Metal Complex Formation Non-redox Systems

One of the most important aspects of the interaction between metals and polynucleotides is that which leads to compactness of structure in the polynucleotides. As polyions, they exhibit structures in solution which are strongly dependent on the concentration and valence of the cations (typically, the compact native structures are favoured by high salt concentrations and particularly by bivalent ions), but even with the transfer RNAs (among the most widely studied nucleic acids and despite analysis of the X-ray structures) the role and location of bound bivalent cations are uncertain. Leroy et al. have used various physical techniques to explore the structure of the central region in a couple of tRNAs from E. coli and thereby obtain evidence on the binding of Mn + and other metal ions. Their interpretation is that simple manganese-phosphate binding is supplemented by electrostatic interaction with distant phosphates. 

In contrast to Mg + and Mn +, which stabilize secondary structures in DNA and RNA, Cu + destabilizes DNA and RNA double helices, and this is attributed to the ability of copper to bind to the nucleic acid bases. Chao and Kearns have recently explored the possibility that this binding, as detected by electron and nuclear magnetic resonance spectroscopy, might be used to probe certain structural features of nucleic acid molecules, such as the looped out regions of tRNAs. The nature of the Cu complexes formed with nucleosides and nucleotides varies with the specific nucleic acid derivatives used and also the pH. Thus, in the pH range 8.5—10.0, copper forms a water-soluble complex with the ribose OH groups of the ribonu-cleosides and 5 -ribonucleotides, but these complexes cannot form with any of the deoxynucleosides or the 2 - and 3 -ribonucleotides. It is suggested that copper(ii) could stabilize unusual polynucleotide structures or interactions in certain enzymatic systems the latter could, for example, be responsible for translational errors in the RNA,DNA polymerase system which are known to be induced by transition metals. 

Thel 1 complexes ofCo +,Ni +,Mg +, and Ca +with ATP have been investigated by n.m.r. By varying the pD so as to have the adenine ring either protonated or unprotonated, and the phosphate chain triply or fully ionized, it was confirmed that the metal ions bind predominantly through the phosphates, and that ring protona- 

It is generally accepted that the bivalent activating metal ion does not have a bridging role in reactions of creatine kinase thus, the M +-nucleotide complex is a substrate but there is minimal interaction between and the enzyme. Gabriel and 

Davis have used the c.d. of the cobalt(ii)-activated enzyme to investigate the local environment of the metal at the active site of the rabbit muscle enzyme. It appears that the metal-nucleotide interaction remains essentially the same throughout the enzyme turnover as that found in the free complex. E.s.r. evidence has been provided for a similar non-bridging role for the metal in reactions of ATP phospho-ribosyltransferase from E. coli. 

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