Mimosa pudica apyrase requires polysaccharide and Ca2+ for the activity

Purification and biochemical characterization of a novel ecto-apyrase, MP67, from Mimosa pudica

We have previously reported the presence of an apyrase in Mimosa pudica. However, only limited information is available for this enzyme. Thus, in this study, the apyrase was purified to homogeneity. The purified enzyme had a molecular mass of around 67 kD and was able to hydrolyze both nucleotide triphosphate and nucleotide diphosphate as substrates. The ratio of ATP to ADP hydrolysis velocity of the purified protein was 0.01 in the presence of calcium ion, showing extremely high substrate specificity toward ADP. Thus, we designated this novel apyrase as MP67. A cDNA clone of MP67 was obtained using primers designed from the amino acid sequence of trypsin-digested fragments of the protein. In addition, rapid amplification of cDNA ends-polymerase chain reaction was performed to clone a conventional apyrase (MpAPY2). Comparison of the deduced amino acid sequences showed that MP67 is similar to ecto-apyrases; however, it was distinct from conventional apyrase based on phylogenetic classification. MP67 and MpAPY2 were expressed in Escherichia coli, and the recombinant proteins were purified. The recombinant MP67 showed high substrate specificity toward ADP rather than ATP. A polyclonal antibody raised against the recombinant MP67 was used to examine the tissue distribution and localization of native MP67 in the plant. The results showed that MP67 was ubiquitously distributed in various tissues, most abundantly in leaves, and was localized to plasma membranes. Thus, MP67 is a novel ecto-apyrase with extremely high substrate specificity for ADP.

Differences in nucleotide-binding site of isoapyrases deduced from tryptophan fluorescence

Comparative studies of intrinsic and extrinsic fluorescence of apyrases purified from two potato tuber varieties (Pimpernel and Desirée) were performed to determine differences in the microenvironment of the nucleotide binding site. The dissociation constants (K(d)) of Pimpernel apyrase for the binding of different fluorescent substrate analogs: methylanthranoyl (MANT-), trinitrophenyl (TNP-), and epsilon -derivatives of ATP and ADP were determined from the quenching of Trp fluorescence, and compared with K(d) values previously reported for Desirée enzyme. Binding of non-fluorescent substrate analogues decreased the Trp emission of both isoapyrases, indicating conformational changes in the vicinity of these residues. Similar effect was observed with fluorescent derivatives where, in the quenching effect, the transfer of energy from tryptophan residues to the fluorophore moiety could be additionally involved. The existence of energy transfer between Trp residues in the Pimpernel enzyme was demonstrated with epsilon -analogues, similar to our previous observations with the Desirée. From these results we deduced that tryptophan residues are close to or in the nucleotide binding site in both enzymes. Experiments with quenchers like acrylamide, Cs(+) and I(-), both in the presence and absence of nucleotide analogues, suggest the existence of differences in the nucleotide binding site of the two enzymes. From the results obtained in this work, we can conclude that the differences found in the microenvironment of the nucleotide binding site can explain, at least in part, the kinetic behaviour of both isoenzymes.

Characterization of diphosphonucleotide phosphatase/phosphodiesterase from yellow lupin (Lupinus luteus) seeds

A phosphatase cleaving the pyrophosphate bond in diphosphonucleotides and phosphodiester bond in various phosphodiesters (pH optimum at 6.25) was purified from yellow lupin (Lupinus luteus L.) seeds. The enzyme is 75 kDa monomeric glycoprotein (pI=6.4) with 4.4% of carbohydrate (mannose, N-acetylglucosamine, fucose and xylose). Analysis of its partial amino acid sequence (8 peptides, 101 amino acid residues) together with no divalent cation requirements for catalysis points out that the purified enzyme is different from known plant pyrophosphate cleaving enzymes (apyrases and inorganic pyrophosphatases). Its physiological role could be related to a regulation of diphosphonucleotides level in plant metabolism.