Dihydrodipicolinate synthase (DHDPS) from most thermophilic and mesophilic bacteria form a homo-tetramer that can be described as a dimer of tight dimers. Mutational studies at the tetramerisation interface show that this dimer-of-dimers structure is important for attenuating the conformational dynamics of the tight dimer interface, where the active sites are located. This suggests that a synergistic link between quaternary structure, protein dynamics and catalytic function exists for this model oligomeric enzyme. We therefore hypothesise that DHDPS from a cold bacterium will adopt the minimal quaternary form required for function (i.e. a dimer) given that protein dynamics are attenuated at cold temperatures.

Biophysical and kinetic studies have been performed on recombinant DHDPS from the psychrophilic bacterium Shewanella benthica (Sb-DHDPS). The enzyme crystallises as a dimer, with the number of contacts present at the interface similar to the homo-tetrameric forms found in other organisms. Sb-DHDPS also exists as an active dimer in solution at biologically-relevant temperatures (4-12°C). This psychrophilic dimer has a dissociation constant that is 20-fold weaker than any previously determined dimer dissociation constant for a DHDPS enzyme. However, the protein aggregates and possesses significantly attenuated enzymatic activity at temperatures greater than 20°C, which was not observed in the mesophilic and thermophilic enzymes. Our studies therefore offer insight into the molecular evolution of DHDPS quaternary structure, stability and dynamics that may represent a paradigm for oligomeric enzymes in general.