A small, thermostable, and monofunctional chorismate mutase from the archeon Methanococcus jannaschii

The gene for chorismate mutase (CM) from the archeon Methanococcus jannaschii, an extreme thermophile, was subcloned and expressed in Escherichia coli. This gene, which belongs to the aroQ class of CMs, encodes a monofunctional enzyme (AroQ(f)) able to complement the CM deficiency of an E. coli mutant strain, The purified protein follows Michaelis-Menten kinetics (k(cat) = 5.7 s(-1) and K-m = 41 mu M at 30 degrees C) and displays pH-independent activity in the range of pH 5-9, Its activation parameters [Delta H double dagger = 16.2 kcal/mol, Delta S double dagger = -1.7 cal/(mol K)] are similar to those of another well characterized AroQ class CM, the mesophilic AroQ(p) domain from E. coli. Like AroQ(p), the thermophilic CM is an alpha-helical dimer, but approximately 5 kcal/mol more stable than its mesophilic counterpart as judged from equilibrium denaturation studies, The possible origins of the thermostability of M. jannaschii AroQ(f), the smallest natural CM characterized to date, are discussed in light of available sequence and tertiary structural information.