期刊
JOURNAL OF BIOLOGICAL CHEMISTRY
卷 283, 期 42, 页码 28641-28648出版社
AMER SOC BIOCHEMISTRY MOLECULAR BIOLOGY INC
DOI: 10.1074/jbc.M802560200
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资金
- Korea Research Institute of Bioscience and Biotechnology Research Initiative
- Ministry of Marine Affairs and Fisheries, Republic of Korea
- Korea Research Foundation Grant [KRF-2006-005-J04703]
- National Research Foundation of Korea [2006-005-J04703, 2008-06529] Funding Source: Korea Institute of Science & Technology Information (KISTI), National Science & Technology Information Service (NTIS)
TreX is an archaeal glycogen-debranching enzyme that exists in two oligomeric states in solution, as a dimer and tetramer. Unlike its homologs, TreX from Sulfolobus solfataricus shows dual activities for alpha-1,4-transferase and alpha-1,6-glucosidase. To understand this bifunctional mechanism, we determined the crystal structure of TreX in complex with an acarbose ligand. The acarbose intermediate was covalently bound to Asp(363), occupying subsites -1 to -3. Although generally similar to the monomeric structure of isoamylase, TreX exhibits two different active-site configurations depending on its oligomeric state. The N terminus of one subunit is located at the active site of the other molecule, resulting in a reshaping of the active site in the tetramer. This is accompanied by a large shift in the flexible loop (amino acids 399-416), creating connected holes inside the tetramer. Mutations in the N-terminal region result in a sharp increase in alpha-1,4-transferase activity and a reduced level of alpha-1,6-glucosidase activity. On the basis of geometrical analysis of the active site and mutational study, we suggest that the structural lid (acids 99-97) at the active site generated by the tetramerization is closely associated with the bifunctionality and in particular with the alpha-1,4-transferase activity. These results provide a structural basis for the modulation of activities upon TreX oligomerization that may represent a common mode of action for other glycogen-debranching enzymes in higher organisms.
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