Introduction

Until the early 1980s, the notion of biocatalysis and enzymatic function was commonly associated with proteins. Beginning in 1982, this point of view started to change after the discovery, in the laboratories of Sid Altman and Tom Cech, of RNAs with catalytic function (Kruger et al., 1982; Guerrier-Takada et al., 1983). While the ability of RNA molecules to catalyze hydrolysis and ligation of phospho-diester bonds has become textbook knowledge, the full range of chemical reactions that RNA can accelerate remains to be elucidated.

Like proteins, nucleic acids can form tertiary structures including binding sites and, consequently, catalytic centers. RNA molecules containing the first artificial binding sites against an artificially selected target were presented in 1990, when Tuerk and Gold published the first RNA aptamer, directed against bacteriophage T4 DNA polymerase, which they had obtained by a new combinatorial technique termed SELEX (systematic evolution of ligands by exponential enrichment). Only in hindsight, with the knowledge on catalytic antibodies in mind, does it seem inevitable that biomolecules with binding properties similar to antibodies would eventually prove to be capable of catalysis. Starting from synthetic combinatorial libraries, the labs of Gold, Szostak, and Joyce isolated RNA molecules, so-called aptamers, featuring binding properties similar to antibodies (Beaudry and Joyce, 1990; Ellington and Szostak, 1990; Tuerk and Gold, 1990). This technique eventually even produced RNA with catalytic activities. Neither of these newly found properties were in any way related to the hitherto presumed biological functions of nucleic acids.

SELEX is a very delicate technique, with the success of the experiment strongly depending on the correct experimental approach. In return SELEX experiments have led to nucleic acids that catalyze a broad range of chemical transformations, ranging from cleavage of amide (Dai et al., 1995) or carboxylic ester bonds (Pic-cirilli et al., 1992) to amide-bond- (Wiegand et al., 1997), C-C- and C-S-bond forming reactions (Seelig and Jaschke, 1999; Sengle et al., 2001; Tarasow et al., 1997)

or the catalysis of o-bond rotation involved in isomerization reactions (Prudent et al., 1994).

The history of ribozymes, in the true sense of the word, is connected to the so-called "RNA world" (Gilbert, 1986; Yarus, 1999), a prebiotic stage in the evolution of life, where the majority of catalytic functions were effected by RNA, and proteins did not yet exist (Woese, 2002). The idea of an RNA world alone implies that RNA should be practically omnipotent in the diversity of its catalytic potential, despite having been outperformed by proteins later on.

This catalytical omnipotence necessarily includes stereoselectivity, a property of outstanding interest to the organic chemist. While intuition predicts stereoselec-tivity because of the chirality of RNA monomers, it was not until 2000 that enan-tioselectivity was described as a property of an in vitro selected ribozyme. The ribozyme described catalyzes the Diels-Alder reaction between anthracene and maleimide (Seelig et al., 2000) and is therefore called a Diels-Alderase. Previously, nucleic acids had been of interest to the organic chemist mainly as target molecules for synthesis, sometimes including more or less complicated modifications. Since the discovery of Diels-Alderase other reactions of significance to the organic chemist have been shown to be catalyzed by ribozymes, including for example Michael additions (Sengle et al., 2001) and even redox reactions (Tsukiji et al., 2003). At present, laboratories are looking for ribozymes accelerating yet other important reactions for organic chemistry (including for example aldol reactions and cycloadditions). In the light of the latest developments in the field, we might soon see the situation reversed, with particular nucleic acids used as customized catalysts in complex organic synthesis.

In what follows, we will discuss the accomplishments, requisites and prerequisites, strategies, and possible applications of catalytically active RNAs. Biomedical applications and related issues are discussed elsewhere (Bramlage et al., 1998; Zinnen et al., 2002; Steele et al., 2003; Trang et al., 2004), as are the latest developments in the field of allosteric regulation and conformational rearrangements of ribozymes (Breaker, 2002; Silverman, 2003). The focus of the present discussion will hence be on the significance of ribozymes to the organic chemist.

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