Essential Tyrosine Residues in 3-Ketosteroid- '-Dehydrogenase from Rhodococcus rhodochrous

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Abstract
Tetranitromethane treatment of 3-ketosteroid-Δ'-dehydrogenase of Rhodococcus rhodochrous caused loss of the catalytic activity in a time- and concentration-dependent manner. Peptides (P-81) and (PN-83) were isolated from tryptic digests of the native and tetranitro-m ethane-treated enzyme proteins, respectively. PN-83 was the nitrated form of P-81. The amino acid sequence was GGAPLmYLESDDDLEFMVYPWPDYFGK (positions 97–124 of the dehydrogenase sequence). PN-83 showed a low yield of PTH-Tyr of position 116, Le. less than 5% of that of P-81, and instead a high yield of PTH-3-nitrotyrosine. This indicated that tetranitromethane modifies Y-116 under the experimental conditions used. Mutation of Y-104, Y-116, and Y-121 to smaller amino acid residues, Phe, Ser, or Ala, significantly changed the catalytic activity of the dehydrogenase. All of the mutants contained FAD and exhibited the same spectrophotometric properties as those of the wild type enzyme. The Km values for 4-androstene-3, 17-dione of the Y-104, Y-116, and Y-121 mutants changed to large values. The most drastic change was observed for Y116A. The Kd values for 1, 4-andro-stadiene-3, 17-dione of the Y116 mutants changed to 1.5–2.6-fold larger values than that of the recombinant enzyme. The Y-121 mutant enzymes exhibited catalytic activities like those of the recombinant enzyme, but the catalytic efficiencies of Y121F and Y121A drastically decreased to 0.014–0.054% of that of the recombinant enzyme. The present results indicate that Y-121 plays an important role in the catalytic function, and that Y-116 and Y-104 act on binding of the substrate steroid.

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