Glutamate 270 plays an essential role in K+-activation and domain closure of Thermus thermophilus isopropylmalate dehydrogenase.

The mutant E270A of Thermus thermophilus 3-isopropylmalate dehydrogenase exhibits largely reduced (~1%) catalytic activity and negligible activation by K⁺ compared to the wild-type enzyme. A 3-4 kcal/mol increase in the activation energy of the catalysed reaction upon this mutation could also be predicted by QM/MM calculations. In the X-ray structure of the E270A mutant a water molecule was observed to take the place of K⁺. SAXS and FRET experiments revealed the essential role of E270 in stabilisation of the active domain-closed conformation of the enzyme. In addition, E270 seems to position K⁺ into close proximity of the nicotinamide ring of NAD⁺ and the electron-withdrawing effect of K⁺ may help to polarise the aromatic ring in order to aid the hydride-transfer. [ABSTRACT FROM AUTHOR]