The Activation of MDH by Ammonia and Amines

As usnally prepared, MDH has an absolute requirement for ammonium or methylammonium salts in the dye-linked assay system. The relatively higher concentrations required at lower pH values suggest that the 

FIGURE 3. Kinetic scheme for the activation of Ba-MDH by ammonia. The combination of enzyme (E) with substrate (S) is not independent of its combination with activator (a), so E and EA have different affinities for the substrate. The equation for the reaction is as follows V = V /(l + KVS)[1 + K /a (1 + S/Ks)/(1+ S/K%)]. The values given are for the dissociation constants for each complex (mM). 

Kinetic analysis has also shown that there is a second (inhibitory) binding site for ammonia with Kjof SOnlOO mM spectroscopic evidence suggests that this may be due to formation of an imino adduct with the PQQ (Harris and Davidson, 1993 Goodwin and Anthony, 1996). 

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