Quaternary structure of pigeon liver malic enzyme.

Pigeon liver malic enzyme (EC 1.1.1.40) has a double dimer quaternary structure. The NADP+ analogs, aminopyridine adenine dinucleotide phosphate and nicotinamide-1,N6-ethenoadenosine dinucleotide phosphate, bind to the enzyme anti-cooperatively. In the presence of non-cooperative competing ligand NADP+, the binding parameter Hill coefficients of these analogues changed very little. Binding of L-malate with enzyme-AADP+ complex first enhanced then reduced the nucleotide fluorescence. Two L-malate binding sites, with Kd values of 23-30 and 270-400 microM, respectively. for the tight and weak binding sites were postulated. A hybrid model between the sequential and pre-existing asymmetrical models was proposed for the pigeon liver malic enzyme.