Inorganic polyphosphate chelates Ca2+ but cannot substitute for ATP as an energy source for SERCA.

Inorganic polyphosphate (polyP) is a high-energy polymer with phosphoanhydride bonds analogous to ATP, but whether it can serve as a phosphoryl donor for mammalian ATP-dependent enzymes remains unclear. Because PMCA was previously reported to utilize polyP in human erythrocytes, we tested whether the closely related sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) can use polyP to drive Ca2+ transport or support polyphosphate kinase-like activity. Protein BLAST identified SERCA as the closest non-PMCA homolog in the human proteome, and docking simulations predicted polyP-14 binding to SERCA with affinities comparable to ATP. However, functional assays with isolated SR vesicles showed that polyP neither supported SERCA-mediated Ca2+ uptake nor detectable polyphosphate kinase-like activity. Instead, fluorescence assays and 500 ns molecular dynamics simulations demonstrated direct Ca2+ chelation by polyP, with stable coordination of 7-9 Ca2+ ions per polyP-14 chain. Unexpectedly, ADP, but not AMP, induced concentration-dependent, thapsigargin-sensitive Ca2+ uptake. Thus, polyP is a potent Ca2+ chelator but cannot substitute for ATP as an energy source for SERCA.