Chicken Liver Amidophosphoribosyltransferase:Ligand-induced alterations in molecular and kinetic properties

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Homogeneous amidophosphoribosyltransferase (EC 2.4.2.14)preparation was obtained from chicken liver. The enzyme preparation was effectively stabilyzed with PRPP or Pi in the presence of Mg2+. From the result of sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the subunit molecular weight was estimated to be approximately 58,000. In Tris-Cl buffer, the predominant form of the enzyme had an s20, ω of 6.5, Stokes radius of 40Å, and estimated molecular weight of 110,000. Incubation with PRPP or Pi resulted in an increase in the s20, ω to 9.1-9.5, Stokes radius to 50Å, and estimated molecular weight to 200,000. Incubation of the larger form with AMP led to a decrease in the molecular weight of the enzyme. One-millimoler AMP produced sigmoidal kinetics with respect to the substrate PRPP, with Hill coefficient of 1.8. Inorganic phosphate inhibited the enzyme activity competitively with respect to the substrate PRPP. It is concluded that chicken liver amidophosphoribosyltransferase is an allosteric protein whose activity is regulated by a series of conformational changes induced by a number of ligands.
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