Analysis of myo-inositol hexakisphosphate hydrolysis by Bacillus phytase: indication of a novel reaction mechanism.

Phytic acid (myo-inositol hexakisphosphate, InsP(6)) hydrolysis by Bacillus phytase (PhyC) was studied. The enzyme hydrolyses only three phosphates from phytic acid. Moreover, the enzyme seems to prefer the hydrolysis of every second phosphate over that of adjacent ones. Furthermore, it is very likely that the enzyme has two alternative pathways for the hydrolysis of phytic acid, resulting in two different myo-inositol trisphosphate end products: Ins(2,4,6)P(3) and Ins(1,3,5)P(3). These results, together with inhibition studies with fluoride, vanadate, substrate and a substrate analogue, indicate a reaction mechanism different from that of other phytases. By combining the data presented in this study with (1) structural information obtained from the crystal structure of Bacillus amyloliquefaciens phytase [Ha, Oh, Shin, Kim, Oh, Kim, Choi and Oh (2000) Nat. Struct. Biol. 7, 147-153], and (2) computer-modelling analyses of enzyme-substrate complexes, a novel mode of phytic acid hydrolysis is proposed.

Extreme halophiles synthesize betaine from glycine by methylation.

Glycine betaine is a compatible solute, which is able to restore and maintain osmotic balance of living cells. It is synthesized and accumulated in response to abiotic stress. Betaine acts also as a methyl group donor and has a number of important applications including its use as a feed additive. The known biosynthetic pathways of betaine are universal and very well characterized. A number of enzymes catalyzing the two-step oxidation of choline to betaine have been isolated. In this work we have studied a novel betaine biosynthetic pathway in two phylogenically distant extreme halophiles, Actinopolyspora halophila and Ectothiorhodospira halochloris. We have identified a three-step series of methylation reactions from glycine to betaine, which is catalyzed by two methyltransferases, glycine sarcosine methyltransferase and sarcosine dimethylglycine methyltransferase, with partially overlapping substrate specificity. The methyltransferases from the two organisms show high sequence homology. E. halochloris methyltransferase genes were successfully expressed in Escherichia coli, and betaine accumulation and improved salt tolerance were demonstrated.

Expression of Bacillus subtilis phytase in Lactobacillus plantarum 755.

Phytase enzymes can increase the nutritional value of food and feed by liberating inorganic phosphate from phytate, the major storage form of phosphorus in plants. The phytase (phyC) from Bacillus subtilis VTT E-68013 was expressed in Lactobacillus plantarum strain 755 using Lact. amylovorus alpha-amylase secretion signals. In an overnight cultivation in MRS medium containing cellobiose for induction of the alpha-amylase promoter, catalytically active phytase was secreted as a predominant extracellular protein. However, Western blot analysis revealed unprocessed and processed phytase in the cell fraction. Pulse chase experiments showed that the recombinant phytase was secreted at a slower rate in comparison to the native proteins of Lact. plantarum 755.

The metal dependence of Bacillus subtilis phytase.

The metal ion requirement of a Bacillus subtilis phytase has been studied. Removal of metal ions from the enzyme by EDTA resulted in complete inactivation. Circular dichroism spectroscopy was used to study the effect of metal ion removal on the protein conformation. The loss of enzymatic activity is most likely due to a conformational change, as the circular dichroism spectra of holoenzyme and metal-depleted enzyme were different. Metal-depleted enzyme was partially able to restore the active conformation when incubated in the presence of calcium. Only minor reactivation was detected with other divalent metal ions and their combinations. Based on the data we conclude that B. subtilis phytase requires calcium for active conformation. Calcium has also a strong stabilizing effect on the enzyme against thermal denaturation. However, the conformational change resulted by calcium depletion does not affect the protease susceptibility.

Isolation, characterization, molecular gene cloning, and sequencing of a novel phytase from Bacillus subtilis.

The Bacillus subtilis strain VTT E-68013 was chosen for purification and characterization of its excreted phytase. Purified enzyme had maximal phytase activity at pH 7 and 55 degrees C. Isolated enzyme required calcium for its activity and/or stability and was readily inhibited by EDTA. The enzyme proved to be highly specific since, of the substrates tested, only phytate, ADP, and ATP were hydrolyzed (100, 75, and 50% of the relative activity, respectively). The phytase gene (phyC) was cloned from the B. subtilis VTT E-68013 genomic library. The deduced amino acid sequence (383 residues) showed no homology to the sequences of other phytases nor to those of any known phosphatases. PhyC did not have the conserved RHGXRXP sequence found in the active site of known phytases, and therefore PhyC appears not to be a member of the phytase subfamily of histidine acid phosphatases but a novel enzyme having phytase activity. Due to its pH profile and optimum, it could be an interesting candidate for feed applications.

Three-dimensional structures of three engineered cellulose-binding domains of cellobiohydrolase I from Trichoderma reesei.

Three-dimensional solution structures for three engineered, synthetic CBDs (Y5A, Y31A, and Y32A) of cellobiohydrolase I (CBHI) from Trichoderma reesei were studied with nuclear magnetic resonance (NMR) and circular dichroism (CD) spectroscopy. According to CD measurements the antiparallel beta-sheet structure of the CBD fold was preserved in all engineered peptides. The three-dimensional NMR-based structures of Y31A and Y32A revealed only small local changes due to mutations in the flat face of CBD, which is expected to bind to crystalline cellulose. Therefore, the structural roles of Y31 and Y32 are minor, but their functional importance is obvious because these mutants do not bind strongly to cellulose. In the case of Y5A, the disruption of the structural framework at the N-terminus and the complete loss of binding affinity implies that Y5 has both structural and functional significance. The number of aromatic residues and their precise spatial arrangement in the flat face of the type I CBD fold appears to be critical for specific binding. A model for the CBD binding in which the three aligned aromatic rings stack onto every other glucose ring of the cellulose polymer is discussed.