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The Bacillus subtilis Sfp protein activates the peptidyl carrier protein (PCP) domains of surfactin synthetase by transferring the 4′-phosphopantetheinyl moiety of coenzyme A (CoA) to a serine residue conserved in all PCPs. Its wide PCP substrate spectrum renders Sfp a biotechnologically valuable enzyme for use in combinatorial non-ribosomal peptide synthesis. The structure of the Sfp-CoA complex determined at 1.8 Å resolution reveals a novel α/β-fold exhibiting an unexpected intra-molecular 2-fold pseudosymmetry. This suggests a similar fold and dimerization mode for the homo-dimeric phosphopantetheinyl transferases such as acyl carrier protein synthase. The active site of Sfp accommodates a magnesium ion, which is complexed by the CoA pyrophosphate, the side chains of three acidic amino acids and one water molecule. CoA is bound in a fashion that differs in many aspects from all known CoA-protein complex structures. The structure reveals regions likely to be involved in the interaction with the PCP substrate. [ABSTRACT FROM AUTHOR]