The human neuroendocrine enzyme glutamate decarboxylase (GAD) catalyses the synthesis of the inhibitory neurotransmitter GABA, using pyridoxal-5’-phosphate as cofactor. GAD exists as two isoforms named according to their respective molecular weights, GAD65 and GAD67. GAD65, but not GAD67, is a prevalent autoantigen, with autoantibodies to GAD65 being detected at high frequency in patients with autoimmune (type 1) diabetes and certain other autoimmune disorders. Using Small Angle X-ray Scattering and mutagenesis we describe an investigation into the structure of a complex between GAD65 and a recombinant Fab fragment derived from the human monoclonal antibody b96.11.