Critical Role of Arginine 160 of the EutB Protein Subunit for Active Site Structure and Radical Catalysis in Coenzyme B₁₂-Dependent Ethanolamine Ammonia-lyase^†

The protein chemical, kinetic, and electron paramagnetic resonance (EPR) and electron spin−echo envelope modulation (ESEEM) spectroscopic properties of ethanolamine ammonia-lyase (EAL) from Salmonella typhimurium with site-directed mutations in a conserved arginine residue (R160) of the active site containing EutB protein subunit have been characterized. R160 was predicted by a comparative model of EutB to play a critical role in protein structure and catalysis [Sun, L., and Warncke, K. (2006) Proteins: Struct., Funct., Bioinf. 64, 308−319]. R160I and R160E mutants fail to assemble into an EAL oligomer that can be isolated by the standard enzyme purification procedure. The R160K and R160A mutants assemble, but R160A EAL is catalytically inactive and reacts with substrates to form magnetically isolated Co^II and unidentified radical species. R160A EAL activity is resurrected by externally added guanidinium to 2.3% of wild-type EAL. R160K EAL displays catalytic turnover of aminoethanol, with a 180-fold lower value of k_cat/K_M relative to wild-type enzyme. R160K EAL also forms Co^II−substrate radical pair intermediate states during turnover on aminoethanol and (S)-2-aminopropanol substrates. Simulations of the X-band EPR spectra show that the Co^II−substrate radical pair separation distances are increased by 2.1 ± 1.0 Å in R160K EAL relative to wild-type EAL, which corresponds to the predicted 1.6 Å change in arginine versus lysine side chain length. ¹⁴N ESEEM from a hyperfine-coupled protein nitrogen in wild type is absent in R160K EAL, which indicates that a guanidinium ¹⁴N of R160 interacts directly with the substrate radical through a hydrogen bond. ESEEM of the ²H-labeled substrate radical states in wild-type and R160K EAL shows that the native separation distances among the substrate C1 and C2, and coenzyme C5′ reactant centers, are conserved in the mutant protein. The EPR and ESEEM measurements evince a protein-mediated force on the C5′-methyl center that is directed toward the reacting substrate species during the hydrogen atom transfer and radical rearrangement reactions. The results indicate that the positive charge at the residue 160 side chain terminus is required for proper folding of EutB, assembly of a stable EAL oligomer, and catalysis in the assembled oligomer.