Catalytic roles of substrate-binding residues in coenzyme B 12-dependent ethanolamine ammonia-lyase

Koichi Mori, Toshihiro Oiwa, Satoshi Kawaguchi, Kyosuke Kondo, Yusuke Takahashi, Tetsuo Toraya

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Abstract

Ethanolamine ammonia-lyase (EAL) catalyzes the adenosylcobalamin-dependent conversion of ethanolamine to acetaldehyde and ammonia. 1-OH of the substrate is hydrogen-bonded with Gluα287, Argα160, and Asnα193 and 2-NH2 with Gluα287, Glnα162, and Aspα362. The active site somewhat resembles that of diol dehydratase. All five residues were important for the high-affinity binding of the substrate and for catalysis. The -COO- group at residue α287 was absolutely required for activity and coenzyme Co-C bond cleavage, and there was a spatially optimal position for it, suggesting that Gluα287 contributes to Co-C bond homolysis, stabilizes the transition state for the migration of NH2 from C2 to C1 through partial deprotonation of spectator OH, and functions as a base in the elimination of ammonia. A positive charge and/or the hydrogen bond at position α160 and the hydrogen bonds at positions α162 and α193 with the substrate are important for catalysis and for preventing a radical intermediate from undergoing side reactions. Argα160 would stabilize the trigonal transition state in NH2 migration by electrostatic catalysis and hydrogen bonding with spectator OH. Asnα193 would contribute to maintaining the appropriate position and direction of the guanidinium group of Argα160, as well. Hydrogen bond acceptors were necessary at position α162, but hydrogen bond donors were rather harmful. Glnα162 might stabilize the trigonal transition state by accepting a hydrogen bond from migrating NH3+. The activity was very sensitive to the position of -COO- at α362. Aspα362 would assist Co-C bond homolysis indirectly and stabilize the trigonal transition state by accepting a hydrogen bond from migrating NH3+ and electrostatic interaction.

Original languageEnglish
Pages (from-to)2661-2671
Number of pages11
JournalBiochemistry
Volume53
Issue number16
DOIs
Publication statusPublished - Apr 29 2014

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ASJC Scopus subject areas

  • Biochemistry

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