The Structural Model of Salmonella typhimurium Ethanolamine Ammonia-Lyase Directs a Rational Approach to the Assembly of the Functional [(EutB–EutC)2]3 Oligomer from Isolated Subunits

Ethanolamine ammonia-lyase (EAL) is a 5'-deoxyadenosylcobalamin-dependent bacterial enzyme that catalyzes the deamination of the short-chain vicinal amino alcohols, aminoethanol and (S)- and (R)-2-aminopropanol. The coding sequence for EAL is located within the 17-gene eut operon, which encodes the broad spectrum of proteins that comprise the ethanolamine utilization (eut) metabolosome suborganelle structure. A high-resolution structure of the ∼500 kDa EAL [(EutB-EutC)₂]₃ oligomer from Escherichia coli has been determined by X-ray crystallography, but high-resolution spectroscopic determinations of reactant intermediate-state structures and detailed kinetic and thermodynamic studies of EAL have been conducted for the Salmonella typhimurium enzyme. Therefore, a statistically robust homology model for the S. typhimurium EAL is constructed from the E. coli structure. The model structure is used to describe the hierarchy of EutB and EutC subunit interactions that construct the native EAL oligomer and, specifically, to address the long-standing challenge of reconstitution of the functional oligomer from isolated, purified subunits. Model prediction that the (EutB₂)₃ oligomer assembly will occur from isolated EutB, and that this hexameric structure will template the formation of the complete, native [(EutB-EutC)₂]₃ oligomer, is verified by biochemical methods. Prediction that cysteine residues on the exposed subunit-subunit contact surfaces of isolated EutB and EutC will interfere with assembly by cystine formation is verified by activating effects of disulfide reducing agents. Angstrom-scale congruence of the reconstituted and native EAL in the active site region is shown by electron paramagnetic resonance spectroscopy. Overall, the hierarchy of subunit interactions and microscopic features of the contact surfaces, which are revealed by the homology model, guide and provide a rationale for a refined genetic and biochemical approach to reconstitution of the functional [(EutB-EutC)₂]₃ EAL oligomer. The results establish a platform for further advances in understanding the molecular mechanism of EAL catalysis and for insights into therapy-targeted manipulation of the bacterial eut metabolosome.