Members of this family transfers the 4'-phosphopantetheine (4'-PP) moiety from coenzyme A (CoA) to the invariant serine of pfam00550. This post-translational modification renders holo-ACP capable of acyl group activation via thioesterification of the cysteamine thiol of 4'-PP. This superfamily consists of two subtypes: The ACPS type and the Sfp type. The structure of the Sfp type is known, which shows the active site accommodates a magnesium ion. The most highly conserved regions of the alignment are involved in binding the magnesium ion.