Ten putative Trichoderma reesei β-glucosidase (BGL) isozymes were heterologously expressed in Escherichia coli and Aspergillus oryzae and purified to homogeneity. Catalytic properties of nine enzymes which showed hydrolytic activity on cellobiose and p-nitrophenyl-β-D-glucopyranoside (pNPG) were investigated. Three BGLs, encoded by the genes cel3A, cel3B, and cel3E, contained a predicted signal peptide, showed higher hydrolytic activity on cello-oligosaccharides than on pNPG, and preferred longer oligosaccharides. Another three putative extracellular BGLs, Cel3B, Cel3F, and Cel3G, and two intracellular enzymes, Cel3C and Cel3D, exhibited preference for pNPG. Intracellular Cel1A showed the highest affinity for cellobiose as a typical cellobiase. Four BGLs, Cel3A, Cel3B, Cel3E, Cel1A, that showed high activity against cello-oligosaccharides were capable of catalyzing transglycosylation reactions from cellobiose, leading to formation of cellotriose and isomeric glucobioses. While Cel3A, Cel3B, and Cel3E synthesized mainly gentiobiose, glycosyl transfer reactions of Cel1A led mainly to sophorose and laminaribiose. Conversion of cellobiose to sophorose by Cel1A reached about 3.6 and 10 % at 1 and 10 % cellobiose concentration, respectively. The formation and persistence of individual cellobiose isomers in incubation mixtures of four BGLs (Cel3A, Cel3B, Cel3E, and Cel1A) with cellobiose correlated well with the k cat values for isomeric glucobioses. Cel1A also showed the lowest sensitivity to inhibition by glucose. Based on all studied catalytic properties, Cel1A appears to be unambiguously the best candidate for site-directed mutations or directed evolution toward improvement of activity, thermostability, and, eventually, efficiency of sophorose synthesis.

The nature and enzymic properties of starch-branching enzyme (SBE) are two of the dominant factors influencing the fine structure of starch. To understand the role of this enzyme's activity in the formation of starch in kidney bean (Phaseolus ulgaris L.), a study was undertaken to identify the major SBE sequences expressed during seed development and to characterize the enzymic properties of the coded recombinant enzymes. Two SBE cDNA species (designated p sbe2 and p sbe1) that displayed significant similarity (more than 70 %) to other family A and B SBEs respectively were isolated. Northern blot analysis revealed that p sbe1 and p sbe2 were differentially expressed during seed development. p sbe2 showed maximum steady-state transcript levels at the mid-stage of seed maturation, whereas p sbe1 reached peak levels at a later stage. Western blot analysis with antisera raised against both recombinant proteins (rPvSBE1 and rPvSBE2) showed that these two SBEs were located in different

Glucose sensitivity and pH and thermal stabilities of Trichoderma reesei Cel1A (Bgl II) were improved by site-directed mutagenesis of only two amino acid residues (L167W or P172L) at the entrance of the active site. The Cel1A mutant showed high glucose tolerance (50% of inhibitory concentration = 650 mM), glucose stimulation (2.0 fold at 50 mM glucose), and enhanced specific activity (2.4-fold) compared with those of the wild-type Cel1A. Furthermore, the mutant enzyme showed stability at a wide pH range of 4.5–9.0 and possessed high thermal stability up to 50°C with 80% of the residual activities compared with the stability seen at the pH range of 6.5–7.0 and temperatures of up to 40°C in the wild-type Cel1A. Kinetic studies for hydrolysis revealed that the Cel1A mutant was competitively inhibited by glucose at similar levels as the wild-type enzyme. Additionally, the mutant enzyme exhibited substrate inhibition, which gradually disappeared with an increasing glucose concentration. These data suggest that the glucose stimulation was caused by relieve the substrate inhibition in the presence of glucose. To conclude, all the properties improved by the mutagenesis would be great advantages in degradation of cellulosic biomass together with cellulases.

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Starch-branching enzymes (SBE) have a dominant role for amylopectin structure as they define chain length and frequency of branch points. We have previously shown that one of the SBE isoforms of kidney bean (Phaseolus vulgaris L.), designated PvSBE2, has a molecular mass (82 kDa) significantly smaller than those reported for isologous SBEs from pea (SBEI), maize (BEIIb), and rice (RBE3). Additionally, in contrast to the dual location of the pea SBEI in both the soluble and starch granule fractions, PvSBE2 was found only in the soluble fraction during seed development. Analysis of a pvsbe2 cDNA suggested that PvSBE2 is generated from a larger precursor with a putative plastid targeting sequence of 156 residues. Here we describe the occurrence of a larger 100-kDa form (LF-PvSBE2) of PvSBE2 found both in the soluble and starch granule fractions of the developing seeds. The determined N-terminal sequence, VKSSHDSD, of LFPvSBE2 corresponded to a peptide sequence located 111 amino acids upstream from the N terminus of purified PvSBE2, suggesting that LF-PvSBE2 and PvSBE2 are products of the same gene. Analysis of the products by 5-RACE (rapid amplification of cDNA ends) and reverse transcription PCR indicated that the two transcripts for pre-LF-PvSBE2 and pre-PvSBE2 are generated by alternative splicing. Recombinant LF-PvSBE2 (rLF-PvSBE2) was purified from Escherichia coli and the kinetic properties were compared with those of recombinant PvSBE2 (rPvSBE2). rLF-PvSBE2 had much higher affinity for amylopectin (K m ‫؍‬ 4.4 mg/ml) than rPvSBE2 (18.4 mg/ml), whereas the V max of rLF-PvSBE2 (135 units/mg) for this substrate was much lower than that of rPvSBE2 (561 units/mg). These results suggest that the N-terminal extension of LF-PvSBE2 plays a critical role for localization in starch granules by altering its enzymatic properties.Starch, one of the most important carbon reserves in plants, is composed of two glucan polymers, amylose and amylopectin.