Acyl carrier proteins [ACP] are an important component in both fatty acid and polyketide biosynthesis, with the growing chain attached during synthesis as a thiol ester at the distal thiol of a 4'-phosphopantetheine moiety.
The protein is expressed in the inactive apo form and the 4'-phosphopantetheine moiety must be post-translationally attached to a conserved serine residue in the [ACP] amino acid sequence.

The 4'-phosphopantetheinyl transferase (holo-acyl carrier protein synthase (ACPS), EC 2.7.8.7) is in charge of producing the active holo-[ACP], i.e. the form with an O-(pantetheine-4ʼ-phosphoryl)-L-seryl residue. This reaction is described by RHEA:12068. It is a prerequisite for all reactions involving an [ACP] as acyl chain carrier.

To model apo- and holo-[ACP], Rhea focuses on the reactive parts, i.e. the L-serine residue of apo-[ACP] which is transformed into an O-(pantetheine-4ʼ-phosphoryl)-L-serine residue in holo-[ACP] :

Rhea identifier	Rhea name	Reactive part
RHEA-COMP:9690	apo-[ACP]	CHEBI:29999 (L-serine residue)
RHEA-COMP:9685	holo-[ACP]	CHEBI:64479 (O-(pantetheine-4ʼ-phosphoryl)-L-serine residue)

[peptidyl-carrier protein] is another example of carrier protein. Similarly to [ACP], it needs an O-(pantetheine-4ʼ-phosphoryl)-L-seryl residue to be active. Consequently, RHEA-COMP:11480 (holo-[peptidyl-carrier protein]) has the same reactive part as holo-[ACP], i.e. CHEBI:64479.