Ward, T.R.

J. Am. Chem. Soc. 2014, 136, 8928-8932, 10.1021/ja500613n

We report our efforts to enable transition-metal catalysis in the presence of cellular debris, notably Escherichia coli cell free extracts and cell lysates. This challenging goal is hampered by the presence of thiols, mainly present in the form of glutathione (GSH), which poison precious metal catalysts. To overcome this, we evaluated a selection of oxidizing agents and electrophiles toward their potential to neutralize the detrimental effect of GSH on a Ir-based transfer hydrogenation catalyst. While the bare catalyst was severely inhibited by cellular debris, embedding the organometallic moiety within a host protein led to promising results in the presence of some neutralizing agents. In view of its complementary to natural enzymes, the asymmetric imine reductase based on the incorporation of a biotinylated iridium pianostool complex within streptavidin (Sav) isoforms was selected as a model reaction. Compared to purified protein samples, we show that pretreatment of cell free extracts and cell lysates containing Sav mutants with diamide affords up to >100 TON’s and only a modest erosion of enantioselectivity.

Maréchal, J.-D.; Ward, T.R.

J. Am. Chem. Soc. 2014, 136, 15676-15683, 10.1021/ja508258t

An artificial imine reductase results upon incorporation of a biotinylated Cp*Ir moiety (Cp* = C5Me5–) within homotetrameric streptavidin (Sav) (referred to as Cp*Ir(Biot-p-L)Cl] ⊂ Sav). Mutation of S112 reveals a marked effect of the Ir/streptavidin ratio on both the saturation kinetics as well as the enantioselectivity for the production of salsolidine. For [Cp*Ir(Biot-p-L)Cl] ⊂ S112A Sav, both the reaction rate and the selectivity (up to 96% ee (R)-salsolidine, kcat 14–4 min–1 vs [Ir], KM 65–370 mM) decrease upon fully saturating all biotin binding sites (the ee varying between 96% ee and 45% ee R). In contrast, for [Cp*Ir(Biot-p-L)Cl] ⊂ S112K Sav, both the rate and the selectivity remain nearly constant upon varying the Ir/streptavidin ratio [up to 78% ee (S)-salsolidine, kcat 2.6 min–1, KM 95 mM]. X-ray analysis complemented with docking studies highlight a marked preference of the S112A and S112K Sav mutants for the SIr and RIr enantiomeric forms of the cofactor, respectively. Combining both docking and saturation kinetic studies led to the formulation of an enantioselection mechanism relying on an “induced lock-and-key” hypothesis: the host protein dictates the configuration of the biotinylated Ir-cofactor which, in turn, by and large determines the enantioselectivity of the imine reductase.

Lu, Y.

J. Am. Chem. Soc. 2014, 136, 11882-11885, 10.1021/ja5054863

Cytochrome c Oxidase (CcO) is known to catalyze the reduction of O2 to H2O efficiently with a much lower overpotential than most other O2 reduction catalysts. However, methods by which the enzyme fine-tunes the reduction potential (E°) of its active site and the corresponding influence on the O2 reduction activity are not well understood. In this work, we report systematic tuning of the heme E° in a functional model of CcO in myoglobin containing three histidines and one tyrosine in the distal pocket of heme. By removing hydrogen-bonding interactions between Ser92 and the proximal His ligand and a heme propionate, and increasing hydrophobicity of the heme pocket through Ser92Ala mutation, we have increased the heme E° from 95 ± 2 to 123 ± 3 mV. Additionally, replacing the native heme b in the CcO mimic with heme a analogs, diacetyl, monoformyl, and diformyl hemes, that posses electron-withdrawing groups, resulted in higher E° values of 175 ± 5, 210 ± 6, and 320 ± 10 mV, respectively. Furthermore, O2 consumption studies on these CcO mimics revealed a strong enhancement in O2 reduction rates with increasing heme E°. Such methods of tuning the heme E° through a combination of secondary sphere mutations and heme substitutions can be applied to tune E° of other heme proteins, allowing for comprehensive investigations of the relationship between E° and enzymatic activity.