日本放射光学会誌    Journal of JSSRR
Vol.33,No.1/Jan. 2020

【表紙の説明】 
ラジカルB
12酵素エタノールアミンアンモニアリアーゼ(EAL)((a), (b))とジオールデヒドラターゼ(DD)((c), (d))における、基質結合によるアデノシルB12のアデノシル基とa側鎖の構造変化。a側鎖はラジカル発生後のリボース部の立体構造を安定に保つ。ただし、DDではすでに失活していたため基質結合前後でほとんど構造変化していない。

Fo-Fc omit maps around the adenosyl group of radical B
12-dependent enzymes ethanolamine ammonia-lyase (EAL) and diol dehydratase (DD) complexed with adenosylcobalamin (AdoCbl) in the absence and presence of substrates (2-amino-1-propanol (2-AP) for EAL and 1,2-propanediol (1,2-PDO) for DD). EAL/AdoCbl (a), EAL/AdoCbl/2-AP (b), DD/AdoCbl (c), and DD/AdoCbl/1,2-PDO (d). The EAL/AdoCbl structure displayed that the C5´ atom of the adenosyl group remains on the cobalt atom with a Co-C distance of 3.00 Å (a). The Co-C bond was completely cleaved and the ribose group of the adenosyl group exists at the “catalytic position” where the adenosyl radical can abstract the hydrogen atom from the substrate molecule in the substrate-bound forms of EAL (b) and DD (d). Note that DD/AdoCbl actually represents suicidally-inactivated through undesirable side reactions during catalysis with contaminating substrate in the purified sample; the C5´ atom of the adenosyl group was placed adjacent to the catalytic position (c). In the case of EAL, structural shifts of the a-side chain and the adenosyl group of AdoCbl were observed after 10 min of soaking of substrate (a, b), although the electron densities of EAL/AdoCbl/2-AP are relatively ambiguous compared with the substrate-free structure; the a-side chain and the ribose group stay at the outward position and on the cobalt atom, respectively, in the resting pre-homolysis state, and they move to the inward position and to the catalytic position, respectively, in the post-homolysis state in the presence of substrate.
      
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