Water-Schiff base interactions

The enzyme phosphotriesterase hydrolyzes many different organophosphorus triesters, including several acetylcholinesterase inhibitors. The chemical mechanism involves an activated water molecule that directly attacks the phosphorus center with a resultant inversion of configuration [708]. Thus the overall reaction mechanism does not involve the formation of a phosphorylated-enzyme intermediate, but it does involve a Schiff base-type interaction between carbon dioxide and a lysine residue giving a carbamate group in the active site. 

See structure 1. The reaction is conducted in dimethyl sulfoxide (DMSO), because DMSO is a very good solvent for the growing oligomers and its strong interaction with water prevents hydrolysis of the zirconium(IV) and favors the forward Schiff-base condensation reaction. 

Asp-96, a key residue in the middle of the relatively hydrophobic cytoplasmic half-channel about 11 A from the protonated Schilf base, is protonated in the ground state. Its only polar interaction stems from a hydrogen bond to the hydroxyl of Thr-46. Otherwise it is surrounded by a hydrophobic barrel whose walls are formed by residues IleA5, Leu-223, and Leu-224, and the lids by Phe-42, Leu-99, and Leu-100 on the cytoplasmic side, and by Val-49, Leu-93, and Phe-219 Schiff base side. In the M to N transition of the photocycle, reprotonation of the Schiffbase from Asp-96 can occur only after this hydrophobic region between the Schiffbase and Asp-96 has been populated by a chain of water molecules (Luecke et al., 2000b). 

I.3. Reduced Forms of the Enzyme. The hydrolysis of the product Schiff-base to release product and generate the aminoquinol form of the cofactor may involve water which has been retained in the active site. Dooley and co-workers have provided direct evidence for copper reduction during the interaction of amine oxidases with substrate under anaerobic conditions (Dooley et al., 1991). By varying the temperature at which EPR spectra were recorded, it was shown for amine oxidases from several sources that there is a temperature dependent equilibrium between Cu V aminoquinol TPQ and Cu / TPQ semiquinone. The Cu / TPQ semi-quinone form was found to be stabilised in the presence of cyanide. The... 

In the steroids diaziridine formation occurs most successfully in the sterically unhindered 3-position, less favorably in the 2-position, in which formation of an aminal-like intermediate is hindered by 1,3-diaxial interaction with a methyl group. In the sterically most hindered 17-position, diaziridine synthesis may be effected by formation of a Schiff base with cyclohexylamine, thus eliminating water before introduction of the aminating... 

As pointed out already, proton transfer in bR is activated by photoisomerization of all the tran -retinal to the 13-cis form, followed by proton transfer from the retinal Schiff base to Asp 85, release of a proton from residues or water molecule(s) at the extracellular surface, and uptake from the cytoplasmic surface through reprotonation of the Schiff base by Asp 96. This results in a proton transfer from the cytoplasmic to the extracellular side. It appears that this process proceeds with induced conformation and/or dynamic changes at both the extracellular and cytoplasmic side owing to protonation of Asp 85. This means that the information of the protonation at Asp 85 should be transmitted to both the extracelluar and the cytoplasmic regions, through specific side-chains or through backbone interactions. 

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