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Džupponová V, Tomášková N, Antošová A, Sedlák E, Žoldák G. Salt-Specific Suppression of the Cold Denaturation of Thermophilic Multidomain Initiation Factor 2. Int J Mol Sci 2023; 24:ijms24076787. [PMID: 37047761 PMCID: PMC10094840 DOI: 10.3390/ijms24076787] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/03/2023] [Revised: 03/30/2023] [Accepted: 04/03/2023] [Indexed: 04/08/2023] Open
Abstract
Thermophilic proteins and enzymes are attractive for use in industrial applications due to their resistance against heat and denaturants. Here, we report on a thermophilic protein that is stable at high temperatures (Ttrs, hot 67 °C) but undergoes significant unfolding at room temperature due to cold denaturation. Little is known about the cold denaturation of thermophilic proteins, although it can significantly limit their applications. We investigated the cold denaturation of thermophilic multidomain protein translation initiation factor 2 (IF2) from Thermus thermophilus. IF2 is a GTPase that binds to ribosomal subunits and initiator fMet-tRNAfMet during the initiation of protein biosynthesis. In the presence of 9 M urea, measurements in the far-UV region by circular dichroism were used to capture details about the secondary structure of full-length IF2 protein and its domains during cold and hot denaturation. Cold denaturation can be suppressed by salt, depending on the type, due to the decreased heat capacity. Thermodynamic analysis and mathematical modeling of the denaturation process showed that salts reduce the cooperativity of denaturation of the IF2 domains, which might be associated with the high frustration between domains. This characteristic of high interdomain frustration may be the key to satisfying numerous diverse contacts with ribosomal subunits, translation factors, and tRNA.
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Affiliation(s)
- Veronika Džupponová
- Department of Biophysics, Faculty of Science, P. J. Šafárik University, Jesenna 5, 04001 Košice, Slovakia
| | - Nataša Tomášková
- Department of Biochemistry, Faculty of Science, P. J. Šafárik University in Košice, Moyzesova 11, 04001 Košice, Slovakia
| | - Andrea Antošová
- Department of Biophysics, Institute of Experimental Physics, Slovak Academy of Sciences, Watsonova 47, 04001 Košice, Slovakia
| | - Erik Sedlák
- Department of Biochemistry, Faculty of Science, P. J. Šafárik University in Košice, Moyzesova 11, 04001 Košice, Slovakia
- Center for Interdisciplinary Biosciences, Technology and Innovation Park P.J. Šafárik University, Trieda SNP 1, 04011 Košice, Slovakia
| | - Gabriel Žoldák
- Center for Interdisciplinary Biosciences, Technology and Innovation Park P.J. Šafárik University, Trieda SNP 1, 04011 Košice, Slovakia
- Center for Interdisciplinary Biosciences, Cassovia New Industry Cluster, Trieda SNP 1, 04011 Košice, Slovakia
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2
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Tajoddin NN, Konermann L. Structural Dynamics of a Thermally Stressed Monoclonal Antibody Characterized by Temperature-Dependent H/D Exchange Mass Spectrometry. Anal Chem 2022; 94:15499-15509. [DOI: 10.1021/acs.analchem.2c03931] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Nastaran N. Tajoddin
- Department of Chemistry, The University of Western Ontario, London, Ontario N6A 5B7, Canada
| | - Lars Konermann
- Department of Chemistry, The University of Western Ontario, London, Ontario N6A 5B7, Canada
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3
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Martin JA, Palmer AG. Comparisons of Ribonuclease HI Homologs and Mutants Uncover a Multistate Model for Substrate Recognition. J Am Chem Soc 2022; 144:5342-5349. [PMID: 35312304 PMCID: PMC9149773 DOI: 10.1021/jacs.1c11897] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Ribonuclease HI (RNHI) nonspecifically cleaves the RNA strand of RNA:DNA hybrid duplexes in a myriad of biological processes. Several RNHI homologs contain an extended domain, termed the handle region, which is critical to substrate binding. Nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) simulations have suggested a kinetic model in which the handle region can exist in open (substrate-binding competent) or closed (substrate-binding incompetent) states in homologs containing arginine or lysine at position 88 (using sequence numbering of E. coli RNHI), while the handle region populates states intermediate between the open and closed conformers in homologs with asparagine at residue 88 [Stafford, K. A., et al., PLoS Comput. Biol. 2013, 9, 1-10]. NMR parameters characterizing handle region dynamics are highly correlated with enzymatic activity for RNHI homologs with two-state (open/closed) handle regions [Martin, J. A., et al., Biochemistry 2020, 59, 3201-3205]. The work presented herein shows that homologs containing asparagine 88 display distinct structural features compared with their counterparts containing arginine or lysine 88. Comparisons of RNHI homologs and site-directed mutants with asparagine 88 support a kinetic model for handle region dynamics that includes 12 unique transitions between eight conformations. Overall, these findings present an example of the structure-function relationships of enzymes and spotlight the use of NMR spectroscopy and MD simulations in uncovering fine details of conformational preferences.
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Affiliation(s)
- James A Martin
- Department of Biological Sciences, Columbia University, New York, New York 10027, United States
| | - Arthur G Palmer
- Department of Biochemistry and Molecular Biophysics, Columbia University, New York, New York 10032, United States
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4
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Tajoddin NN, Konermann L. Analysis of Temperature-Dependent H/D Exchange Mass Spectrometry Experiments. Anal Chem 2020; 92:10058-10067. [DOI: 10.1021/acs.analchem.0c01828] [Citation(s) in RCA: 15] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Affiliation(s)
- Nastaran N. Tajoddin
- Department of Chemistry, The University of Western Ontario, London, Ontario N6A 5B7, Canada
| | - Lars Konermann
- Department of Chemistry, The University of Western Ontario, London, Ontario N6A 5B7, Canada
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Sever AIM, Konermann L. Gas Phase Protein Folding Triggered by Proton Stripping Generates Inside-Out Structures: A Molecular Dynamics Simulation Study. J Phys Chem B 2020; 124:3667-3677. [DOI: 10.1021/acs.jpcb.0c01934] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Affiliation(s)
- Alexander I. M. Sever
- Department of Chemistry, The University of Western Ontario, London, Ontario N6A 5B7, Canada
| | - Lars Konermann
- Department of Chemistry, The University of Western Ontario, London, Ontario N6A 5B7, Canada
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Lim SA, Marqusee S. The burst-phase folding intermediate of ribonuclease H changes conformation over evolutionary history. Biopolymers 2018; 109:e23086. [PMID: 29152711 PMCID: PMC6047922 DOI: 10.1002/bip.23086] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/03/2017] [Revised: 10/23/2017] [Accepted: 10/30/2017] [Indexed: 11/06/2022]
Abstract
The amino acid sequence encodes the energy landscape of a protein. Therefore, we expect evolutionary mutations to change features of the protein energy landscape, including the conformations adopted by a polypeptide as it folds to its native state. Ribonucleases H (RNase H) from Escherichia coli and Thermus thermophilus both fold via a partially folded intermediate in which the core region of the protein (helices A-D and strands 4-5) is structured. Strand 1, however, uniquely contributes to the T. thermophilus RNase H folding intermediate (Icore+1 ), but not the E. coli RNase H intermediate (Icore ) (Rosen & Marqusee, PLoS One 2015). We explore the origin of this difference by characterizing the folding intermediate of seven ancestral RNases H spanning the evolutionary history of these two homologs. Using fragment models with or without strand 1 and FRET probes to characterize the folding intermediate of each ancestor, we find a distinct evolutionary trend across the family-the involvement of strand 1 in the folding intermediate is an ancestral feature that is maintained in the thermophilic lineage and is gradually lost in the mesophilic lineage. Evolutionary sequence changes indeed modulate the conformations present on the folding landscape and altered the folding trajectory of RNase H.
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Affiliation(s)
- Shion An Lim
- Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, CA, United States
- Institute for Quantitative Biosciences (QB3), University of California, Berkeley, Berkeley, CA, United States
| | - Susan Marqusee
- Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, CA, United States
- Institute for Quantitative Biosciences (QB3), University of California, Berkeley, Berkeley, CA, United States
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7
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Zaidi S, Haque MA, Ubaid-ullah S, Prakash A, Hassan MI, Islam A, Batra JK, Ahmad F. Denatured states of yeast cytochrome c induced by heat and guanidinium chloride are structurally and thermodynamically different. J Biomol Struct Dyn 2016; 35:1420-1435. [DOI: 10.1080/07391102.2016.1185039] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/21/2022]
Affiliation(s)
- Sobia Zaidi
- Centre for Interdisciplinary Research in Basic Sciences, Jamia Millia Islamia, New Delhi 110025, India
| | - Md. Anzarul Haque
- Centre for Interdisciplinary Research in Basic Sciences, Jamia Millia Islamia, New Delhi 110025, India
| | - Shah Ubaid-ullah
- Centre for Interdisciplinary Research in Basic Sciences, Jamia Millia Islamia, New Delhi 110025, India
- Department of Biotechnology, School of Life Sciences, Central University of Kashmir (CUK), Sonwar Campus, Srinagar 190004, India
| | - Amresh Prakash
- Centre for Interdisciplinary Research in Basic Sciences, Jamia Millia Islamia, New Delhi 110025, India
| | - Md. Imtaiyaz Hassan
- Centre for Interdisciplinary Research in Basic Sciences, Jamia Millia Islamia, New Delhi 110025, India
| | - Asimul Islam
- Centre for Interdisciplinary Research in Basic Sciences, Jamia Millia Islamia, New Delhi 110025, India
| | - Janendra K. Batra
- Immunochemistry Lab, National Institute of Immunology, Aruna Asaf Ali Marg, New Delhi 110067, India
| | - Faizan Ahmad
- Centre for Interdisciplinary Research in Basic Sciences, Jamia Millia Islamia, New Delhi 110025, India
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Uversky VN. Under-folded proteins: Conformational ensembles and their roles in protein folding, function, and pathogenesis. Biopolymers 2016; 99:870-87. [PMID: 23754493 PMCID: PMC7161862 DOI: 10.1002/bip.22298] [Citation(s) in RCA: 35] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/20/2013] [Revised: 05/21/2013] [Accepted: 05/30/2013] [Indexed: 11/16/2022]
Abstract
For decades, protein function was intimately linked to the presence of a unique, aperiodic crystal‐like structure in a functional protein. The two only places for conformational ensembles of under‐folded (or partially folded) protein forms in this picture were either the end points of the protein denaturation processes or transiently populated folding intermediates. Recent years witnessed dramatic change in this perception and conformational ensembles, which the under‐folded proteins are, have moved from the shadow. Accumulated to date data suggest that a protein can exist in at least three global forms–functional and folded, functional and intrinsically disordered (nonfolded), and nonfunctional and misfolded/aggregated. Under‐folded protein states are crucial for each of these forms, serving as important folding intermediates of ordered proteins, or as functional states of intrinsically disordered proteins (IDPs) and IDP regions (IDPRs), or as pathology triggers of misfolded proteins. Based on these observations, conformational ensembles of under‐folded proteins can be classified as transient (folding and misfolding intermediates) and permanent (IDPs and stable misfolded proteins). Permanently under‐folded proteins can further be split into intentionally designed (IDPs and IDPRs) and unintentionally designed (misfolded proteins). Although intrinsic flexibility, dynamics, and pliability are crucial for all under‐folded proteins, the different categories of under‐foldedness are differently encoded in protein amino acid sequences. © 2013 Wiley Periodicals, Inc. Biopolymers 99: 870–887, 2013.
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Affiliation(s)
- Vladimir N Uversky
- Department of Molecular Medicine, USF Health Byrd Alzheimer's Research Institute, Morsani College of Medicine, University of South Florida, Tampa, FL, 33612; Institute for Biological Instrumentation, Russian Academy of Sciences, Pushchino, 142292, Moscow Region, Russia
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9
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Broom A, Gosavi S, Meiering EM. Protein unfolding rates correlate as strongly as folding rates with native structure. Protein Sci 2014; 24:580-7. [PMID: 25422093 DOI: 10.1002/pro.2606] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/10/2014] [Revised: 11/03/2014] [Accepted: 11/04/2014] [Indexed: 01/19/2023]
Abstract
Although the folding rates of proteins have been studied extensively, both experimentally and theoretically, and many native state topological parameters have been proposed to correlate with or predict these rates, unfolding rates have received much less attention. Moreover, unfolding rates have generally been thought either to not relate to native topology in the same manner as folding rates, perhaps depending on different topological parameters, or to be more difficult to predict. Using a dataset of 108 proteins including two-state and multistate folders, we find that both unfolding and folding rates correlate strongly, and comparably well, with well-established measures of native topology, the absolute contact order and the long range order, with correlation coefficient values of 0.75 or higher. In addition, compared to folding rates, the absolute values of unfolding rates vary more strongly with native topology, have a larger range of values, and correlate better with thermodynamic stability. Similar trends are observed for subsets of different protein structural classes. Taken together, these results suggest that choosing a scaffold for protein engineering may require a compromise between a simple topology that will fold sufficiently quickly but also unfold quickly, and a complex topology that will unfold slowly and hence have kinetic stability, but fold slowly. These observations, together with the established role of kinetic stability in determining resistance to thermal and chemical denaturation as well as proteases, have important implications for understanding fundamental aspects of protein unfolding and folding and for protein engineering and design.
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Affiliation(s)
- Aron Broom
- Department of Chemistry, Guelph-Waterloo Centre for Graduate Studies in Chemistry and Biochemistry, University of Waterloo, Waterloo, Ontario, Canada, N2L 1W2
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10
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Thermal adaptation of conformational dynamics in ribonuclease H. PLoS Comput Biol 2013; 9:e1003218. [PMID: 24098095 PMCID: PMC3789780 DOI: 10.1371/journal.pcbi.1003218] [Citation(s) in RCA: 24] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/07/2013] [Accepted: 07/24/2013] [Indexed: 11/23/2022] Open
Abstract
The relationship between inherent internal conformational processes and enzymatic activity or thermodynamic stability of proteins has proven difficult to characterize. The study of homologous proteins with differing thermostabilities offers an especially useful approach for understanding the functional aspects of conformational dynamics. In particular, ribonuclease HI (RNase H), an 18 kD globular protein that hydrolyzes the RNA strand of RNA:DNA hybrid substrates, has been extensively studied by NMR spectroscopy to characterize the differences in dynamics between homologs from the mesophilic organism E. coli and the thermophilic organism T. thermophilus. Herein, molecular dynamics simulations are reported for five homologous RNase H proteins of varying thermostabilities and enzymatic activities from organisms of markedly different preferred growth temperatures. For the E. coli and T. thermophilus proteins, strong agreement is obtained between simulated and experimental values for NMR order parameters and for dynamically averaged chemical shifts, suggesting that these simulations can be a productive platform for predicting the effects of individual amino acid residues on dynamic behavior. Analyses of the simulations reveal that a single residue differentiates between two different and otherwise conserved dynamic processes in a region of the protein known to form part of the substrate-binding interface. Additional key residues within these two categories are identified through the temperature-dependence of these conformational processes. The relationship between enzymatic activity and protein stability has long been a difficult problem in the study of protein biochemistry. Enzymes may undergo structural changes in order to bind substrates, catalyze chemical reactions, and release products, but flexibility often is inversely correlated with thermodynamic stability. Proteins from organisms that are adapted to high temperature can be both more rigid and less active at ambient temperature than their homologs from organisms that grow at lower temperatures. For this reason, studying homologous pairs of proteins from organisms adapted to different thermal environments is a productive way to identify functionally important motions. In this work we perform comparative analyses of molecular dynamics simulations for five ribonuclease H proteins of varying thermal stabilities, isolated from organisms that grow in varying thermal environments. We identify two different mechanisms of motion in a region of the protein that interacts with substrate molecules, suggesting at least two forms of thermal adaptation in this protein family.
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11
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Stepwise protein folding at near amino acid resolution by hydrogen exchange and mass spectrometry. Proc Natl Acad Sci U S A 2013; 110:7684-9. [PMID: 23603271 DOI: 10.1073/pnas.1305887110] [Citation(s) in RCA: 143] [Impact Index Per Article: 13.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The kinetic folding of ribonuclease H was studied by hydrogen exchange (HX) pulse labeling with analysis by an advanced fragment separation mass spectrometry technology. The results show that folding proceeds through distinct intermediates in a stepwise pathway that sequentially incorporates cooperative native-like structural elements to build the native protein. Each step is seen as a concerted transition of one or more segments from an HX-unprotected to an HX-protected state. Deconvolution of the data to near amino acid resolution shows that each step corresponds to the folding of a secondary structural element of the native protein, termed a "foldon." Each folded segment is retained through subsequent steps of foldon addition, revealing a stepwise buildup of the native structure via a single dominant pathway. Analysis of the pertinent literature suggests that this model is consistent with experimental results for many proteins and some current theoretical results. Two biophysical principles appear to dictate this behavior. The principle of cooperativity determines the central role of native-like foldon units. An interaction principle termed "sequential stabilization" based on native-like interfoldon interactions orders the pathway.
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12
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Nickson AA, Wensley BG, Clarke J. Take home lessons from studies of related proteins. Curr Opin Struct Biol 2012; 23:66-74. [PMID: 23265640 PMCID: PMC3578095 DOI: 10.1016/j.sbi.2012.11.009] [Citation(s) in RCA: 31] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/05/2012] [Revised: 11/26/2012] [Accepted: 11/27/2012] [Indexed: 11/30/2022]
Abstract
The 'Fold Approach' involves a detailed analysis of the folding of several topologically, structurally and/or evolutionarily related proteins. Such studies can reveal determinants of the folding mechanism beyond the gross topology, and can dissect the residues required for folding from those required for stability or function. While this approach has not yet matured to the point where we can predict the native conformation of any polypeptide chain in silico, it has been able to highlight, amongst others, the specific residues that are responsible for nucleation, pathway malleability, kinetic intermediates, chain knotting, internal friction and Paracelsus switches. Some of the most interesting discoveries have resulted from the attempt to explain differences between homologues.
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Affiliation(s)
- Adrian A Nickson
- Department of Chemistry, University of Cambridge, Lensfield Rd, Cambridge CB2 1EW, UK.
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13
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Zou T, Ozkan SB. Local and non-local native topologies reveal the underlying folding landscape of proteins. Phys Biol 2011; 8:066011. [DOI: 10.1088/1478-3975/8/6/066011] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022]
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14
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Bowler BE. Residual structure in unfolded proteins. Curr Opin Struct Biol 2011; 22:4-13. [PMID: 21978577 DOI: 10.1016/j.sbi.2011.09.002] [Citation(s) in RCA: 66] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/02/2011] [Accepted: 09/07/2011] [Indexed: 11/27/2022]
Abstract
The denatured state ensemble (DSE) of unfolded proteins, once considered to be well-modeled by an energetically featureless random coil, is now well-known to contain flickering elements of residual structure. The position and nature of DSE residual structure may provide clues toward deciphering the protein folding code. This review focuses on recent advances in our understanding of the nature of DSE collapse under folding conditions, the quantification of the stability of residual structure in the DSE, the determination of the location and types of residues involved in thermodynamically significant residual structure and advances in detection of long-range interactions in the DSE.
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Affiliation(s)
- Bruce E Bowler
- Department of Chemistry and Biochemistry and Center for Biomolecular Structure and Dynamics, The University of Montana, Missoula, MT 59812, USA.
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15
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Stocks BB, Rezvanpour A, Shaw GS, Konermann L. Temporal Development of Protein Structure during S100A11 Folding and Dimerization Probed by Oxidative Labeling and Mass Spectrometry. J Mol Biol 2011; 409:669-79. [DOI: 10.1016/j.jmb.2011.04.028] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/22/2011] [Revised: 04/01/2011] [Accepted: 04/11/2011] [Indexed: 10/18/2022]
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Sosnick TR, Barrick D. The folding of single domain proteins--have we reached a consensus? Curr Opin Struct Biol 2010; 21:12-24. [PMID: 21144739 DOI: 10.1016/j.sbi.2010.11.002] [Citation(s) in RCA: 120] [Impact Index Per Article: 8.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/26/2010] [Revised: 11/03/2010] [Accepted: 11/04/2010] [Indexed: 10/18/2022]
Abstract
Rather than stressing the most recent advances in the field, this review highlights the fundamental topics where disagreement remains and where adequate experimental data are lacking. These topics include properties of the denatured state and the role of residual structure, the nature of the fundamental steps and barriers, the extent of pathway heterogeneity and non-native interactions, recent comparisons between theory and experiment, and finally, dynamical properties of the folding reaction.
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Affiliation(s)
- Tobin R Sosnick
- Department of Biochemistry and Molecular Biology, Institute for Biophysical Dynamics, University of Chicago, Chicago, IL 60637, USA.
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Rieger R, Kobitski A, Sielaff H, Nienhaus GU. Evidence of a Folding Intermediate in RNase H from Single‐Molecule FRET Experiments. Chemphyschem 2010; 12:627-33. [DOI: 10.1002/cphc.201000693] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/27/2010] [Indexed: 11/10/2022]
Affiliation(s)
- Robert Rieger
- Institute of Applied Physics and Center for Functional Nanostructures (CFN), Karlsruhe Institute of Technology (KIT), 76128 Karlsruhe (Germany), Fax: (+49) 721‐608 84 80
| | - Andrei Kobitski
- Institute of Applied Physics and Center for Functional Nanostructures (CFN), Karlsruhe Institute of Technology (KIT), 76128 Karlsruhe (Germany), Fax: (+49) 721‐608 84 80
| | - Hendrik Sielaff
- Institute of Applied Physics and Center for Functional Nanostructures (CFN), Karlsruhe Institute of Technology (KIT), 76128 Karlsruhe (Germany), Fax: (+49) 721‐608 84 80
| | - G. Ulrich Nienhaus
- Institute of Applied Physics and Center for Functional Nanostructures (CFN), Karlsruhe Institute of Technology (KIT), 76128 Karlsruhe (Germany), Fax: (+49) 721‐608 84 80
- Department of Physics, University of Illinois at Urbana‐Champaign, Urbana, 61801 (USA)
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