Polymerization of Monomers to Give Macromolecules

Polymerization of monomers to give biological macromolecules usually requires the removal of H2O. Clearly, water is in excess in the oceans and removal of H2O from dissolved molecules is therefore unfavorable. Consequently, the assembly of macromol-ecules such as proteins and nucleic acids needs energy to form the linkages and/or remove the water. Before the high-energy phosphates used in modern cells were available, some other form of energy was needed.

Imitation protein polymers, containing randomly linked amino acids, are known as "proteinoids." They can be formed by heating dry amino acid mixtures at around 150°C for a few hours (Fig. 20.03A).Whereas biological proteins are bonded using only the a-NH2 and a-COOH groups of amino acids these "primeval polypeptides" contain substantial numbers of bonds involving side chain residues. They contain up to 250 amino acids and can sometimes perform primitive enzymatic activities. Such dry heat could have occurred near volcanoes or when pools left behind by a changing coastline evaporated. Much of the early work on proteinoids was done by Sydney Fox who proposed their thermal origin. However, another way to randomly polymerize amino acids is by using clay minerals with special binding properties (Fig. 20.03B). Binding of small molecules to the surface of catalytic minerals can promote many reactions. For example, certain clays, such as Montmorillonite, will condense amino acids to form polypeptides up to 200 residues long.

Polymerization of amino acids may have also occurred in solution, but another component, a condensing agent, is required to withdraw water. Several possible primeval condensing agents have been proposed, including reactive cyanide derivatives and, more biologically relevant, polyphosphates. Inorganic polyphosphates would have been present in primeval times (formed by volcanic heat from phosphates for example). Polyphosphates can react with many organic molecules to give organic phosphates. Amino acids give two possible products (Fig. 20.04). Acyl phosphates have the phosphate group attached to the carboxyl group of the amino acid (NH2CHRCO-OPO3H2) and phosphoramidates have the phosphate attached to the amino group of the amino acid (H2O3P-NH-CHR-COOH). Gentle heating or irradiation such derivatives will give polypeptides. Modern life uses acyl phosphate derivatives during protein acyl phosphate Phosphate derivative in which the phosphate is attached to a carboxyl group phosphoramidate Phosphate derivative in which the phosphate group is attached to an amino group polyphosphate Compound consisting of multiple phosphate groups linked by high energy phosphate bonds proteinoid Artificially synthesized polypeptide containing randomly linked amino acids

FIGURE 20.03 Formation of Proteinoids by Mild Heat or Clay Catalysis

(A) A mixture of separate amino acids will form artificial polypeptide chains or "proteinoids" when subjected to heat in the absence of water for a few hours. (B) Amino acids can also form bonds when they bind to certain types of clay. The clay has binding sites for amino acids in close proximity, therefore, once bound the amino acids condense into a proteinoid.

FIGURE 20.03 Formation of Proteinoids by Mild Heat or Clay Catalysis

(A) A mixture of separate amino acids will form artificial polypeptide chains or "proteinoids" when subjected to heat in the absence of water for a few hours. (B) Amino acids can also form bonds when they bind to certain types of clay. The clay has binding sites for amino acids in close proximity, therefore, once bound the amino acids condense into a proteinoid.

FIGURE 20.04 Formation of Acyl Phosphates and Phosphoramidates

Condensing single amino acids into polypeptide chains could have occurred in solution as long as a condensing agent was present. One possible condensing agent was a polyphosphate that would react with the amino group or carboxyl group of individual amino acids. The two possible products, phosphoramidates and acyl phosphates, can form polypeptide chains by heating in solution.

Amino acid r

Amino group

Amino group

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