title: Dissecting the multistep reaction pathway of an RNA enzyme by single-molecule kinetic "fingerprinting".
abstract: Single-molecule FRET is a powerful tool for probing the kinetic mechanism of a complex enzymatic reaction. However, not every reaction intermediate can be identified via a distinct FRET value, making it difficult to fully dissect a multistep reaction pathway. Here, we demonstrate a method using sequential kinetic experiments to differentiate each reaction intermediate by a distinct time sequence of FRET signal (a kinetic "fingerprint"). Our model system, the two-way junction hairpin ribozyme, catalyzes a multistep reversible RNA cleavage reaction, which comprises two structural transition steps (docking/undocking) and one chemical reaction step (cleavage/ligation). Whereas the docked and undocked forms of the enzyme display distinct FRET values, the cleaved and ligated forms do not. To overcome this difficulty, we used Mg(2+) pulse-chase experiments to differentiate each reaction intermediate by a distinct kinetic fingerprint at the single-molecule level. This method allowed us to unambiguously determine the rate constant of each reaction step and fully characterize the reaction pathway by using the chemically competent enzyme-substrate complex. We found that the ligated form of the enzyme highly favors the docked state, whereas undocking becomes accelerated upon cleavage by two orders of magnitude, a result different from that obtained with chemically blocked substrate and product analogs. The overall cleavage reaction is rate-limited by the docking/undocking kinetics and the internal cleavage/ligation equilibrium, contrasting the rate-limiting mechanism of the four-way junction ribozyme. These results underscore the kinetic interdependence of reversible steps on an enzymatic reaction pathway and demonstrate a potentially general route to dissect them.
authors.affiliation: Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138, USA.
authors.forename: Shixin
authors.initials: S
authors.lastname: Liu
authors.forename: Gregory
authors.initials: G
authors.lastname: Bokinsky
authors.forename: Nils G
authors.initials: NG
authors.lastname: Walter
authors.forename: Xiaowei
authors.initials: X
authors.lastname: Zhuang
mesh_terms: D002384:Catalysis; D031541:Fluorescence Resonance Energy Transfer; D005821:Genetic Techniques; D007700:Kinetics; D009690:Nucleic Acid Conformation; D011485:Protein Binding; D012313:RNA / Q000737:chemistry / Q000378:metabolism; D016337:RNA, Catalytic / Q000737:chemistry* / Q000378:metabolism*; D013816:Thermodynamics
chemical_list: D016337:RNA, Catalytic; D012313:RNA