Deoxyribozymes or DNA enzymes or catalytic DNA, or DNAzymes are DNA molecules that have the ability to perform a chemical reaction, such as catalytic action. In contrast to the RNA ribozymes, which have many catalytic capabilities, in nature DNA is only associated with gene replication and nothing else. The reasons are that DNA lacks the 2′-hydroxyl group of RNA, which diminishes its chemical reactivity and its ability to form complex tertiary structures, and that nearly all biological DNA exists in the double helix conformation in which potential catalytic sites are shielded. In comparison to proteins built up from 20 different monomers both RNA and DNA have a much more restricted set of monomers (4) to choose from which limits the construction of interesting catalytic sites. For these reasons DNAzymes exist only in the laboratory.
With the aid of combinatorial chemistry techniques a great many DNA sequences (up to 1016 of them) can be generated in a single experiment with 20 to 200 base pairs each, that can be screened for a specific catalytic task. In this way the sheer number of DNA candidates make up for DNA being more appropriate for information storage than for catalysis. An inherent disadvantage of DNA enzymes is product inhibition and single-turnover behavior. It may therefore be argued if DNA enzymes can be counted as true catalysts. On the other hand low catalytic turnover is observed with many natural (non-DNA) occurring enzymes. Although the discovery of RNA enzymes predates that of DNA enzymes the latter have some distinct advantages. DNA has better cost-effectiveness and DNA can be made with longer sequence length and can be made with higher purity in Solid-phase synthesis.
Chirality is another property that a DNAzyme can exploit. DNA occurs in nature as a right-handed double helix and in asymmetric synthesis a chiral catalyst is a valuable tool in the synthesis of chiral molecules from an achiral source. In one application an artificial DNA catalyst was prepared by attaching a copper ion to it through a spacer. The copper – DNA complex catalysed a Diels-Alder reaction in water between cyclopentadiene and an aza chalcone. The reaction products (endo and exo) were found to be present in an enantiomeric excess of 50%. Later it was found that an enantiomeric excess of 99% could be induced, and that both the rate and the enantioselectivity were related to the DNA sequence.