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    Inhibition of Escherichia coli porphobilinogen synthase using analogs of postulated intermediates
    (2000)
    Jarret, Caroline
    ;
    Stauffer, Frédéric
    ;
    Henz, Matthias
    ;
    Marty, Maurus
    ;
    Lüönd, Rainer Martin
    ;
    Bobalova, Janette
    ;
    ;
    Neier, Reinard
    Background: Porphobilinogen synthase is the second enzyme involved in the biosynthesis of natural tetrapyrrolic compounds, and condenses two molecules of 5-aminolevulinic acid (ALA) through a nonsymmetrical pathway to form porphobilinogen. Each substrate is recognized individually at two different active site positions to be regioselectively introduced into the product. According to pulse-labeling experiments, the substrate forming the propionic acid sidechain of porphobilinogen is recognized first. Two different mechanisms for the first bond-forming step between the two substrates have been proposed. The first involves carbon-carbon bond formation (an aldol-type reaction) and the second carbon-nitrogen bond formation, leading to an iminium ion. Results: With the help of kinetic studies, we determined the Michaelis constants for each substrate recognition site. These results explain the Michaelis-Menten behavior of substrate analog inhibitors - they act as competitive inhibitors. Under standard conditions, however, another set of inhibitors demonstrates uncompetitive, mixed, pure irreversible, slow-binding or even quasi-irreversible inhibition behavior. Conclusions: Analysis of the different classes of inhibition behavior allowed us to make a correlation between the type of inhibition and a specific site of interaction. Analyzing the inhibition behavior of analogs of postulated intermediates strongly suggests that carbon-nitrogen bond formation occurs first.