The Structure of a Plant Tyrosinase from Walnut Leaves Reveals the Importance of “Substrate‐Guiding Residues” for Enzymatic Specificity

Tyrosinases and catechol oxidases are members of the class of type III copper enzymes. While tyrosinases accept both mono‐ and o‐diphenols as substrates, only the latter substrate is converted by catechol oxidases. Researchers have been working for decades to elucidate the monophenolase/diphenolase specificity on a structural level and have introduced an early hypothesis that states that the reason for the lack of monophenolase activity in catechol oxidases may be its structurally restricted active site. However, recent structural and biochemical studies of this enzyme class have raised doubts about this theory. Herein, the first crystal structure of a plant tyrosinase (from Juglans regia) is presented. The structure reveals that the distinction between mono‐ and diphenolase activity does not depend on the degree of restriction of the active site, and thus a more important role for amino acid residues located at the entrance to and in the second shell of the active site is proposed.     

New Insights into the Nature of Observable Reaction Intermediates in Cytochrome P450 NO Reductase by Using a Combination of Spectroscopy and Quantum Mechanics/Molecular Mechanics Calculations

Cytochrome P450 NO reductase is an unusual member of the cytochrome P450 superfamily. It catalyzes the reduction of nitric oxide to nitrous oxide. The reaction intermediates were studied in detail by a combination of experimental and computational methods. They have been characterized experimentally by UV/Vis, EPR, Mössbauer, and MCD spectroscopy. In conjunction with quantum mechanics/molecular mechanics (QM/MM) calculations, we sought to characterize the resting state and the two detectable intermediates in detail and to elucidate the nature of the key intermediate I of the reaction. Six possible candidates were taken into account for the unknown key intermediate in the computational study, differing in protonation state and electronic structure. Two out of the six candidates could be identified as putative intermediates I with the help of the spectroscopic data: singlet diradicals Fe^III‐NHO ^{. ?} and Fe^III‐NHOH ^. . In a companion publication (C. Riplinger, F. Neese, ChemPhysChem­ 2011, 12, 3192 ) we have used QM/MM models based on these structures and performed a kinetic simulation. The combination of these two studies shows the nature of the key intermediate to be the singlet diradical, Fe^III‐NHOH ^. .     

Chemical Twinning of Salt and Metal in the Subnitridometalates Ba23Na11(MN4)4 with M=V,Nb, Ta

The subnitridometalates Ba₂₃Na₁₁(MN₄)₄ (M=V, Nb, Ta) crystallize in a new structure type, which shows ionic ortho‐nitridometalate anions and motifs from simple (inter)metallic packings: Na‐centered [Na₈] cubes as cutouts of the bcc structure of elemental Na and Na‐centered [Ba₁₀Na₂] icosahedra as found in Laves phases, for example. Single‐crystal and powder X‐ray diffraction studies in combination with quantum‐chemical calculations of the electronic structure and Raman spectroscopy support the characterization of the subnitridometalates as “chemical twins”. They consist of independent building units with locally prevalent ionic or metallic bonding in an overall metallic compound.