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J. Biochem, 1992, Vol. 112, No. 6 811-815
© 1992 Japanese Biochemical Society

research-article

Further Studies on Aspartate Aminotransferase of Thermophilic Methanogens by Analysis of General Properties, Bound Cofactors, and Subunit Structures

Toahio Tanaka*,1, Satoahi Yamamoto*, Makoto Taniguchi*, Hideyuki Hayashi**, Seiki Kuramitsu***, Hiroyuki kagamiyama** and Susumu Oi*

*Department of Bioloigy, Faculty of Science, Osaka City University 3-3-138 Sugimoto, Sumyoshi-ku, Osaka 658
**Department of Medical Chemistry, Osaka Medical College Daigaku-cho, Takatuuki, Osaka 569
***Department of Biology, Osaka University College of Science Toyonoka Osaka 560

2To whom correspondence should b addressed

Aspartate aminotransferase (AspAT) [EC 2.6.1.1 [EC] ] of thermophilic methanogen was further characterized with the enzyme from Methanobacterium thermoautotrophicum strain FTF-INRA as well as M. thermoformicicm strain SF-4. AspAT of strain FTF-INRA was similar in the amino donor specificity to the enzyme of M. thermoformicicum strain SF-4, in that it was active on L-cysteine and L-cysteine sulfinate in addition to L-glutamate and L-aspartate. The enzymes gave similar absorption spectra having maxima at around 328 and 416 nm with no pH-dependent shift but were found to contain 1 mol of tightly bound pyridoxal 5'-phosphate (PLP) per subunit. Reconstitution of each apoenzyme with added PLP resulted in partial recovery of the original enzymatic activity, suggesting a significant conformational change of the active site region upon removal of the cofactor. Polyacrylamide gel electrophoresis (PAGE) and gel filtration analyses revealed a tetrameric structure (180 kDa) of identical subunits with a molecular mass of 43 kDa for each of these enzymes. Electric current was found to affect the interaction or affinity of each subunit, promoting dissociation of the native enzyme into the monomeric form. Alkaline treatment was effective only for dissociation of the enzyme from strain SF-4. They were distinguishable by the more rapid reassociation of the monomer to the native aggregated form in the enzyme of strain FTF-INRA.


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