Interaction of Trimeresurus flavoviridis Phospholipase A2 and Its Fragment with Calcium Ion

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Abstract
Dimeric T. flavoviridis phospholipase A2 has been studied in terms of the interaction with essential Ca2+ by equilibrium gel filtration, ultraviolet difference spectroscopy, fluorescence measurements, and chemical modifications with p-bromophenacyl bromide. The subunit bound to Ca2+ with a 1 : 1 molar ratio and no cooperative binding was observed. The hypochromic effect produced upon the binding of Ca2+ is due to perturbation of (a) specific tryptophan residues) located in the vicinity of the active site and appears to be characteristic of this enzyme. On the basis of the pH dependence of the dissociation constants, it has been found that the α-amino group (pK2 8.7) controls the binding of Ca2+. Deprotonation of the a-amino group is possibly accompanied by conformational transition to the active form which is able to bind Ca2+. This is in contrast to the case of bovine pancreatic phospholipase A2 in which Asp-49 (pKa 5.2) is responsible for the metal ion binding (Fleer et al. (1981) Eur. J. Biochem. 113, 283–288). Des-octapeptide(l-8>phospho-lipase A, (L-fragment) was found to be capable of binding Ca2+ under the control of a group with a pKa of 7.6. This pKa value was similar to an apparent pKa of 7.5 determined for the. histidine residue in the active site of the native enzyme by way of p-bromophenacyl bromide modification. It appears that the N-terminal (octapeptide) sequence affects the binding mode of Ca2+, possibly because of conformational transition arising from its removal. The reinvestigation showed that the N-terminal octapeptide sequence is Gly-Leu-Trp-Gln-Phe-Glu-Asn-Met.

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