Part of the propionyl coenzyme A carboxylase (PCC) complex involved in propionate utilization and in the production of the poly(3-hydroxybutyrate-co-3-hydroxyvalerate)(PHBV), which is a water-insoluble biopolymer used as intracellular energy reserve material when cells grow under conditions of nutrient limitation. The complex catalyzes the carboxylation of propionyl-CoA to methylmalonyl-CoA. PCC is also able to catalyze the carboxylation of acetyl-CoA. PccX could be responsible for the interaction of the biotin carboxylase and carboxyltransferase subunits. ATP + hydrogencarbonate + propanoyl-CoA = (S)-methylmalonyl-CoA + ADP + H(+) + phosphate Metabolic intermediate metabolism; propanoyl-CoA degradation; succinyl-CoA from propanoyl-CoA: step 1/3. The propionyl coenzyme A carboxylase (PCC) complex is composed of three subunits: PccA (biotin carboxylase and biotin-carboxyl carrier), PccB (carboxyltransferase) and PccX. Cells lacking pccB and pccX genes accumulate a high level of propionyl-CoA, which would then inhibit CoA-dependent enzymes such as succinyl-CoA synthetase and could partially inhibit the TCA cycle. Also affected in growth and glucose utilization. Cells are unable to produce PHBV and show morphological abnormalitiess such as a decrease of PHBV granules.