Catalytic modules of assembly-line polyketide synthases (PKSs) have previously been observed in two very different conformations an "extended" architecture and an "arch-shaped" architecture although the catalytic relevance of neither has been directly established. By the use of a fully human naive antigen-binding fragment (F-ab) library, a high-affinity antibody was identified that bound to the extended conformation of a PKS module, as verified by X-ray crystallography and tandem size-exclusion chromatography small-angle X-ray scattering (SEC-SAXS). Kinetic analysis proved that this antibody stabilized module conformation was fully competent for catalysis of intermodular polyketide chain translocation as well as intramodular polyketide chain elongation and functional group modification of a growing polyketide chain. Thus, the extended conformation of a PKS module is fully competent for all of its essential catalytic functions.