Living cells and organelles are surrounded by membranes that contain a variety of embedded proteins catalyzing processes that are essential for cells to grow and maintain their activities. We investigate the structures and mechanisms of membrane proteins catalyzing transport reactions such as nutrient uptake, drug extrusion, or lipid flipping. We also study the process of N-linked protein glycosylation, catalyzed by oligosaccharyltransferase. Our goal is to assemble molecular "movies" of these essential biochemical reactions by combining high resolution protein structures, obtained by X-ray crystallography, with biochemical and biophysical studies. For a description of ongoing projects, please check out "Research."