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Electrostatic interactions across the dimer-dimer interface contribute to the pH-dependent stability of a tetrameric
Alexandra Bjørk1, Dimitrios Mantzilas, Reidun Sirevåg
1Department of Biology, University of Oslo, Box 1031 Blindern, N-0316 Oslo, Norway. alexandra.bjork@bio.uio.no
Abstract:
Malate dehydrogenase (MDH) from the moderately thermophilic bacterium Chloroflexus aurantiacus (CaMDH) is a tetrameric enzyme, while MDHs from mesophilic bacteria usually are dimers. Using site-directed mutagenesis, we show here that a network of electrostatic interactions across the extra dimer-dimer interface in CaMDH is important for thermal stability and oligomeric integrity. Stability effects of single point mutations (E25Q, E25K, D56N, D56K) varied from -1.2 degrees C to -26.8 degrees C, and depended strongly on pH. Gel-filtration experiments indicated that the 26.8 degrees C loss in stability observed for the D56K mutant at low pH was accompanied by a shift towards a lower oligomerization state.