G Identify which of the following statements about human glycogen debranching enzyme are true based on the HPLC results. The (α‑1→6) glucosidase catalytic center is in the C‑terminal half. The transferase catalytic center can hydrolyze α‑1,6 glycosidic bonds. The oligo‑(α1→4)‑(α1→4) glucanotransferase catalytic center is in the C‑terminal half. Oligo‑(α1→4)‑(α1→4) glucanotransferase activity creates a substrate for (α‑1→6) glucosidase. Based on the peaks in the HPLC charts, what do you think is the most likely substrate for the oligo‑(α1→4)‑(α1→4) glucanotransferase catalytic center of glycogen debranching enzyme? maltotetraosyl and glucosyl residues 6‑O‑α‑glucosyl cyclomaltoheptaose maltosyl and maltotriosyl residues maltoheptaosyl and maltooctaosyl residues cyclomaltoheptaose (β‑cyclodextrin)

A gene, treX, encoding a debranching enzyme previously cloned from the trehalose biosynthesis gene cluster of Sulfolobus solfataricus P2 was expressed in Escherichia coli as a His-tagged protein and the biochemical properties were studied. The specific activity of the S. solfataricus debranching enzyme (TreX) was highest at 75°C and pH 5.5. The enzyme exhibited hydrolysing activity toward α-1,6-glycosidic linkages of amylopectin, glycogen, pullulan, and other branched substrates, and glycogen was the preferred substrate. TreX has a high specificity for hydrolysis of maltohexaosyl α-1,6-β-cyclodextrin, indicating the high preference for side chains consisting of 6 glucose residues or more. The enzyme also exhibited 4-α-sulfoxide-glucan transferase activity, catalysing transfer of α-1,4-glucan oligosaccharides from one chain to another. Dimethyl sulfoxide (10%, v/v) increased the hydrolytic activity of TreX. Gel permeation chromatography and sedimentation equilibrium analytical ultracentrifugation revealed that the enzyme exists mostly as a dimer at pH 7.0, and as a mixture of dimers and tetramers at pH 5.5. Interestingly, TreX existed as a tetramer in the presence of DMSO at pH 5.5–6.5. The tetramer showed a 4-fold higher catalytic efficiency than the dimer. The enzyme catalysed not only intermolecular trans-glycosylation of malto-oligosaccharides (disproportionation) to produce linear α-1,4-glucans, but also intramolecular trans-glycosylation of glycogen. The results presented in this study indicated that TreX may be associated with glycogen metabolism by selective cleavage of the outer side chain.

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