Structures of two histidine ammonia‐lyase modifications and implications for the catalytic mechanism

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Abstract
Histidine ammonia‐lyase (EC 4.3.1.3) catalyzes the nonoxidative elimination of the α‐amino group of histidine using a 4‐methylidene‐imidazole‐5‐one (MIO), which is formed autocatalytically from the internal peptide segment 142Ala‐Ser‐Gly. The structure of the enzyme inhibited by a reaction with l‐cysteine was established at the very high resolution of 1.0 Å. Five active center mutants were produced and their catalytic activities were measured. Among them, mutant Tyr280→Phe could be crystallized and its structure could be determined at 1.7 Å resolution. It contains a planar sp2‐hybridized 144‐N atom of MIO, in contrast to the pyramidal sp3‐hybridized 144‐N of the wild‐type. With the planar 144‐N atom, MIO assumes the conformation of a putative intermediate aromatic state of the reaction, demonstrating that the conformational barrier between aromatic and wild‐type states is very low. The data led to a new proposal for the geometry for the catalyzed reaction, which also applies to the closely related phenylalanine ammonia‐lyase (EC 4.3.1.5). Moreover, it suggested an intermediate binding site for the released ammonia.