NGAL is a 178 amino acids human protein, belonging to the siderocalin family. It is found in secretory vesicles of granulocytes. Siderocalins are a subclass of lipocalins. This protein family is characterized structurally by the formation of an eightstranded, antiparallel β-barrel and functionally by binding small, hydrophobic ligands. The ligands of siderocalins are so called siderophores, iron-chelating molecules. Siderophores are for instance excreted by bacteria, in order to capture iron

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... m its host and transport it into the cell. Due to its low bioavailability, iron often accounts as limiting factor for bacterial growth. In this context siderocalins can be seen as the host's mechanism to counteract iron-depletion. Besides this function in bacterial infection, NGAL plays a role in various other processes, e.g. inflammation, iron delivery, tissue differentiation and cancer progression. These functions are most likely intimately connected to the ligand binding properties. Q83 is the quail homologue of NGAL and is essentially less examined. In spite of low sequence identity with NGAL, it shows very similar functions and an analog structural fold. Q83's newfound secondary binding site for fatty acids (46) might be a key feature for deciphering its pleiotropic functions. In this thesis the binding properties of NGAL and Q83 to different ligands were investigated. Fluorescence quenching experiments and macromolecular NMR spectroscopy on various Q83 mutants provided detailed information on the siderophore binding mechanism. Furthermore the fatty acid binding site of Q83 was characterized. An insight into the dynamics of free NGAL and the protein-ligand complex, with the bound siderophore enterobactin, was given using NMR spectroscopic methods.