A one-step procedure of immobilizing soluble and aggregated preparations of D-amino acid oxidase from Trigonopsis variabilis (TvDAO) is reported where carrier-free enzyme was entrapped in semipermeable microcapsules produced from the polycation poly(methylene-co-guanidine) in combination with CaCl2 and the polyanions alginate and cellulose sulfate. The yield of immobilization, expressed as the fraction of original activity present in microcapsules, was approximately 52 +/- 5%. The effectiveness of the entrapped oxidase for O2-dependent conversion of D-methionine at 25 degrees C was 85 +/- 10% of the free enzyme preparation. Because continuous spectrophotometric assays are generally not well compatible with insoluble enzymes, we employed a dynamic method for the rapid in situ estimation of activity and relatedly, stability of free and encapsulated oxidases using on-line measurements of the concentration of dissolved O2. Integral and differential modes of data acquisition were utilized to examine cases of fast and slow inactivation of the enzyme, respectively. With a half-life of 60 h, encapsulated TvDAO was approximately 720-fold more stable than the free enzyme under conditions of bubble aeration at 25 degrees C. The soluble oxidase was stabilized by added FAD only at temperatures of 35 degrees C or greater.

One of the primary sources of enzyme instability is protein oxidative modification triggering activity loss or denaturation. We show here that the side chain of Cys108 is the main site undergoing stress-induced oxidation in Trigonopsis variabilis D-amino acid oxidase, a flavoenzyme employed industrially for the conversion of cephalosporin C. High-resolution anion-exchange chromatography was used to separate the reduced and oxidized protein forms, which constitute, in a molar ratio of about 3:1, the active biocatalyst isolated from the yeast. Comparative analysis of their tryptic peptides by electrospray tandem mass spectrometry allowed unequivocal assignment of the modification as the oxidation of Cys108 into cysteine sulfinic acid. Cys108 is likely located on a surface-exposed protein region within the flavin adenine dinucleotide (FAD) binding domain, but remote from the active center. Its oxidized side chain was remarkably stable in solution, thus enabling the relative biochemical characterization of native and modified enzyme forms. The oxidation of Cys108 causes a global conformational response that affects the protein environment of the FAD cofactor. In comparison with the native enzyme, it results in a fourfold-decreased specific activity, reflecting a catalytic efficiency for reduction of dioxygen lowered by about the same factor, and a markedly decreased propensity to aggregate under conditions of thermal denaturation. These results open up unprecedented routes for stabilization of the oxidase and underscore the possible significance of protein chemical heterogeneity for biocatalyst function and stability.

Trigonopsis variabilis D-amino acid oxidase (TvDAO) is a long-known flavoenzyme whose most important biocatalytic application is currently the industrial production of 7-amino-cephalosporanic acid (7-ACA) from cephalosporin C. Lacking mechanistic foundation, rational stabilization of TvDAO for improved process performance remains a problem. We report on results of thermal denaturation studies at 50 degrees C in which two purified TvDAO forms were compared: the native enzyme, and a site-specifically oxidized protein variant that had the side chain of cysteine108 converted into a sulfinic acid and lost 75% of original specific activity. Although inactivation time courses for both enzymes are fairly well described by simple single-exponential decays, the underlying denaturation mechanisms are shown by experiments and modeling to be complex. One main path leading to inactivation is FAD release, a process whose net rate is determined by the reverse association rate constant (k), which is 25-fold lower in the oxidized form of TvDAO. Cofactor dissociation is kinetically coupled to aggregation and can be blocked completely by the addition of free FAD. Aggregation is markedly attenuated in the less stable Cys108-SO(2)H-containing enzyme, suggesting that it is a step accompanying but not causing the inactivation. A second parallel path, characterized by a k-value of 0.26/h that is not dependent on protein concentration and identical for both enzymes, likely reflects thermal unfolding reactions. A third, however, slow process is the conversion of the native enzyme into the oxidized form (k < 0.03/h). The results fully explain the different stabilities of native and oxidized TvDAO and provide an inactivation mechanism-based tool for the stabilization of the soluble oxidase.