Amino acyladenylates

Polymerization of amino acids in aqueous solution has, in general, only been partially successful. An exception is the remarkable reaction of amino-acyladenylates in the presence of a suspension of montmorillonite to produce chains of up to hfty amino acid residues in length (Paecht-Horowitz, 1974). By contrast, in the absence of the clay mineral, the aminoacyladeny-lates hydrolyze rapidly and produce, at most, short chains of four or five amino acids. The mechanism of this reaction is unclear and the possible origin of these highly unstable compounds on the primitive Earth has not been demonstrated directly. Possibly such activated eunino acids could be formed under evaporating conditions from the free amino acids in the presence of adenosine cyclic-2, 3 -phosphate (Lohrmann and Orgel, 1973). 

Figure 21 Two step activation of an amino acid substrate in non-ribosomal biosynthesis as aminoacyl adenylate and enzyme thioester. Both steps are catalysed by the same enzyme which catalyses amino acyladenylate formation (top) and covalently hinds its amino acid substrate as thioester (bottom)...

Synthesis of aminoacyl-tRNA (HI) from amino acids (I) requires activation of the amino acid carboxyl group with formation of an intermediate enzyme-bound amino-acyladenylate (II). 

Another motif recurs in this activation reaction. The enzyme-bound acyladenylate intermediate is not unique to the synthesis of acyl CuA. Acyl adenylates are frequently formed when carboxyl groups are acti vated in biochemical reactions. Amino acids are activated for protein synthesis by a similar mechanism (p. 862), although the enzymes that catalyze this process are not homologous to acyl CoA synthetase. Thus, activation by adenylation recurs in part because of convergent evolution. 

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