Structural and biochemical characterization of the nonliganding residue glutamine 143 near the manganese of human Mn superoxide dismutase (hMnSOD), a homotetramer of 22 kDa, reveals a functional role for this residue. In the wild-type protein, the side-chain amide group of Gln 143 is about 5 A from the metal and is hydrogen-bonded to Tyr 34, which is a second prominent side chain adjacent to the metal. We have prepared the site-specific mutant of hMnSOD with the conservative replacement of Gln 143 --> Asn (Q143N). The crystal structure of Q143N shows that the side-chain amide nitrogen of residue 143 is 1.7 A more distant from the manganese than in the wild-type enzyme. The Tyr 34 side-chain hydroxyl in Q143N is also moved to become 0.6 A more distant from the metal due to an additional water molecule. Differential scanning calorimetry showed that Q143N is slightly more stable than the wild-type enzyme with Tm for the main unfolding transition increased by 2 degrees C to 90.7 degrees C. Pulse radiolysis and stopped-flow spectrophotometry reveal that unlike wild-type hMnSOD, which is strongly inhibited by peroxide, Q143N MnSOD exhibits no product inhibition even at concentrations of O2. - in the millimolar range, and its catalysis follows Michaelis kinetics with no evidence of cooperativity. However, the overall catalytic activity of this mutant was decreased 2-3 orders of magnitude compared with the wild-type MnSOD, which can account for its lack of product inhibition. Q143N MnSOD lacked the visible absorption spectrum typical of wild-type Mn(III)SOD. Also, unlike the wild-type Mn(III)SOD, which is electron paramagnetic resonance (EPR) silent, Q143N MnSOD has a complex EPR spectrum with many resonances in the region below 2250 G. We conclude that the Gln 143 --> Asn mutation has increased the reduction potential of manganese to stabilize Mn(II), indicating that Gln 143 has a substantial role in maintaining a reduction potential favorable for the oxidation and reduction cycles in the catalytic disproportionation of superoxide. A solvent hydrogen isotope effect near 2 for kcat in catalysis by Q143N hMnSOD indicates rate-contributing proton transfers to form product hydroperoxide anion or hydrogen peroxide. The data demonstrate a prominent role for Gln 143 in maintaining the microenvironment of the manganese and in efficient catalysis of superoxide dismutation to oxygen and hydrogen peroxide.

In this present study we report millimeter and submillimeter high-field CW EPR spectra of P700•+, the primary electron donor in photosystem I. The data shown are the first well-resolved chlorophyll radical spectra observed without prior deuteration. The signal was generated from isolated plant photosystem I (in both digitonin and Triton-X100 preparations) by photooxidation. At 325 GHz and higher frequencies, the spectrum was resolvable into the three principal components of its g-matrix. They were obtained to high accuracy by spectral simulation and were found to be g xx = 2.003 17(±7 × 10-5), g yy = 2.002 64(±7 × 10-5), g zz = 2.002 26(±7 × 10-5) at 40 K and g xx = 2.003 07(±7 × 10-5), g yy = 2.002 60(±7 × 10-5), g zz = 2.002 26(±7 × 10-5) at 200 K. These values indicate a temperature dependence of the g-values in P700•+ that was measured over a temperature range of 5−260 K using both preparations. The results showed that g xx changed with temperature while g yy and g zz remain constant within the error margin, suggesting that the P700•+ radical becomes less anisotropic with increasing temperature.