The Subunit Structure and Subunit Interactions of Cytidine Triphosphate Synthetase

作者：Cedric W. Long, Alexander Levitzki, Daniel E. Koshland · 发表于：Journal of Biological Chemistry · 年份：1970 · DOI：10.1016/s0021-9258(18)63424-6 · 被引用次数：112 · 研究领域：Biochemical and Molecular Research、Enzyme Structure and Function、Protein Structure and Dynamics

Abstract Cytidine triphosphate synthetase, when purified, has been shown to be a dimer of 105,000 molecular weight, composed of two seemingly identical 50,000 subunits. In the presence of the substrates ATP, UTP, and Mg2+, under assay conditions, the dimer aggregates to the enzymatically active tetramer of 210,000 molecular weight. Two of the four glutamine-binding sites have been covalently labeled using the affinity label, 6-diazo-5-oxo-l-norleucine, which attaches to a cysteine residue. The unavailability of all glutamine sites to the affinity label is discussed in terms of the negative cooperativity with respect to glutamine and 6-diazo-5-oxo-l-norleucine. The finding of four identical subunits in the aggregated tetramer explains the high Hill coefficients and the kinetic requirement for four interacting sites for each substrate. It thus appears that the conformational changes depend on the ligand inducing the changes and hence that different subunit interaction patterns are induced by the different ligands.