Efeito espacial do grupo imidazol nas reações de diésteres de fosfato e fosfoimidazóis: modelos não-miméticos da fosfolipase D

Abstract

The reactions involving phosphoryl transfer are among the most studied in chemistry and biochemistry because their relevance in many chemical and biological processes. The control and increase of the reaction rate of these reactions are governed by extensive enzymatic machinery using side chains of some amino acid residues present in the active site as nucleophiles. Enzymes are macromolecules and their structural complexity, sensitivity to temperature and pH interfere in studies of its mechanism of catalysis. In order of better understand the catalytic power of phospholipase D, this work focuses on the study of the reaction mechanism of the phosphate diesters, p-nitrophenyl phosphate ortho-(1H-imidazol-2-yl)naphthyles, named 1Ni2P-pNP, 2Ni1P-pNP and 3Ni2P-pNP, which are three new non-mimetic models of the mechanism of phospholipase D. We evaluated the effects of directionality and proximity of the imidazole group in the reaction of the three phosphate diesters. These isomers react to their monoesters, ortho-(1H-imidazol-2-yl)naphthyl phosphate in two steps: i. phosphodiester cleavage with intramolecular nucleophilic attack of the neutral or anionic imidazole group on the phosphorus atom with formation of p-nitrophenolate and an phosphoimidazol intermediate; hydrolysis of cyclic phosphoimidazol (protonated imidazole group) to yield the respective monoesters of 1Ni2P-pNP, 2Ni1P-pNP and 3Ni2P-pNP. In this case, it was observed that the cyclic phosphoimidazol (deprotonated imidazole group) reacts slowly with water or other oxygen nucleophiles. Among the three diesters, the 3Ni2PpNP isomer was the most reactive in the first st ep, presenting an effective molarity 6.5-fold greater than 1Ni2P-pNP. The first step was studied computationally, with a good agreement with the experimental thermodynamic parameters, showing that the reaction of the three isomers presents a synchronous transition state. In the hydrolysis step of the cyclic phosphoimidazol, low NUCvalues indicate an early transition state. Finally, based on the results of this and other studies, it is proposed that, according to the spatiotemporal theory, the reaction cone to the nucleophilic attack on the phosphorous atom is less dependent on directionality in relation to the attack on the carbon atom.