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Title: Interplay of catalysis, fidelity, threading, and processivity in the exo- and endonucleolytic reactions of human exonuclease I

Abstract

Human exonuclease 1 (hExo1) is a member of the RAD2/XPG structure-specific 5'-nuclease superfamily. Its dominant, processive 5'–3' exonuclease and secondary 5'-flap endonuclease activities participate in various DNA repair, recombination, and replication processes. A single active site processes both recessed ends and 5'-flap substrates. By initiating enzyme reactions in crystals, we have trapped hExo1 reaction intermediates that reveal structures of these substrates before and after their exo- and endonucleolytic cleavage, as well as structures of uncleaved, unthreaded, and partially threaded 5' flaps. Their distinctive 5' ends are accommodated by a small, mobile arch in the active site that binds recessed ends at its base and threads 5' flaps through a narrow aperture within its interior. A sequence of successive, interlocking conformational changes guides the two substrate types into a shared reaction mechanism that catalyzes their cleavage by an elaborated variant of the two-metal, in-line hydrolysis mechanism. Coupling of substrate-dependent arch motions to transition-state stabilization suppresses inappropriate or premature cleavage, enhancing processing fidelity. The striking reduction in flap conformational entropy is catalyzed, in part, by arch motions and transient binding interactions between the flap and unprocessed DNA strand. At the end of the observed reaction sequence, hExo1 resets without relinquishing DNA binding,more » suggesting a structural basis for its processivity.« less

Authors:
; ;
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States). Advanced Photon Source (APS)
Sponsoring Org.:
National Institutes of Health (NIH)
OSTI Identifier:
1373796
Resource Type:
Journal Article
Resource Relation:
Journal Name: Proceedings of the National Academy of Sciences of the United States of America; Journal Volume: 114; Journal Issue: 23
Country of Publication:
United States
Language:
ENGLISH
Subject:
59 BASIC BIOLOGICAL SCIENCES; 37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY

Citation Formats

Shi, Yuqian, Hellinga, Homme W., and Beese, Lorena S. Interplay of catalysis, fidelity, threading, and processivity in the exo- and endonucleolytic reactions of human exonuclease I. United States: N. p., 2017. Web. doi:10.1073/pnas.1704845114.
Shi, Yuqian, Hellinga, Homme W., & Beese, Lorena S. Interplay of catalysis, fidelity, threading, and processivity in the exo- and endonucleolytic reactions of human exonuclease I. United States. doi:10.1073/pnas.1704845114.
Shi, Yuqian, Hellinga, Homme W., and Beese, Lorena S. 2017. "Interplay of catalysis, fidelity, threading, and processivity in the exo- and endonucleolytic reactions of human exonuclease I". United States. doi:10.1073/pnas.1704845114.
@article{osti_1373796,
title = {Interplay of catalysis, fidelity, threading, and processivity in the exo- and endonucleolytic reactions of human exonuclease I},
author = {Shi, Yuqian and Hellinga, Homme W. and Beese, Lorena S.},
abstractNote = {Human exonuclease 1 (hExo1) is a member of the RAD2/XPG structure-specific 5'-nuclease superfamily. Its dominant, processive 5'–3' exonuclease and secondary 5'-flap endonuclease activities participate in various DNA repair, recombination, and replication processes. A single active site processes both recessed ends and 5'-flap substrates. By initiating enzyme reactions in crystals, we have trapped hExo1 reaction intermediates that reveal structures of these substrates before and after their exo- and endonucleolytic cleavage, as well as structures of uncleaved, unthreaded, and partially threaded 5' flaps. Their distinctive 5' ends are accommodated by a small, mobile arch in the active site that binds recessed ends at its base and threads 5' flaps through a narrow aperture within its interior. A sequence of successive, interlocking conformational changes guides the two substrate types into a shared reaction mechanism that catalyzes their cleavage by an elaborated variant of the two-metal, in-line hydrolysis mechanism. Coupling of substrate-dependent arch motions to transition-state stabilization suppresses inappropriate or premature cleavage, enhancing processing fidelity. The striking reduction in flap conformational entropy is catalyzed, in part, by arch motions and transient binding interactions between the flap and unprocessed DNA strand. At the end of the observed reaction sequence, hExo1 resets without relinquishing DNA binding, suggesting a structural basis for its processivity.},
doi = {10.1073/pnas.1704845114},
journal = {Proceedings of the National Academy of Sciences of the United States of America},
number = 23,
volume = 114,
place = {United States},
year = 2017,
month = 5
}
  • No abstract prepared.
  • The {lambda} exonuclease is an ATP-independent enzyme that binds to dsDNA ends and processively digests the 5'-ended strand to form 5' mononucleotides and a long 3' overhang. The crystal structure of {lambda} exonuclease revealed a toroidal homotrimer with a central funnel-shaped channel for tracking along the DNA, and a mechanism for processivity based on topological linkage of the trimer to the DNA was proposed. Here, we have determined the crystal structure of {lambda} exonuclease in complex with DNA at 1.88-{angstrom} resolution. The structure reveals that the enzyme unwinds the DNA prior to cleavage, such that two nucleotides of the 5'-endedmore » strand insert into the active site of one subunit of the trimer, while the 3'-ended strand passes through the central channel to emerge out the back of the trimer. Unwinding of the DNA is facilitated by several apolar residues, including Leu78, that wedge into the base pairs at the single/double-strand junction to form favorable hydrophobic interactions. The terminal 5' phosphate of the DNA binds to a positively charged pocket buried at the end of the active site, while the scissile phosphate bridges two active site Mg{sup 2+} ions. Our data suggest a mechanism for processivity in which wedging of Leu78 and other apolar residues into the base pairs of the DNA restricts backward movement, whereas attraction of the 5' phosphate to the positively charged pocket drives forward movement of the enzyme along the DNA at each cycle of the reaction. Thus, processivity of {lambda} exonuclease operates not only at the level of the trimer, but also at the level of the monomer.« less
  • An exonuclease, DNase VII, has been purified 6000-fold from human placenta. The enzyme has an apparent molecular weight of 43,000, requires Mg/sup 2 +/ for activity, and has a pH optimum of 7.8. The enzyme hydrolyzes single-stranded and nicked duplex DNA at the same rate proceeding in a 3' ..-->.. 5' direction liberating 5'-mononucleotides. It does not measurably hydrolyze polyribonucleotides.