Piecing together nonribosomal peptide synthesis.

Nonribosomal peptide synthetases (NRPSs) produce peptide products with wide-ranging biological activities. NRPSs are macromolecular machines with modular assembly-line logic, a complex catalytic cycle, moving parts and multiple active sites. They are organized into repeating sets of domains, called modules. Each module contains all functionality to introduce a building block into the growing peptide, many also perform cosynthetic tailoring. Structures of individual domains have provided insights into their catalytic mechanisms, but with one exception, larger NRPS proteins were refractory to structure determination. Recently, structure determination succeeded for four multi-domain NRPS proteins: an alternative formylating initiation and two termination modules as well as a large cross-module construct. This review highlights how these data, together with novel didomain structures, contribute to a holistic view of the architecture, domain-domain interactions and conformational changes in NRPS megaenzymes.