AMP Deaminase from yeast. Role in AMP degradation, large scale purification, and properties of the native and proteolyzed enzymes

Eukaryotes have been proposed to depend on AMP deaminase as a primary step in the regulation of intracellular adenine nucleotide pools. This report describes 1) the role of AMP deaminase in adenylate metabolism in yeast cell extracts, 2) a method for large scale purification of the enzyme, 3) the kinetic properties of native and proteolyzed enzymes, 4) the kinetic reaction mechanism, and 5) regulatory interactions with ATP, GTP, MgATP, ADP, and PO₄. Allosteric regulation of yeast AMP deaminase is of physiological significance, since expression of the gene is constitutive (Meyer, S.L., Kvalnes-Krick, K.L., and Schramm, V.L. (1989) Biochemistry 28, 8734-8743). The metabolism of ATP in cell-free extracts of yeast demonstrates that AMP deaminase is the sole pathway of AMP catabolism in these extracts. Purification of the enzyme from bakers' yeast yields a proteolytically cleaved enzyme, M(r) 86,000, which is missing 192 amino acids from the N-terminal region. Extracts of Escherichia coli containing a plasmid with the gene for yeast AMP deaminase contained only the unproteolyzed enzyme, M(r) 100,000. The unproteolyzed enzyme is highly unstable during purification. Substrate saturation plots for proteolyzed AMP deaminase are sigmoidal. In the presence of ATP, the allosteric activator, the enzyme exhibits normal saturation kinetics. ATP activates the proteolyzed AMP deaminase by increasing the affinity for AMP from 1.3 to 0.2 mM without affecting V(M). Activation by ATP is more efficient than MgATP, with half-maximum activation constants of 6 and 80 μM, respectively. The kinetic properties of the proteolyzed and unproteolyzed AMP deaminase are similar. Thus, the N-terminal region is not required for catalysis or allosteric activation. AMP deaminase is competitively inhibited by GTP and PO₄ with respect to AMP. The inhibition constants for these inhibitors decrease in the presence of ATP. ATP, therefore, tightens the binding of GTP, PO₄, and AMP. The products of the reaction, NH₃ and IMP, are competitive inhibitors against substrate, consistent with a rapid equilibrium random kinetic mechanism. Kinetic dissociation constants are reported for the binary and ternary substrate and product complexes and the allosteric modulators.