CRYSTAL STRUCTURE OF THE AMINO TERMINAL DOMAIN OF ENZYME I OF THE E. COLI PHOSPHOENOLPYRUVATE:SUGAR PHOSPHOTRANSFERASE SYSTEM. Der-Ing Liao¹, Enid Silverton¹, Yeong-Jae Seok², Byeong Ryong Lee^2,3, Alan Peterkofsky², David R. Davies¹. ¹Laboratory of Molecular Biology, NIDDK, National Institutes of Health, Bethesda, MD 20892; ²Laboratory of Biochemical Genetics, NHLBI, National Institutes of Health, Bethesda, MD 20892; ³Present address: Department of Biological Education, Seo-Won University, Mochung-Dong, Chong-Ju City, Chung-Buk, South Korea.

The crystal structure of the amino terminal domain of enzyme I (EIN) from E. coli has been determined and refined at 2.5Å resolution with the crystallographic R factor of 22.5%. Enzyme I catalyzes the first reaction in the multi-protein phosphoenolpyruvate: sugar phosphotransferase system (PTS), which couples the sugar translocation across bacterial cell membranes and phosphorylation. Enzyme I transfers the phosphoryl group from phosphoenolpyruvate (PEP) to a histidine containing phosphocarrier protein HPr through phosphorylation and dephosphorylation of a histidine residue HIS 189. EIN contains this phosphorylation site histidine and can transfer phosphoryl group back and forth between HPr and itself. Unlike the intact enzyme I, EIN can not be autophosphorylated by PEP. It has an elongated two-domain structure consisting of a four- helix [[alpha]]-domain and an [[alpha]]/ß domain. In addition, there is a helical linker to the missing C-terminal domain of the intact enzyme I. The [[alpha]]/ß domain of EIN is topologically similar to the phospho- histidine domain of the enzyme pyruvate-phosphate dikinase (PPDK), which can be phosphorylated by PEP on a histidyl residue. The crystal structure of EIN as well as its comparisons with PPDK and EIIA, another PTS protein which also interacts with HPr, will be presented. A model of the interaction between EIN and HPr will also be purposed.