Phosphopantetheine adenylyltransferase (PPAT). PPAT is an essential enzyme in bacteria, responsible for catalyzing the rate-limiting step in coenzyme A (CoA) biosynthesis. The dinucleotide-binding fold of PPAT is homologous to class I aminoacyl-tRNA synthetases. CoA has been shown to inhibit PPAT and competes with ATP, PhP, and dPCoA. PPAT is a homohexamer in E. coli.