Adenosylcobalamin-dependent glutamate mutase: properties of a fusion protein in which the cobalamin-binding subunit is linked to the catalytic subunit.

Adenosylcobalamin-dependent glutamate mutase (EC 5.4.99.1) from Clostridium tetanomorphum comprises two protein components, MutE and MutS. The formation of the holoenzyme is a kinetically complex process that involves the co-operative association of MutS, MutE and adenosylcobalamin. The MutS portion of the cobalamin-binding site is conserved within a group of adenosylcobalamin-dependent enzymes that catalyse similar isomerizations. However, in contrast with glutamate mutase, in these other enzymes the cobalamin-binding region represented by MutS is present as a C-terminal domain. We have investigated the effect on the structural and kinetic properties of glutamate mutase of linking MutS to the C-terminus of MutE. Kinetic analysis of this protein, MutES, showed, unexpectedly, that enzyme activity was still co-operatively dependent on protein concentration. The Km for L-glutamate was unchanged from the wild type, whereas Vmax was decreased to approx. one-thirtieth and the Km for coenzyme increased approx. 10-fold. Investigation of the quaternary structure of MutES by equilibrium ultra-centrifugation indicated that the protein existed in equilibrium between monomeric and dimeric forms. Thus linking MutE and MutS together seems to substantially weaken the contacts that are responsible for the dimerization of MutE. The two domains of the MutES monomer seem unable to communicate, so that active enzyme is formed by the intermolecular association of two MutES subunits in a co-operative manner.