Structure, catalysis and supramolecular assembly of adenylate kinase from maize

The crystal structure of adenylate kinase from maize ligated with an inhibitor has been determined by molecular replacement and refined to 3.5-A resolution. The enzyme keeps the ATP/ADP/AMP equilibrium in the cell. In the C4 plant maize, it has the special task to recycle the AMP produced in large amounts in primary CO2 assimilation. The established structure explains the side reaction with CMP. Moreover, it shows infinite rods that can be readily discerned in the crystal packing. In comparison with homologues, two structural differences that are crucial for this supramolecular assembly are evident. We propose that the rods represent a natural inactive storage form that assembles at night when maize stops CO2 assimilation and thus most of the AMP production in its C4 cycle. The enzyme is particularly abundant in mesophyll chloroplasts, where such an assembly would release appreciable amounts of water that can be used in other processes during the night.