Mechanism of adenylate kinase. Are the essential lysines essential?

Using site-specific mutagenesis, we have probed the structural and functional roles of lysine-21 and lysine-27 of adenylate kinase (AK) from chicken muscle expressed in Escherichia coli. The two residues were chosen since according to the nuclear magnetic resonance (NMR) model [Mildvan, A. S., & Fry, D. C. (1987) Adv. Enzymol. 58, 241-313], they are located near the alpha- and the gamma-phosphates, respectively, of adenosine 5'-triphosphate (ATP) in the AK-MgATP complex. In addition, a lysine residue (Lys-21 in the case of AK) along with a glycine-rich loop is considered "essential" in the catalysis of kinases and other nucleotide binding proteins. The Lys-27 to methionine (K27M) mutant showed only slight increases in kcat and Km, but a substantial increase (1.8 kcal/mol) in the free energy of unfolding, relative to the WT AK. For proper interpretation of the steady-state kinetic data, viscosity-dependent kinetics was used to show that the chemical step is partially rate-limiting in the catalysis of AK. Computer modeling suggested that the folded form of K27M could gain stability (relative to the wild type) via hydrophobic interactions of Met-27 with Val-179 and Phe-183 and/or formation of a charge-transfer complex between Met-27 and Phe-183. The latter was supported by an upfield shift of the methyl protons of Met-27 in 1H NMR. Other than this, the 1H NMR spectrum of K27M is very similar to that of WT, suggesting little perturbation in the global or even local conformations.(ABSTRACT TRUNCATED AT 250 WORDS)