Nucleophilic Substitution at Phosphorus in Ribonuclease

Nucleases, such as ribonuclease (RNase) A or T, catalyze the hydrolysis of phospho-diester bonds in nucleic acids to produce oligonucleotides. Whereas the mammalian-type nuclease RNase A requires a pyrimidine base on the (3 )-side of the substrate, the microbial enzyme RNase T, requires a guanine [116]. Although the two families show hardly any sequence homology or structural relation, the cleavage mechanisms are believed to be similar [117-119]. 

3 Expansion of Coordination and Change of Electronic State at Iron in Cytochrome P-450can, 

In the substrate-free situation, Fe is in a low-spin state. The crystal structure indicates that several water molecules or hydroxide ions are present in the active site. One of them occupies the sixth coordination site at Fe. The hexacoordinated low-spin state is presumably achieved by coordinating the strong-field hydroxide ligand, OH , rather than the weak-field ligand, H2O [137]. The low redox potential is due to stabilization of the ferric state by an environment of high dielectric constant, i.e. by the water molecules in the active site [137]. When the camphor substrate diffuses into the enzyme, the change towards a high-spin complex can be followed spectroscopically. A transition to square pyramidal coordination at Fe occurs, and ordered 

6-exo 47%, 3-exo 8%) than the other two (a single product). From modeling studies [140] it is estimated that the missing hydrophobic interactions allow rotation of norcamphor about its C = 0... HO axis by ca. 130° into an orientation where hydroxylation at the 3-position would be possible. Adamantanone may perform a comparable rotation about this axis, but, due to its high symmetry, only one product is formed. 

A second, more pronounced difference between the various substrates is that the iron remains hexacoordinated in the norcamphor, camphane and adamantane complexes, the sixth ligand being H2O or OH , but becomes pentacoordinated for camphor and adamantanone. Since camphane permits the aqua ligand to remain, and the slightly smaller adamantanone displaces this ligand, substrate size is not the only factor that determines the displacement of the sixth ligand [132,137]. The 

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