Fold Structures Research Articles

Comprehensive Analysis and Application Research of Advanced Computational Algorithms in Protein Folding Simulations

Protein engineering stands at the forefront of biotechnology, aiming to modify natural proteins or create new ones tailored to specific functional requirements. The three-dimensional structures of proteins, particularly their folding patterns, are critical in defining their biological roles. Accurate prediction and detailed examination of these protein folding structures are crucial in protein engineering. The close relationship between protein structure and function highlights the importance of understanding protein folding dynamics to successfully manipulate protein designs for intended uses. Genetic algorithms (GA), taking inspiration from natural evolutionary principles, employ a heuristic search approach that integrates elements of randomness. In contrast, simulated annealing (SA) leverages stochastic optimization techniques based on the Monte Carlo method, theoretically capable of approximating the global optimum with a high degree of accuracy. Additionally, generalized ensemble methods are increasingly used to explore protein folding processes. This paper explores the fundamental principles and practical applications of these algorithms in simulating protein folding dynamics, aiming to enhance the methodologies used in protein engineering. This exploration not only aids in the refinement of protein design but also extends the potential applications of engineered proteins in various scientific and industrial fields.

Benzimidazole resistance-associated mutations improve the in silico dimerization of hookworm tubulin: An additional resistance mechanism

Background and Aim: Mutations in the β-tubulin genes of helminths confer benzimidazole (BZ) resistance by reducing the drug’s binding efficiency to the expressed protein. However, the effects of these resistance-associated mutations on tubulin dimer formation in soil-transmitted helminths remain unknown. Therefore, this study aimed to investigate the impact of these mutations on the in silico dimerization of hookworm α- and β-tubulins using open-source bioinformatics tools. Materials and Methods: Using AlphaFold 3, the α- and β-tubulin amino acid sequences of Ancylostoma ceylanicum were used to predict the structural fold of the hookworm tubulin heterodimer. The modeled complexes were subjected to several protein structure quality assurance checks. The binding free energies, overall binding affinity, dissociation constant, and interacting amino acids of the complex were determined. The dimer’s structural flexibility and motion were simulated through molecular dynamics. Results: BZ resistance-associated amino acid substitutions in the β-tubulin isotype 1 protein of hookworms altered tubulin dimerization. The E198K, E198V, and F200Y mutations conferred the strongest and most stable binding between the α and β subunits, surpassing that of the wild-type. In contrast, complexes with the Q134H and F200L mutations exhibited the opposite effect. Molecular dynamics simulations showed that wild-type and mutant tubulin dimers exhibited similar dynamic behavior, with slight deviations in those carrying the F200L and E198K mutations. Conclusion: Resistance-associated mutations in hookworms impair BZ binding to β-tubulin and enhance tubulin dimer interactions, thereby increasing the parasite’s ability to withstand treatment. Conversely, other mutations weaken these interactions, potentially compromising hookworm viability. These findings offer novel insights into helminth tubulin dimerization and provide a valuable foundation for developing anthelmintics targeting this crucial biological process. Keywords: Ancylostoma, anthelmintic resistance, microtubules, soil-transmitted helminths.

Thermodynamic and Conformational Analysis of GTRNase and Lysozyme Proteins Under Thermal Variations using Molecular Dynamics Simulations

This study examines the thermodynamic and conformational dynamics of GTRNase and lysozyme proteins under varying temperature conditions using molecular dynamics (MD) simulations. The objective is to evaluate their structural stability, folding behavior, and thermodynamic properties to understand their responses to thermal fluctuations. Protein structures were retrieved from the Protein Data Bank (PDB), refined to remove extraneous molecules, and simulated using GROMACS 2022.2 under NVT and NPT ensembles, spanning temperatures from 270 K to 380 K. The results revealed distinct behaviors for the two proteins. GTRNase exhibited a slight escalation in radius of gyration (Rg) from 1.434 nm at 270 K to 1.445 nm at 380 K, suggesting a marginal conformational expansion. In contrast, lysozyme maintained a consistent Rg of 1.38 nm over the same temperature range, indicating structural compactness. Root mean square deviation (RMSD) data demonstrated increased flexibility in both proteins, with GTRNase escalating from 0.115 nm at 270 K to 0.179 nm at 380 K and lysozyme rising from 0.102 nm to 0.142 nm across the temperature range. Solvent-accessible surface area (SASA) for GTRNase fluctuated between 69 nm² and 73 nm², with the lowest value observed at 300 K and the highest at 370 K. These findings highlight that GTRNase is more susceptible to thermal perturbations than lysozyme, showing greater conformational flexibility and expansion. This research underscores the utility of MD simulations in exploring protein behavior and provides valuable insights for applications in protein engineering and drug design.

Structural Evaluation of Interleukin-19 Cytokine and Interleukin-19-Bound Receptor Complex Using Computational Immuno-Engineering Approach

Interleukin 19 (IL-19) is an anti-inflammatory cytokine that belongs to the IL-10 family, where IL-20 and IL-24 also exist. While IL-19 and IL-20 share some comparable structural folds, there are certain structural divergences in their N-terminal ends. To date, there are no reported IL-19 receptors; although, it has been suggested in the literature that IL-19 would bind to lL-20 receptor (IL-20R) and trigger the JAK-STAT signaling pathways. The present report examines the structure of the IL-19 cytokine and its receptor complex using a computational approach. Specifically, the postulated modes of interactions for IL-20R as an IL-19 receptor are examined on the basis of a set of computational findings. The author has used molecular docking and molecular dynamics simulation to generate a 3D model for the IL-19 complex with IL-20R. When a protein’s crystal structure is not available in the literature, predictive modeling is often employed to determine the protein’s 3D structure. The model assessment can be based on various factors, which include stability analysis using RMSD calculations, tracking changes in time-based secondary structures and the associated Gibbs energies, ΔG. Since one model complex (referred to as model A throughout this paper) can be used as a working hypothesis for future experiments, this structure has been explored here in detail to check its stability, subunit interfaces, and binding residues. The information gathered in this approach can potentially help to design specific experiments to test the validity of the model protein structure. Additionally, the results of this research should be relevant for understanding anti-inflammatory mechanisms and, eventually, could contribute to the efforts for therapeutic developments and targeted therapy.

Revealing the correlation between asymmetric structure and low thermal conductivity in Janus-graphene via machine learning force constant potential

Understanding the fundamental link between structure and functionalization is crucial for designing and optimizing functional materials, since different structural configurations could trigger materials to demonstrate diverse physical and chemical properties. However, the correlation between crystal structure and thermal conductivity (κ) remains unclear. In this study, taking two-dimensional (2D) carbon allotropes Janus-graphene and graphene as study cases, we utilize phonon Boltzmann transport equation combined with machine learning potential to thoroughly investigate the complex folding structure of pure sp2 hybridized Janus-graphene from the perspective of crystal structure, phonon modal resolved thermal transport, and atomic interactions, with the goal of identifying the underlying relationship between 2D geometry and κ. The results reveal that the folded structure in Janus-graphene causes strong symmetry breaking, significantly reduces phonon group velocities, increases phonon–phonon scattering, and ultimately leads to low κ. These findings enhance our understanding of how atomic structure folding affects thermal transport and the relationship between structure and functionalization.

High fitness paths can connect proteins with low sequence overlap.

The structure and function of a protein are determined by its amino acid sequence. While random mutations change a protein's sequence, evolutionary forces shape its structural fold and biological activity. Studies have shown that neutral networks can connect a local region of sequence space by single residue mutations that preserve viability. However, the larger-scale connectedness of protein morphospace remains poorly understood. Recent advances in artificial intelligence have enabled us to computationally predict a protein's structure and quantify its functional plausibility. Here we build on these tools to develop an algorithm that generates viable paths between distantly related extant protein pairs. The intermediate sequences in these paths differ by single residue changes over subsequent steps - substitutions, insertions and deletions are admissible moves. Their fitness is evaluated using the protein language model ESM2, and maintained as high as possible subject to the constraints of the traversal. We document the qualitative variation across paths generated between progressively divergent protein pairs, some of which do not even acquire the same structural fold. The ease of interpolating between two sequences could be used as a proxy for the likelihood of homology between them.

Sustaining 500,000 Folding Cycles Through Bioinspired Stress Dispersion Design in Sodium‐Ion Batteries

AbstractDeveloping super‐foldable electronic materials and devices presents a significant challenge, as intrinsic conductive materials are unable to achieve numerous true‐folding operations (super‐foldable) due to limitations from short‐range forces of chemical bonds. Consequently, super‐foldable batteries remain unexplored. This work focused on sodium‐ion batteries as a breakthrough point to advance super‐foldable devices. By employing a “2+1” bioinspired strategy, we stepwise designed and assembled super‐foldable components, from substrates to electrodes, and to ultimately device. This bioinspired approach completely disperses folding stress and thus prevents the breakage of chemical bonds, enabling the successful fabrication of the first super‐foldable ion battery. This battery can withstand true‐folding at any angle, in any direction, and for an unprecedented number of cycles—far outperforming current foldable phones with hinge structures. Remarkably, after 500,000 true‐folding cycles, the battery's microstructure remains intact with no significant degradation of electrochemical performance. Real‐time dynamic folding observations reveal an M‐shaped folding structure within the bioinspired materials, which effectively disperses stress via bulged layers, dispersed arcs, and slidable microgrooves that work together across different directions and dimensions to achieve super‐foldability. Mechanical simulations vividly verify this principle. This work represents a breakthrough in super‐foldable devices, offering valuable insights and promoting practical application for future super‐foldable devices.

Sustaining 500,000 Folding Cycles Through Bioinspired Stress Dispersion Design in Sodium-Ion Batteries.

Developing super-foldable electronic materials and devices presents a significant challenge, as intrinsic conductive materials are unable to achieve numerous true-folding operations (super-foldable) due to limitations from short-range forces of chemical bonds. Consequently, super-foldable batteries remain unexplored. This work focused on sodium-ion batteries as a breakthrough point to advance super-foldable devices. By employing a "2+1" bioinspired strategy, we stepwise designed and assembled super-foldable components, from substrates to electrodes, and to ultimately device. This bioinspired approach completely disperses folding stress and thus prevents the breakage of chemical bonds, enabling the successful fabrication of the first super-foldable ion battery. This battery can withstand true-folding at any angle, in any direction, and for an unprecedented number of cycles-far outperforming current foldable phones with hinge structures. Remarkably, after 500,000 true-folding cycles, the battery's microstructure remains intact with no significant degradation of electrochemical performance. Real-time dynamic folding observations reveal an M-shaped folding structure within the bioinspired materials, which effectively disperses stress via bulged layers, dispersed arcs, and slidable microgrooves that work together across different directions and dimensions to achieve super-foldability. Mechanical simulations vividly verify this principle. This work represents a breakthrough in super-foldable devices, offering valuable insights and promoting practical application for future super-foldable devices.

Differentiation and quantification of bovine and pork gelatin using UPLC-QTOF and ATR-FTIR spectroscopy: Addressing challenges in mixed gelatin analysis and detection

The amino acid sequence dictates protein folding and spatial structure, essential for understanding their behavior. Gelatin, important in industry and biomedicine, demands accurate source authentication due to ethical and dietary concerns. This study explores differentiating and quantifying bovine and pork gelatin using UPLC-QTOF and ATR-FTIR spectroscopy. UPLC-QTOF identified distinct ions for pure bovine (m/z + 962.1471, NRLHFFK) and pork gelatin (m/z + 653.0289, YNEVK) but struggled with mixed gelatins due to protein-protein interactions altering peptide ion profiles. ATR-FTIR was employed to leverage ethanol's differential effects on gelatin's amide bands for quantifying pork gelatin contamination in bovine gelatin. The method showed a strong linear correlation (R2 = 0.9994) with contamination levels and amide band transmission, with detection and quantification limits of 0.85 and 2.85 mg/100 mg, respectively. It effectively identified pork gelatin in halal candy, with recovery rates from 50.05 % to 103.69 %, highlighting ATR-FTIR's potential for accurate gelatin analysis.

3D chromatin maps of a brown alga reveal U/V sex chromosome spatial organization.

Nuclear three dimensional (3D) folding of chromatin structure has been linked to gene expression regulation and correct developmental programs, but little is known about the 3D architecture of sex chromosomes within the nucleus, and how that impacts their role in sex determination. Here, we determine the sex-specific 3D organization of the model brown alga Ectocarpus chromosomes at 2 kb resolution, by mapping long-range chromosomal interactions using Hi-C coupled with Oxford Nanopore long reads. We report that Ectocarpus interphase chromatin exhibits a non-Rabl conformation, with strong contacts among telomeres and among centromeres, which feature centromere-specific LTR retrotransposons. The Ectocarpus chromosomes do not contain large local interactive domains that resemble TADs described in animals, but their 3D genome organization is largely shaped by post-translational modifications of histone proteins. We show that the sex determining region (SDR)within the U and V chromosomes are insulated and span the centromeres andwe link sex-specific chromatin dynamics and gene expression levels to the 3D chromatin structure of the U and V chromosomes. Finally, we uncover the unique conformation of a large genomic region on chromosome 6 harboring an endogenous viral element, providing insights regarding the impact of a latent giant dsDNA virus on the host genome's 3D chromosomal folding.

Teleseismic virtual-source reverse time migration of upper crustal structures in the Three Gorges region, China

Summary The Three Gorges (TG) in central China is an earthquake prone region that lacks active-source seismic surveys, hence requiring high-resolution passive seismic imaging to reveal crustal structures in detail. While teleseismic virtual-source reflection (TVR) profiling is effective for imaging gently dipping upper crustal structures, it is unsuitable for mapping steep structures beneath rugged topography in the TG region. Here we develop a teleseismic virtual-source reverse time migration (TV-RTM) to improve the imaging of steep structures. Synthetic tests are conducted to demonstrate the validity and resolution of the TV-RTM method. To mitigate the imaging inaccuracy due to sparse station spacing, we interpolate direct-arrival waveforms to achieve a doubling of the minimally required station spacing from 2 km to 4 km. The TV-RTM images of the upper crustal structure beneath a 2D array in the TG region reveal significant fold and thrust structures that correlate well with the surface geology and tectonic framework. The resolution of the images is assessed using 2D and 3D synthetic models with the station and source geometry of field data. The TV-RTM method provides a new passive seismic solution for studying upper crustal structures in regions that lack active-source seismic data.

Molecular Lineages and Spatial Distributions of Subplate Neurons in the Human Fetal Cerebral Cortex.

The expansion of neural progenitors and production of distinct neurons are crucial for architectural assembly and formation of connectivity in human brains. Subplate neurons (SPNs) are among the firstborn neurons in the human fetal cerebral cortex, and play a critical role in establishing intra- and extracortical connections. However, little is known about SPN origin and developmental lineages. In this study, spatial landscapes and molecular trajectories of SPNs in the human fetal cortices from gestational weeks (GW) 10 to 25 are created by performing spatial transcriptomics and single-cell RNA sequencing. Genes known to be evolutionarily human-specific and genes associated with extracellular matrices (ECMs) are found to maintain stable proportions of subplate neurons among other neuronal types. Enriched ECM gene expression in SPNs varies in distinct cortical regions, with the highest level in the frontal lobe of human fetal brains. This study reveals molecular origin and lineage specification of subplate neurons in the human fetal cerebral cortices, and highlights underpinnings of SPNs to cortical neurogenesis and early structural folding.

Form-Finding and Structural Analysis of Deployable Rigid Folding Structures using Synchronized Vector-based Graphic Statics and Finite Element Method

This paper presents a computational framework for designing and analysing deployable rigid folding plate structures by integrating Vector-based Graphic Statics (VGS) and Finite Element Method (FEM). To ensure consistent data flow from crease pattern generating to structural analysis and form-finding, a matrix-based data structure is utilized to represent the topology and geometry of the folding structure. An algorithm applied to this data structure enables simulation of the folding kinematics within a particle-based physical system, and solving for static equilibrium in the folded states to construct the discrete stress field model with VGS. This model provides a concise representation of edge and in-plane static equilibrium in a folded state. Additionally, the data structure maintains uniform detail levels between VGS and FEM by aligning mesh subdivision in FEM with the discrete stress field in VGS. Two case studies demonstrate the effectiveness and intuitiveness of the proposed method in limit state analysis by evaluating the multiple folded states of a continuous folding procedure, and in topological navigating and optimizing the folding crease pattern related to the performance of the deployable form. The research highlights advancements in the rapid, visually informed design process and concludes that this integrated approach serves as both a tool for engineers and a collaborative framework for architects and engineers in architectural design.

Catalyzing PET-RAFT Polymerizations Using Inherently Photoactive Zinc Myoglobin.

Protein photocatalysts provide a modular platform to access new reaction pathways and affect product outcomes, but their use in polymer synthesis is limited because co-catalysts and/or co-reductants are required to complete catalytic cycles. Herein, we report using zinc myoglobin (ZnMb), an inherently photoactive protein, to mediate photoinduced electron/energy transfer (PET) reversible addition-fragmentation chain transfer (RAFT) polymerizations. Using ZnMb as the sole reagent for catalysis, photomediated polymerizations of N,N-dimethylacrylamide in PBS were achieved with predictable molecular weights, dispersity values approaching 1.1, and high chain-end fidelity. We found that initial apparent rate constants of polymerization increased from 4.6×10-5 s-1 for zinc mesoporpyhrin IX (ZnMIX) to 6.5×10-5 s-1 when ZnMIX was incorporated into myoglobin to yield ZnMb, indicating that the protein binding site enhanced catalytic activity. Chain extension reactions comparing ZnMb-mediated RAFT polymerizations to thermally-initiated RAFT polymerizations showed minimal differences in block copolymer molecular weights and dispersities. This work enables studies to elucidate how protein modifications (e.g., secondary structure folding, site-directed mutagenesis, directed evolution) can be used to modulate polymerization outcomes (e.g., selective monomer additions towards sequence control, tacticity control, molar mass distributions).

Catalyzing PET‐RAFT Polymerizations Using Inherently Photoactive Zinc Myoglobin

AbstractProtein photocatalysts provide a modular platform to access new reaction pathways and affect product outcomes, but their use in polymer synthesis is limited because co‐catalysts and/or co‐reductants are required to complete catalytic cycles. Herein, we report using zinc myoglobin (ZnMb), an inherently photoactive protein, to mediate photoinduced electron/energy transfer (PET) reversible addition‐fragmentation chain transfer (RAFT) polymerizations. Using ZnMb as the sole reagent for catalysis, photomediated polymerizations of N,N‐dimethylacrylamide in PBS were achieved with predictable molecular weights, dispersity values approaching 1.1, and high chain‐end fidelity. We found that initial apparent rate constants of polymerization increased from 4.6×10–5 s−1 for zinc mesoporpyhrin IX (ZnMIX) to 6.5×10–5 s−1 when ZnMIX was incorporated into myoglobin to yield ZnMb, indicating that the protein binding site enhanced catalytic activity. Chain extension reactions comparing ZnMb‐mediated RAFT polymerizations to thermally‐initiated RAFT polymerizations showed minimal differences in block copolymer molecular weights and dispersities. This work enables studies to elucidate how protein modifications (e.g., secondary structure folding, site‐directed mutagenesis, directed evolution) can be used to modulate polymerization outcomes (e.g., selective monomer additions towards sequence control, tacticity control, molar mass distributions).

NMR residual dipolar couplings investigation in the topology of house dust mite Group V allergens

Blo t 5 is an important major allergen protein from Blomia tropicalis mites, which are prevalent in tropical and subtropical regions, including Taiwan. It is a coiled-coil triple helical bundle, but there currently is ambiguity around its structural fold and packing of the three helices. We have relied on NMR residual dipolar coupling data collected from four different alignment media to confirm that Blo t 5 has left-handed helical topology and further used that data to refine its solution structure. Earlier we had described conformational epitope for a detection monoclonal antibody by exclusive use of TROSY NMR experiments that studied Blo t 5 binding with the antibody FAB’ fragment. Here, we confirm those findings with an extensive mutagenesis and biophysical study to validate the NMR epitope mapping approach proposed by us.

Linear strain gradient-regulated bifurcation of circular bilayer plates

Bilayer structures with controllable self-folding capability have found applications in a variety of cutting-edge fields such as flexible electrics, wearable devices and soft robotics. The folding of bilayer structures occurs when the mismatch strain between the two layers exceeds the bifurcation threshold, resulting in a deformation transition from an axisymmetric to a folded state. Previous efforts have predominantly focused on bilayer structures with uniform and/or anisotropic strain distributions. However, the role of non-uniform in-plane strain distributions in regulating the bifurcation of bilayer structures has not been fully understood. In this study, the effects of linear in-plane strain gradients on the bifurcation of circular bilayer plates, both with and without geometric mismatch, are systematically investigated by combining theoretical analysis, finite element simulations and experiments. Our results reveal that both the mismatch strain gradient and the geometric mismatch between the two layers play crucial roles in regulating bifurcation. Notably, linear mismatch strain gradients with larger strain at the center delay bifurcation, while those with larger strain along the edge promote bifurcation. This work offers new insights into the design of controllable self-folding bilayer structures, which is of great significance for advanced applications.

Proteome-scale structural prediction of the giant Marseillevirus reveals conserved folds and putative homologs of the hypothetical proteins.

A significant proportion of the highly divergent and novel proteins of giant viruses are termed "hypothetical" due to the absence of detectable homologous sequences in the existing databases. The quality of genome and proteome annotations often relies on the identification of signature sequences and motifs in order to assign putative functions to the gene products. These annotations serve as the first set of information for researchers to develop workable hypotheses for further experimental research. The structure-function relationship of proteins suggests that proteins with similar functions may also exhibit similar folding patterns. Here, we report the first proteome-wide structure prediction of the giant Marseillevirus. We use AlphaFold-predicted structures and their comparative analysis with the experimental structures in the PDB database to preliminarily annotate the viral proteins. Our work highlights the conservation of structural folds in proteins with highly divergent sequences and reveals potentially paralogous relationships among them. We also provide evidence for gene duplication and fusion as contributing factors to giant viral genome expansion and evolution. With the easily accessible AlphaFold and other advanced bioinformatics tools for high-confidence de novo structure prediction, we propose a combined sequence and predicted-structure-based proteome annotation approach for the initial characterization of novel and complex organisms or viruses.

Metformin modulates the TXNIP-NLRP3-GSDMD pathway to improve diabetic bladder dysfunction

To validate the therapeutic efficacy of metformin on diabetic bladder dysfunction (DBD) and further elucidate whether the TXNIP-NLRP3-GSDMD axis serves as a target for metformin in ameliorating DBD. C57BL/6J mice were induced with diet-induced obesity by being fed a high-fat diet (HFD) for 16 weeks. After establishing the model, the mice were treated with metformin for 4 weeks, and their glucose metabolism-related parameters were assessed. Urine spot assays and urodynamic measurements were conducted to reflect the bladder function and urinary behavior in mice, while histological examination was performed to observe morphological changes. Western blot analysis was employed to measure the expression levels of pyroptotic factors such as TXNIP, NLRP3, GSDMD, and tight junction proteins. Metformin treatment significantly improved glucose tolerance and insulin sensitivity in mice. Moreover, it showed promise in decreasing urinary spot occurrence, reducing urination frequency, alleviating non-voiding contractions, and stabilizing peak urinary pressure. Following metformin therapy, mice displayed restored epithelial fold structure, increased thickness of the muscular layer, substantial decrease in muscle fiber content, notably reduced levels of TXNIP and GSDMD proteins in the metformin-treated group compared to the DBD group, and restored expression of tight junction proteins Zo-1, Claudin-1, and Occludin. Metformin ameliorates urothelial cells damage in DBD mice by inhibiting TXNIP generation and reducing NLRP3 and GSDMD production.

Single turnover transient state kinetics reveals processive protein unfolding catalyzed by Escherichia coli ClpB

Escherichia coli ClpB and Saccharomyces cerevisiae Hsp104 are AAA+ motor proteins essential for proteome maintenance and thermal tolerance. ClpB and Hsp104 have been proposed to extract a polypeptide from an aggregate and processively translocate the chain through the axial channel of its hexameric ring structure. However, the mechanism of translocation and if this reaction is processive remains disputed. We reported that Hsp104 and ClpB are non-processive on unfolded model substrates. Others have reported that ClpB is able to processively translocate a mechanically unfolded polypeptide chain at rates over 240 amino acids (aa) per second. Here, we report the development of a single turnover stopped-flow fluorescence strategy that reports on processive protein unfolding catalyzed by ClpB. We show that when translocation catalyzed by ClpB is challenged by stably folded protein structure, the motor enzymatically unfolds the substrate at a rate of ~0.9 aa s−1 with a kinetic step-size of ~60 amino acids at sub-saturating [ATP]. We reconcile the apparent controversy by defining enzyme catalyzed protein unfolding and translocation as two distinct reactions with different mechanisms of action. We propose a model where slow unfolding followed by fast translocation represents an important mechanistic feature that allows the motor to rapidly translocate up to the next folded region or rapidly dissociate if no additional fold is encountered.