The trans-form (two separate strands) of the 17E DNAzyme. Most ribonuclease DNAzymes have a similar form, consisting of a separate enzyme strand (blue
) and substrate strand (black
). Two arms of complementary bases flank the catalytic core (cyan
) on the enzyme strand and the single ribonucleotide (red
) on the substrate strand. The arrow shows the ribonucleotide cleavage site.
The most abundant class of deoxyribozymes are ribonucleases, which catalyze the cleavage of a ribonucleotide phosphodiester bond through a transesterification reaction, forming a 2'3'-cyclic phosphate terminus and a 5'-hydroxyl terminus.
Ribonuclease deoxyribozymes typically undergo selection as long, single-stranded oligonucleotides which contain a single ribonucleotide base to act as the cleavage site. Once sequenced, this single-stranded "cis"-form of the deoxyribozyme can be converted to the two-stranded "trans"-form by separating the substrate domain (containing the ribonucleotide cleavage site) and the enzyme domain (containing the catalytic core) into separate strands which can hybridize through two flanking arms consisting of complementary base pairs.
The first known deoxyribozyme was a ribonuclease, discovered in 1994 by Ronald Breaker while a postdoctoral fellow in the laboratory of Gerald Joyce at the Scripps Research Institute.
This deoxyribozyme, later named GR-5,
catalyzes the Pb2+-dependent cleavage of a single ribonucleotide phosphoester at a rate that is more than 100-fold compared to the uncatalyzed reaction. Subsequently, additional RNA-cleaving deoxyribozymes that incorporate different metal cofactors were developed, including the Mg2+-dependent E2 deoxyribozyme
and the Ca2+-dependent Mg5 deoxyribozyme.
These first deoxyribozymes were unable to catalyze a full RNA substrate strand, but by incorporating the full RNA substrate strand into the selection process, deoxyribozymes which functioned with substrates consisting of either full RNA or full DNA with a single RNA base were both able to be utilized.
The first of these more versatile deoxyribozymes, 8-17 and 10-23, are currently the most widely studied deoxyribozymes. In fact, many subsequently discovered deoxyribozymes were found to contain the same catalytic core motif as 8-17, including the previously discovered Mg5, suggesting that this motif represents the "simplest solution for the RNA cleavage problem".
The 10-23 DNAzyme contains a 15-nucleotide catalytic core that is flanked by two substrate recognition domains. This DNAzyme cleaves complementary RNAs efficiently in a sequence specific manner between an unpaired purine and a paired pyrimidine. DNAzymes targeting AU or GU vs. GC or AC are more effective. Furthermore, the RNA cleavage rates have been shown to increase after the introduction of intercalators or the substitution of deoxyguanine with deoxyinosine at the junction of the catalytic loop. Specifically, the addition of 2’-O-methyl modifications to the catalytic proved to significantly increase the cleavage rate both in vitro and in vivo.
Other notable deoxyribozyme ribonucleases are those that are highly selective for a certain cofactor. Among this group are the metal selective deoxyribozymes such as Pb2+-specific 17E,
and Na+-specific A43. First crystal structure of a DNAzyme was reported in 2016.
Of particular interest are DNA ligases. These molecules have demonstrated remarkable chemoselectivity in RNA branching reactions. Although each repeating unit in a RNA strand owns a free hydroxyl group, the DNA ligase takes just one of them as a branching starting point. An accomplishment unattainable with traditional organic chemistry.
Many other deoxyribozymes have since been developed that catalyze DNA phosphorylation, DNA adenylation, DNA
deglycosylation, porphyrin metalation, thymine dimer photoreversion
and DNA cleavage.