Experiments at DESY’s X-ray source PETRA III have revealed the first structure of a biological transporter protein loaded with a pro-drug. Prodrugs are inactive medications that are metabolized into an active, functional form within the body. They can hijack the human nutrient transporter proteins PepT1 or PepT2 that are located in the cell membrane and usually channel peptides derived from digested food into cells. This way, pro-drugs can hitch a ride directly into the cell, accelerating the drug’s absorption. While the pro-drug concept is very effective, little is known at the molecular level about the peptide transporters’ structure and how they recognize, bind and transport pro-drugs. The groups of Christian Loew and Jan Kosinski from the European Molecular Biology Laboratory (EMBL) working at the Centre for Structural Systems Biology (CSSB) on DESY campus in Hamburg are now a step closer to understanding this.