Polyamino acids, thermal

Polyacetylene, vibrational spectra, 42 196 Poly acids, defined, 41 117 Polyalkenamers,24 134,135 Polyamino acids, thermal, 20 374-377 catalysis by, 20 379... 

Honda, Y., Yamanashi, H., Imahori, K., Yuasa, S. Affinity of adenylate derivatives for thermal Polyamino acids, in Origin of Life Proceedings of the Second ISSOL Meeting, the Fifth ICOL Meeting (ed.) Noda, H.,p. 315, Japan, Bus. Cent. Acad. Soc. Japan 1978... 

V. Summary of Catalytic Actions of Thermal Polyamino Acids. 409... 

VI. Significances of the Catalytic Activity of Thermal Polyamino Acids 410... 

Thermal polyamino acids can be classifled for convenience into three large groups, based on the number of types of monomeric residue. These are homopolymers, oligotonic, and heterotonic polymers. Within the latter two variable classes, composition can be controlled between wide limits. 

Homopolymers have been briefly discussed above. Polymers of a few components represent a second loosely defined class of thermal polyamino acids. Although individual neutral amino acids do not homo-polymerize when heated (glycine excepted), they copolymerize with certain non-neutral amino acids. Lysine, aspartic acid, or glutamic acid have commonly been used for cocondensation (7). Glutamic acid, which alone forms pyroglutamic acid on heating and does not homopolymerize, is copolymerized with many neutral amino acids 13). Combinations of... 

The third class of thermal polyamino acid includes those polymers that contain some proportion of each of the 18 amino acids common to protein. These polymers are referred to as proteinoids, a term that indicates a similarity to protein, yet disavows an identity (5). Proteinoids are in turn of three principal types, depending upon the dominant charge. 

Lysine-rich proteinoids, prepared from reactants which are high in lysine content, represent a second type of heterotonic thermal polyamino acid (23). The structure of these has been less extensively studied than that of the acidic proteinoids, but they have been used in several studies of catalytic activity (16-18). Lysine-rich proteinoids are distinguished by high molecular weights, e.g., 100,000, and by more than 40% of lysine (23). One limitation of this type of proteinoid is the fact that hydrolysis seldom affords recovery of more than 60% of the amino acids. The presence of atypical linkages is thereby suggested. 

These polymers are also discussed as models for prebiotic protein this relationship will be elaborated later (p. 411). In evaluating the examples of catalysis by thermal polyamino acids to be presented, one can develop a more rigorous set of criteria by using the multiple perspectives of chemistry, enzymology, and evolution. 

Various definitions of the term catalyst can be categorized into those that require the regeneration of the promoting species, and those that do not. Usage of definitions that fall into either category can be found [e.g., Leuthardt (27) Laidler (28)]. Some of the actions of thermal polyamino acids have been proven, by various criteria, to be catalytic in the... 

The level of activity of thermal polyamino acids has been expressed in several ways. Frequently the activity per unit weight of polymer is used. In at least one case, apparent second-order rate constants are reported. Additionally, comparisons have been made with the equivalent amount of free amino acid(s) or key cofactor, with other types of polyamino acid, or with enzymes. Bough interlaboratory comparisons have sometimes been made rigorous comparison between laboratories is often difficult because of differences in conditions employed. 

At the present time, the types of reaction known to be accelerated by thermal polyamino acids fall into three main categories. These are hydrolyses, decarboxylations, and aminations. Several examples are available for each of these classes. A related deamination reaction provides one example for a fourth category. Photopromoted decarboxylations have also been observed. 

The first reaction reported as catalyzed by thermal polyamino acids was the hydrolysis of p-nitrophenyl acetate (NPA). Such hydrolysis of this substrate has been studied in three laboratories (29-31). The most extensive of these investigations (16, 29) focused attention upon the histidine residues in proteinoids and on the relationship of such contents to activity. 

Inhibition and Reactivation of Hydrolytis of NPA by Thermal Polyamino Acid"... 

The action of proteinoids on glucose has been studied by Fox and Krampitz (46). Although the action was very weak, as well as complex, the results stimulated a series of subsequent investigations with naturally occurring substrates. The action of thermal polyamino acids is more powerful on these other substrates. 

Krampitz and Hardebeck (48) and Hardebeck et al. (19) have found that thermal polyamino acids accelerate the decarboxylation of pyruvic acid. Carbon dioxide (from C-1 of p5n uvate) and acetic acid (from C-2 and C-3 of pyruvate) were the main products. Small amounts of acetaldehyde and acetoin were also detected. The finding of acetic acid as a main product indicates, as the authors pointed out, that an oxidation as well as a decarboxylation must occur. The process is thus somewhat similar to that observed by Fox and Krampitz (46) with... 

AetivUiea of Thermal Polyamino Acids on Oxalacetio Acid ... 

To date, four main types of catalytic activity have been reported in detail for thermal polyamino acids. These are (with the most studied substrates in parentheses) hydrolyses (p-nitrophenyl acetate, p-nitro-phenyl phosphate, ATP), decarboxylations (OAA, glucuronic acid, pyruvic acid), and aminations (a-ketoglutaric acid, OAA, pyruvic acid, phenylpyruvic acid). The fourth type is a deamination reaction yielding a-ketoglutaric acid (51). For some of the actions of the thermal polymers the products are identified quantitatively, and the kinds of amino acid side chain necessary for activity in the polymer elucidated. In others, products have yet to be fully identified. The activities of thermal polyamino acids are manifest on substrates which range from chemically labile to relatively stable. 

The thermal polymers are incapable of mimicing a peptide or protein to the exact extent that the product of a stepwise synthesis does. An exact duplication of a functional protein, however, does little to elucidate the reason for its activity modification and study of the effect on activity are necessary. Systematic synthetic modification of polymeric models is easily achieved in the case of thermal polyamino acids. They are prepared with much ease and in large numbers, and their quantitative compositions can be regulated and controlled simply. Examples are already at hand to illustrate the use of the thermal method to evaluate qualitatively the kinds of amino acid residue that are necessary for, contributory to, or detrimental to, activity. Such studies augment information from enzymes and from nonthermal models. 

The presence of D-amino acids and the occurrence of non-a-peptide linkages are structural features by which thermal polyamino acids may deviate more from typical protein than do, for example, Leuchs polymers or other models. The extent to which these features occur has not been thoroughly evaluated. However, limited testing (7) has shown that the proportion of non-a-peptide bonds can be controlled to some degree, and other experiments (7) have revealed that some amino acids are relatively resistant to racemization during thermal polymerization. 

Since several kinds of reaction have been shown to be catalyzed by thermal polyamino acids, the discovery of other kinds of reaction seems likely to occur on further examination. Those reactions already studied include hydrolysis, decarboxylation, amination, and one instance of deamination. Assemblage of some of the reactions catalyzed reflect metabolic sequences, as laid out in a flow sheet in the section on pathways (Section IV, E). Within these sequences, some specificity in reaction between proteinoid and substrate can be observed. As stated before, some activities of thermal polymers have been shown to be influenced by, or to be dependent on, cofactors. 

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