Distinctive Features of Catalytic and Transport Mechanisms in Mammalian Sarco-endoplasmic Reticulum Ca2+ ATPase (SERCA) and Cu+ (ATP7A/B) ATPases

摘要

Ca2+ (sarco-endoplasmic reticulum Ca2+ ATPase (SERCA)) and Cu+ (ATP7A/B) ATPases utilize ATP through formation of a phosphoenzyme intermediate (E-P) whereby phosphorylation potential affects affinity and orientation of bound cation. SERCA E-P formation is rate-limited by enzyme activation by Ca2+, demonstrated by the addition of ATP and Ca2+ to SERCA deprived of Ca2+ (E2) as compared with ATP to Ca2+-activated enzyme (E1·2Ca2+). Activation by Ca2+ is slower at low pH (2H+·E2 to E1·2Ca2+) and little sensitive to temperature-dependent activation energy. On the other hand, subsequent (forward or reverse) phosphoenzyme processing is sensitive to activation energy, which relieves conformational constraints limiting Ca2+ translocation. A “H+-gated pathway,” demonstrated by experiments on pH variations, charge transfer, and Glu-309 mutation allows luminal Ca2+ release by H+/Ca2+ exchange. As compared with SERCA, initial utilization of ATP by ATP7A/B is much slower and highly sensitive to temperature-dependent activation energy, suggesting conformational constraints of the headpiece domains. Contrary to SERCA, ATP7B phosphoenzyme cleavage shows much lower temperature dependence than EP formation. ATP-dependent charge transfer in ATP7A and -B is observed, with no variation of net charge upon pH changes and no evidence of Cu+/H+ exchange. As opposed to SERCA after Ca2+ chelation, ATP7A/B does not undergo reverse phosphorylation with Pi after copper chelation unless a large N-metal binding extension segment is deleted. This is attributed to the inactivating interaction of the copper-deprived N-metal binding extension with the headpiece domains. We conclude that in addition to common (P-type) phosphoenzyme intermediate formation, SERCA and ATP7A/B possess distinctive features of catalytic and transport mechanisms.