Computational analysis of the autocatalytic posttranslational cyclization observed in histidine ammonia-lyase. A comparison with green fluorescent protein.

Density functional calculations using hybrid functionals (B3LYP) have been performed to study the mechanism of the autocatalytic posttranslational cyclization observed in histidine ammonia-lyase. Two mechanisms were analyzed, the commonly accepted mechanism in which cyclization precedes dehydrogenation (reduced mechanism) and a mechanism in which dehydrogenation precedes cyclization (oxidized mechanism). The reduced pathway is not supported by the calculations, while the alternative oxidized mechanism where a dehydration occurs prior to the formation of the ring yields reasonable energetics for the system. Database searches showed that the oxidative mechanism in which the formation of the dehydro amino acids in residue i + 1 precedes the cyclization is also structurally advantageous as it results in shorter distances between the carbonyl carbon of residue i and the amide nitrogen of residue i + 2 and, therefore, preorganizes the protein for cyclization. Conformational searches showed that these distances were also unusually short and exhibited very little variation in the Delta-Ala143 HAL tetramer, indicating that like GFP the tetrameric form of HAL is rigidly preorganized for cyclization. The monomeric form of HAL is less preorganized than the tetrameric form of HAL. Dehydro amino acids aid in the preorganization, but the main driving force in the rigid tight turn formation is the influence of the surrounding protein.