Mutations in glucocerebrosidase (GCase), the enzyme deficient in Gaucher 
disease, are a common genetic risk factor for the development of Parkinson 
disease and related disorders, implicating the role of this lysosomal hydrolase 
in the disease etiology. A specific physical interaction exists between the 
Parkinson disease-related protein α-synuclein (α-syn) and GCase both in solution 
and on the lipid membrane, resulting in efficient enzyme inhibition. Here, 
neutron reflectometry was employed as a first direct structural characterization 
of GCase and α-syn·GCase complex on a sparsely-tethered lipid bilayer, revealing 
the orientation of the membrane-bound GCase. GCase binds to and partially 
inserts into the bilayer with its active site most likely lying just above the 
membrane-water interface. The interaction was further characterized by intrinsic 
Trp fluorescence, circular dichroism, and surface plasmon resonance 
spectroscopy. Both Trp fluorescence and neutron reflectometry results suggest a 
rearrangement of loops surrounding the catalytic site, where they extend into 
the hydrocarbon chain region of the outer leaflet. Taking advantage of 
contrasting neutron scattering length densities, the use of deuterated α-syn 
versus protiated GCase showed a large change in the membrane-bound structure of 
α-syn in the complex. We propose a model of α-syn·GCase on the membrane, 
providing structural insights into inhibition of GCase by α-syn. The interaction 
displaces GCase away from the membrane, possibly impeding substrate access and 
perturbing the active site. GCase greatly alters membrane-bound α-syn, moving 
helical residues away from the bilayer, which could impact the degradation of 
α-syn in the lysosome where these two proteins interact.