Use of protein engineering to explore subunit interactions in an allosteric enzyme: construction of inter-subunit hybrids in Clostridium symbiosum glutamate dehydrogenase.

Hybrids of different forms of clostridial glutamate dehydrogenase (GDH) have been constructed in order to probe the basis of allosteric interaction in this hexameric enzyme. It was shown that the C320S mutant, which is fully active and shows allosteric behaviour similar to that of the wild-type enzyme, can also be renatured after unfolding in urea. Mixtures of unfolded wild-type and C320S subunits gave rise to hybrids upon refolding. A purely random reassembly would lead to a simple binomial distribution. However there was a slightly better overall recovery of wild-type subunits and there appears to be a tendency for rapidly formed structured wild-type subunits in a mixture to nucleate further refolding in a way that biases the final distribution against the formation of C320S hexamers. Only the wild-type subunits in such hybrid mixtures are able to react with Ellman's reagent, 5,5'-dithiobis-(2-nitrobenzoate) (DTNB). Accordingly, after modification of hybrid hexamers with DTNB only the mutant subunits can bind NAD+. This permits fractionation on an NAD+-agarose affinity column. The elution pattern in itself indicates cooperativity since DTNB modification of just one subunit in a 1:5 wild-type/C320S hybrid largely abolished binding to the column. Kinetic studies were carried out on a fractionated preparation in which hexamers containing only one C320S subunit and five wild-type subunits were the predominant active species. Measurements of activity were made both before and after treatment with an excess of beta-mercaptoethanol to remove the blocking thionitrobenzoate moieties. Before beta-mercaptoethanol treatment this sample, with only one active subunit per hexamer, gave strictly hyperbolic (Michaelis-Menten) kinetics with NAD+ at pH 7.0, whereas after beta-mercaptoethanol (all six subunits now active) the markedly kinked Eadie-Hofstee plot characteristic of wild-type enzyme was obtained. On the other hand the sigmoid response to glutamate at high pH persisted (Hill coefficient=3.6) even without beta-mercaptoethanol, reflecting the fact that the inactive subunits can still bind glutamate. Beta-mercaptoethanol treatment restored full positive cooperativity (Hill coefficient=5.2). These results prove beyond doubt that the non-classical kinetic behaviour of clostridial GDH is a direct result of interaction between NAD+ binding sites on the six (normally) identical subunits of a hexamer.