Enzymatische Acyl‐ und Phosphoryltransferreaktionen unter Beteiligung von zwei Metallionen

Effects of (N7)-Coordinated Nickel(II), Copper(II), or Platinum(II) on the Acid-Base Properties of Guanine Derivatives and Other Related Purines[≠]

“…Only these two kinds of complexes form, since the experiments involving metal ions were carried out at a large M 2 :G ratio (see Sections 4.4.1 ± 4.4.3). Consequently, the experimental data of the potentiometric pH titrations could be fully explained by taking into account Equations (1) and (2) as well as the following two complex-forming equilibria (3) and (4), as long as the evaluation of the data was not carried into the pH range where hydroxo-complex formation occurs.…”

“…[5,6] In contrast, the trans isomer, trans-[(NH 3 ) 2 PtCl 2 ], is found to be inactive. [8,9] As far as labile metal ions are concerned, it may be emphasized that for nucleotide systems involved in transphosphorylations there is now much evidence that in simple [10] as well as in enzymatic reactions [3] two (or even more) metal ions are involved. There is further evidence that in reactive intermediates a metal ion ± N7 interaction occurs not only in the metal ion-promoted hydrolysis [10,11] of ATP but also in enzymatic reactions as proposed recently for Zn 2 ; [12] similarly, adenosine N7 nitrogens are important in a Mg 2 -dependent ribozyme.…”

“…The data also indicate that the effects are similar for guanine and hypoxanthine residues, but that they are more pronounced for adenine derivatives because in the latter case a (N7)-bound M 2 affects a (N1)H site; hence, a further charge effect is operative here. The available material does not yet allow certain prediction of the more subtle differences occurring between the cis and trans isomers of Pt 2 complexes, but replacement of, for example, NH 3 in the coordination sphere of Pt 2 with CH 3 NH 2 has an effect. Of course, as one might expect, the effect of (N7)-bound Pt 2 in guanine nucleotide complexes is smaller on the more remote phosphate groups than it is on the closer (N1)H sites.…”

“…Abstract: The effect of Ni 2 , Cu 2 , and cis-a 2 Pt 2 or trans-a 2 Pt 2 (where a NH 3 or CH 3 NH 2 ), if coordinated to the N7 site of guanine residues, on the acid ± base properties of complexes containing guanine derivatives as ligands is considered. The various acidity constants were determined by potentiometric pH titrations.…”

Recognition of single-stranded DNA by nuclease P1: High resolution crystal structures of complexes with substrate analogs

“…6): the scissile phosphate binds close to Zn2 with its free oxygens replacing two water molecules and the base 5Ј to the cleaved bond stacks against Phe61 and forms hydrogen bonding contacts with Asp63. The water molecule bridging Zn1 and Zn3, which like in other binuclear metallohydrolases is presumably present as a hydroxide ion due to the lowering of its pKa by the metal ions, 30,31 acts as the nucleophile attacking the phosphate in-line with the P-O3Ј bond. Asp45, which also serves as a ligand of Zn1, helps to properly orient the hydroxide for attack.…”