title: The interaction of an ionizing ligand with enzymes having a single ionizing group. Implications for the reaction of folate analogues with dihydrofolate reductase.
abstract: Binding theory has been developed for the reaction of an ionizing enzyme with an ionizing ligand. Consideration has been given to the most general scheme in which all possible reactions and interconversions occur as well as to schemes in which certain interactions do not take place. Equations have been derived in terms of the variation of the apparent dissociation constant (Kiapp) as a function of pH. These equations indicate that plots of pKiapp against pH can be wave-, half-bell- or bell-shaped according to the reactions involved. A wave is obtained whenever there is formation of the enzyme-ligand complexes, ionized enzyme . ionized ligand and protonated enzyme . protonated ligand. The additional formation of singly protonated enzyme-ligand complexes does not affect the wave form of the plot, but can influence the shape of the overall curve. The formation of either ionized enzyme . ionized ligand or protonated enzyme . protonated ligand, with or without singly protonated enzyme-ligand species, gives rise to a half-bell-shaped plot. If only singly protonated enzyme-ligand complexes are formed the plots are bell-shaped, but it is not possible to deduce the ionic forms of the reactants that participate in complex formation. Depending on the reaction pathways, true values for the ionization and dissociation constants may or may not be determined.
mesh_terms: D004798:Enzymes / Q000378:metabolism*; D005492:Folic Acid / Q000031:analogs & derivatives* / Q000494:pharmacology; D006860:Hydrogen Bonding; D007700:Kinetics; D008024:Ligands; D008433:Mathematics; D011485:Protein Binding; D013329:Structure-Activity Relationship; D013762:Tetrahydrofolate Dehydrogenase / Q000378:metabolism*
chemical_list: D004798:Enzymes; D008024:Ligands; D005492:Folic Acid; D013762:Tetrahydrofolate Dehydrogenase