CHARACTERIZATION OF THE TETRAMER DIMER MONOMER EQUILIBRIUM OF THE ENZYMATICALLY ACTIVE SUBUNITS OF PIGEON LIVER MALIC ENZYME

The tetrameric malic enzyme from pigeon liver was reversibly dissociated in the sequence of tetramer-dimer-monomer in an acidic environment (pH 4.5) or when the ionic strength or temperature of the solution was perturbed (0.2 M ammonium sulfate or <10-degrees-C). The dissociated monomer was enzymatically active according to the following criteria: (a) separation and direct activity staining of the monomer in the native gradient polyacrylamide gel, (b) activity staining of the monomer at its pI region in the isoelectric focusing gel, and (c) the enzyme showing lower but definite enzyme activity under conditions where only monomer existed in the solution. The catalytic constant (k(cat)) and specificity constant (k(cat)/K(m)Mal) for the monomer were found to be 19 +/- 6 s-1 and 58 x 10(3) s-1.M-1, respectively, only one-seventh and one-seventeenth of those for the tetramer. Different types of interactions are involved in the monomer-monomer and dimer-dimer associations: (a) Two dissociation processes showed different pH dependences. The monomer-monomer interactions involve an amino acid with a side chain pK(a) value around 5.7, and an amino acid with a side chain pK(a) value of 7.2 is involved in the dimer-dimer association. (b) Ammonium sulfate up to 0.2 M only affects the monomer-monomer but not the dimer-dimer interactions. The Gibb's free energy, enthalpy, and entropy all have negative values for the above subunits' dissociations. The overall dissociation is an enthalpy-driven process. Association of the subunits to form dimers and tetramers involves salt-bridge, van der Waals, and hydrogen-bonding interactions. Small amounts of trimer were also detectable under certain experimental conditions. These experimental results are consistent with our previously proposed preexisting asymmetric model for the tetrameric pigeon liver malic enzyme [Lee, H. J., & Chang, G. G. (1990) FEBS Lett. 277, 175-179]. The possible role of quaternary structure modulation in regulating the malic enzyme activity in vivo is discussed.