NAD-BD sits on the top of Glu-BD. NAD-BD and Glu-BD form the catalytic cleft. During substrate binding, the NAD-BD moves significantly. This movement has two components, rotating along the long axis of a helix at the back of the NAD-BD, called "the pivot helix", and twisting about the antenna in a clockwise fashion. A comparison of the open and closed conformations of GLUD1 reveals changes in the small helix of the descending strand of the antenna, which seems to recoil as the catalytic cleft opens. Closure of one subunit is associated with distortion of the small helix of the descending strand that is pushed into the antenna of the adjacent subunit. R496 is located on this small helix (see Mutations).