A SMALL, THERMOSTABLE, AND MONOFUNCTIONAL CHORISMATE MUTASE FROM THE ARCHEON METHANOCOCCUS-JANNASCHII

The gene for chorismate mutase (CM) from the archeon Methanococcus jan naschii, an extreme thermophile, was subcloned and expressed in Escher ichia coli. This gene, which belongs to the aroQ class of CMs, encodes a monofunctional enzyme (AroQ(f)) able to complement the CM deficienc y 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 acti vation parameters [Delta H double dagger = 16.2 kcal/mol, Delta S doub le dagger = -1.7 cal/(mol K)] are similar to those of another well cha racterized AroQ class CM, the mesophilic AroQ(p) domain from E. coli. Like AroQ(p), the thermophilic CM is an alpha-helical dimer, but appro ximately 5 kcal/mol more stable than its mesophilic counterpart as jud ged from equilibrium denaturation studies, The possible origins of the thermostability of M. jannaschii AroQ(f), the smallest natural CM cha racterized to date, are discussed in light of available sequence and t ertiary structural information.