Fluorodeoxy Sugar Analogues of 2′,5′-Oligoadenylates as Probes of Hydrogen Bonding in Enzymes of the 2-5A System

In order to understand further the contribution of the 3′-hydroxyl groups of 2-5A (ppp5′(A2′p)nA) to its interaction with RNase L, we synthesized a series of analogues in which the 3′-hydroxyl moiety was replaced by fluorine to yield 3′-fluoro-3′-deoxyadenosine (AF). These included ppp5′A2′p5′A2′p5′AF, ppp5′A2′p5AF2′p5′A, ppp5′AF2′p5′A2′p5′A, ppp5′AF2′p5′AF2′p5′AF, ppp5′A2′p5′AF2′p5′AF, ppp5′AF2′p5′A2′p5′AF, and the corresponding monophosphates. When these oligomers were evaluated for their ability to activate RNase L from various sources, we found that the replacement of the second from the 5′-terminus adenosine residue of 2-5A with the fluoro analague caused major reductions in activity. We conclude that the hydroxyl group of this second or middle nucleotide residue of 2-5A trimer may act as a hydrogen bond donor to an acceptor group in RNase L and that this hydrogen bond may be key to what we presume may be a critical conformational change required for nuclease activity of RNase L. We have also found that this substitution of fluorine for hydroxyl in the second or penultimate residue of 2-5A trimer results in an oligomer with a 2′,5′-phosphodiesterase sensitivity comparable to that of 2-5A itself. When viewed in terms of earlier experiments, these results suggest that the role of the 3′-OH group of the penultimate nucleotide of 2-5A may be to anchor the substrate to the phosphodiesterase through its action as a hydrogen bond receptor.