Nuclear magnetic resonance relaxation time studies on the manganese(II) ion complex with succinyl coenzyme A synthetase from Escherichia coli

Abstract

Nuclear magnetic resonance, as well as electron paramagnetic resonance, experiments were carried out in a study of the Mn(II) ion complex with phosphorylated succinyl-CoA synthetase. For high specific activity enzyme samples, there are 3.5 +/- 0.7 metal ion binding sites per enzyme molecule with indistinguishable dissociation constants (KD = 6.9 X 10(-4) M). However, for enzyme samples with a lower specific activity yet equivalent purity, there are 1.6 strong metal ion binding sites (KD = 6.6 X 10(-4) M) and 2.0 weak metal ion binding sites (KD = 4.0 X 10(-3) M), a result that is easily reconciled with the alpha2beta2 subunit structure of the enzyme. Water proton relaxation rate measurements indicate that each strongly bound Mn(II) ion is coordinated to three water molecules. The present results strongly suggest that the existence of nearly 4 high-affinity metal binding sites per enzyme molecule is related to the integrity or configuration of that portion of the molecule which interacts with substrates.