Evolution of quaternary structure in a homotetrameric protein

The reason that many proteins exist as oligomeric species in their native form is one of the major unsolved problems in structural biology. In order to explore the role of quaternary structure, dimeric variants of a homotetrameric protein, Escherichia coli dihydrodipicolinate synthase, were engineered and their properties compared to the wild-type, tetrameric form. X-ray crystallography reveals that the active site is not disturbed when the quaternary structure is disrupted. However, the activity of the dimeric enzymes in solution is substantially reduced, and a tetrahedral adduct of a substrate analogue is observed to be trapped at the active site in the crystal form. Remarkably, heating the dimeric enzymes increases activity. We propose that the homotetrameric structure of DHDPS reduces dynamic fluctuations present in the dimeric forms, and increases specificity for the first substrate, pyruvate. By restricting motion in a key catalytic motif, a competing, non-productive reaction of a substrate analogue is avoided. Support for this idea come