Mechanistic analysis of nonribosomal peptide synthetases Considering the ongoing rise of the multidrug resistant bacterial infections it is essential to expand the available repertoire of therapeutic agents. Microbial natural products are an indispensable source of novel activities and continue to serve as our main provider of antibiotics and chemotherapeutics. Nonribosomal peptides are among the most widespread natural products in bacteria and fungi. Their importance is best illustrated by their complexity and the amounts of resources dedicated to building the underlying biosynthetic machineries nonribosomal peptide synthetases (NRPS). These gigantic multidomain enzymes synthesize peptides by linking individual amino acid units in an assembly line fashion. Six decades of NRPS research have resulted in several remarkable tailoring successes. However the lack of mechanistic understanding of the inner workings of NRPSs has prevented the development of a general workflow which would reliably generate functional enzymes and new drugs. Aspiring to alleviate these obstacles this thesis offers critical insights into adenylation and the interplay with condensation two fundamental NRPS reactions.