate for both natural esterases and esterase models. The imidazole ring of histidine is involved in the active site of a variety of enzymes, including hydrolytic enzymes. Histidine residues of proteinoid play a key role in the hydrolysis, the contribution to activity of residues of lysine and arginine is minor, and no activity is observed for proteinoid containing no basic amino acid 7). 

The proteinoids are inactivated by heating in buffer solution or by treatment with alkali at room temperature, and it is proved that the hydrolysis of cyclic imide bonds, in which aspartic acid residues are initially bound, accompanies the inactivation by heat8). 

Alkali-treated proteinoids containing the 18 common amino acids promote the hydrolysis of the ester bond of p-nitrophenylphosphate 9). In general, the higher the proportion of neutral and basic amino acids proteinoid has, the higher the activity is9>. 

The rate of hydrolysis of p-nitrophenylphosphate by a fraction of neutral proteinoid separated by gel filtration is 3 pmoles/day/mg at 30 °C, while cow milk protein has an activity 15-31 pmoles/day/mg at 37 °C9). 

The characteristics of the proteinoid activity resemble those of natural enzymes in several respects. The activity of the proteinoid is inhibited by typical phosphatase inhibitors such as arsenate or phosphate, the effects of divalent metal ions are similar,... 

O-Methyl fluorescein phosphate is also hydrolysed by the proteinoids 9). 

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