Site-directed mutagenesis of intersubunit boundary residues in histidine decarboxylase, a pH-dependent allosteric enzyme.
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Histidine decarboxylase (HDC) from Lactobacillus 30a forms a trimer around a central cavity or well. Three active sites are formed around the well at the interface of each of two adjacent molecules. HDC exhibits cooperative kinetics at pH 7.6 and can be described in terms of a two-state, T and R, model. At pH 4.8, protons stabilize HDC in the R form. Asp 198 and Asp 53, from a neighboring molecule, are the core of the pH-sensitive mechanism controlling the shift in quaternary state. Eight site-directed mutations have been made to analyze the region. Several mutants, including the conversion of Asp 53 to Asn, cause HDC to exhibit sigmoidal kinetics even at pH 4.8. Others lock the enzyme into the T state. Kinetic analysis suggests that kcat values for T and R states are similar. The Km for the T state, near 8 mM, exceeds that for the R state by 40-fold and shows HDC is primarily regulated by altering its affinity for substrate.