Calcium ion binding to pancreatic phospholipase A2 and its zymogen: a 43Ca NMR study.
Calcium ion binding to phospholipase A2 and its zymogen has been studied by 43Ca NMR. The temperature dependence of the band shape of the calcium-43 NMR signal has been used to calculate the calcium ion exchange rate. The on-rate was calculated to be 5 X 10(6) M-1 s-1, which is 2 orders of magnitude less than the diffusion limit of the hydrated Ca2+ ion in water. The 43Ca quadrupole coupling constant for calcium ions bound to phospholipase, chi = 1.4 MHz, is significantly larger than those found for EF-hand proteins, indicating a less symmetric site. For prophospholipase A2, we found chi = 0.8 MHz, indicating a calcium binding site, which is somewhat more symmetric than the EF-hand sites. The dependence of the 43Ca NMR band shape on the calcium ion concentration showed that there are two cation binding sites on the phospholipase A2 molecule: K1 = 4 X 10(3) M-1 and K2 = 20 M-1. The strong site was found to be affected by a pKa = 6.5 and the weak site by pKa = 4.5.
D000098:Acetophenones / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D008433:Mathematics; D010179:Pancreas / Q000201:enzymology*; D010740:Phospholipases / Q000378:metabolism*; D010741:Phospholipases A / Q000378:metabolism*; D054467:Phospholipases A2; D011485:Protein Binding; D013552:Swine
D000098:Acetophenones; D010740:Phospholipases; D010741:Phospholipases A; D054467:Phospholipases A2; C006821:4-bromophenacyl bromide; D002118:Calcium