Biology subjects

Martinez-Seara, H.

Publications and source records attributed to Martinez-Seara, H..

9 recordsLinked to original sources

bioRxiv2026

Protein Crowding Buffers Membrane Fluidity

Like cholesterol, protein crowding has been shown to alter the temperature dependence of lateral diffusion in biomembranes, eventually eliminating the distinct liquid-gel phase transition. Here, we resolve the molecular origin of this protein effect with coarse-grained and all-atom simulations of protein-crowded phosphatidylcholine bilayers, which give access to lipid structure and dynamics on both sides of the transition temperature. Unlike cholesterol, which is known to induce a continuous liquid ordered phase by having opposite effects on the two phases, we find that membrane proteins decrease lipid order at all temperatures studied. The disordering is stronger at low temperatures, so that once the proteins cover roughly half of the membrane area the low- and high-temperature phases become alike and the transition is lost. Our results suggest that in addition to local cholesterol concentration, local protein concentration may also help dictate membrane fluidity and especially its response to temperature change.

biophysics↗

bioRxiv2026

Quantitative Mapping of Sulfation, Iduronic Acid, and Secondary Structure in Glycosaminoglycans

Glycosaminoglycans (GAGs) are extracellular matrix polysaccharides whose sequence variability and chemical modifications, particularly sulfation, generate substantial structural diversity. However, how sulfation patterns and monosaccharide composition encode secondary structure in GAGs is not systematically resolved, and quantitative metrics for classifying these structures are largely lacking. Here, we employ large-scale all-atom molecular dynamics simulations to investigate the molecular origin of secondary structure in sulfated GAGs. We systematically vary sulfation patterns and monosaccharide composition to isolate the factors that promote changes in three-dimensional structure. We show that GAG helical conformations arise from recurrent local shortening motifs caused primarily by stabilization of O_SCPLOWLC_SCPLOW-iduronic acid in the 1C4 puckering conformation, promoted by 2-O-sulfation or by densely sulfated regions. We also introduce a two-parameter structural metric that objectively classifies GAG secondary structures and distinguishes heparin helices from related conformations. Together, our results establish a quantitative link between monosaccharide identity, sulfation pattern, and three-dimensional organization of polysaccharide chains, providing a framework for future studies of sequence-structure relationships in GAGs.

biophysics↗

bioRxiv2025

Transient Formation of Supramolecular Complexes Between Hyaluronan and Oligopeptides at Submicromolar Concentration

Charged polymer interactions govern critical biological and technological processes by altering the structure and dynamics of the surrounding aqueous environment. How-ever, studying these interactions and the resulting environments across a broad concentration range is challenging, as it demands a combination of multiple techniques with varying resolutions and complex data interpretation. In particular, detecting interactions at submicromolar levels remains technically challenging, with most methods lacking the resolution required for molecular-level characterization. Here, we examined the interactions between high-molecular-weight hyaluronan (HA), a biologically and technologically relevant polymer, and a series of model oligopeptides--nonaarginine, nonalysine, and nonaglycine, chosen for their charge and side-chain chemistry. Using angle-resolved second harmonic scattering (AR-SHS), multi-angle dynamic light scattering, nuclear magnetic resonance spectroscopy, and all-atom molecular dynamics simulations, we captured a detailed molecular picture of HA-peptide interactions across a broad concentration range, including submicromolar concentrations. We found selective and multivalent interactions between HA and positively charged peptides, which are consistently stronger with nonaarginine. These interactions trigger significant solvent and solute remodeling, including nanoscale HA-peptide clustering. Our molecular simulations provide essential atomic-level interpretation of the experimental data, elucidating the transient and dynamic nature of the intermolecular interactions and the underlying processes of molecular aggregation and induced water reorientation. The distinct behavior found in arginine-rich peptides highlights their potential in modulating extracellular environments and as peptide-based drug delivery systems. Moreover, our methodological framework, combining sensitive AR-SHS signals with atomistic simulations and traditional structural techniques, offers unprecedented molecular insight into complex polymer-peptide interactions, laying the groundwork for future research on dynamic supramolecular systems in soft materials and unstructured biological environments such as the extracellular matrix.

biophysics↗

bioRxiv2024

Developing and Benchmarking Sulfate and Sulfamate Force Field Parameters for Glycosaminoglycans via Ab Initio Molecular Dynamics Simulations

Glycosaminoglycans (GAGs) are negatively charged polysaccharides found on cell surfaces, where they regulate transport pathways of foreign molecules toward the cell. The structural and functional diversity of GAGs is largely attributed to varied sulfa-tion patterns along the polymer chains, which makes understanding their molecular recognition mechanisms crucial. Molecular dynamics (MD) simulations, with their un-matched microscopic perspective, have the potential to be a reference tool for exploring the patterns responsible for biologically relevant interactions. However, the capability of molecular dynamics models (i.e., force fields) used in biosimulations to accurately capture sulfation-specific interactions is not well established. In this work, we evalu-ate the performance of molecular dynamics force fields for sulfated GAGs by studying ion pairing of Ca2+ to sulfated moieties -- N-methylsulfamate and methylsulfate -- that resemble N- and O-sulfation found in GAGs, respectively. We tested nonpolariz-able (CHARMM36 and GLYCAM06), explicitly polarizable (Drude and AMOEBA), and implicitly polarizable through charge scaling (prosECCo75 and GLYCAM-ECC75) force fields. The Ca-sulfamate/sulfate interaction free energy profiles obtained with the tested force fields were compared against reference ab initio molecular dynamics (AIMD) simulations. AIMD reveals that the preferential Ca2+ binding mode to sul-fated GAG groups is solvent-shared pairing, and only the charge-scaled models agree satisfactorily with the AIMD data. All other force fields exhibit poorer performance, sometimes even qualitatively. Surprisingly, even explicitly polarizable force fields dis-play a notable shortfall in their performance, attributed to difficulties in their optimiza-tion and possible inherent limitations in depicting high-charge-density ion interactions accurately. Finally, the underperforming force fields lead to unrealistic aggregation of sulfated saccharides, qualitatively distorting our understanding of the soft glycocalyx environment. Our results highlight the importance of accurately treating electronic polarization in MD simulations of sulfated GAGs and caution against over-reliance on currently available models without thorough validation and optimization.

biophysics↗

bioRxiv2024

Effective Inclusion of Electronic Polarization Improves the Description of Electrostatic Interactions: The prosECCo75 Biomolecular Force Field

prosECCo75 is an optimized force field effectively incorporating electronic polarization via charge scaling. It aims to enhance the accuracy of nominally nonpolarizable molecular dynamics (MD) simulations for interactions in biologically relevant systems involving water, ions, proteins, lipids, and saccharides. Recognizing the inherent limitations of nonpolarizable force fields in precisely modeling electrostatic interactions essential for various biological processes, we mitigate these shortcomings by accounting for electronic polarizability in a physical rigorous mean-field way that does not add to computational costs. With this scaling of (both integer and partial) charges within the CHARMM36 framework, prosECCo75 addresses overbinding artifacts. This improves agreement with experimental ion binding data across a broad spectrum of systems -- lipid membranes, proteins (including peptides and amino acids), and saccharides -- without compromising their biomolecular structures. prosECCo75 thus emerges as a computationally efficient tool providing enhanced accuracy and broader applicability in simulating the complex interplay of interactions between ions and biomolecules, pivotal for improving our understanding of many biological processes.

biophysics↗

bioRxiv2023

Can calmodulin bind to lipids of the cytosolic leaflet of plasma membranes?

Calmodulin (CaM) is a ubiquitous calcium-sensitive messenger in eukaryotic cells. It was previously shown that CaM possesses an affinity for diverse lipid moieties, including those found on CaM-binding proteins. These facts together with our observation that CaM accumulates in membrane-rich protrusions of HeLa cells upon increased cytosolic calcium, motivated us to perform a systematic search for unmediated CaM interactions with model lipid membranes mimicking the cytosolic leaflet of plasma membranes. A range of experimental techniques and Molecular Dynamics simulations proves unambiguously that CaM interacts with lipid bilayers in the presence of calcium ions. Lipids phosphatidylserine (PS) and phosphatidylethanolamine (PE) hold the key to CaM-membrane interactions. Calcium induces an essential conformational rearrangement of CaM, but its binding to the headgroup of PS also neutralizes the membrane negative surface charge. More intriguingly, PE plays a dual role - it forms hydrogen bonds with CaM, but also destabilizes the lipid bilayer to increase exposure of hydrophobic acyl chains to the interacting proteins. Our findings suggest that upon increased intracellular calcium concentration, CaM and the cytosolic leaflet of cellular membranes can be functionally connected.

biophysics↗

bioRxiv2023

Influence of Electronic Polarization on the Binding of Anions to a Chloride-Pumping Rhodopsin

The functional properties of some biological ion channels and membrane transport proteins are proposed to exploit anion-hydrophobic interactions. Here, we investigate a chloride-pumping rhodopsin (ClR) as an example of a membrane protein known to contain a defined anion binding site composed predominantly of hydrophobic residues. Using molecular dynamics simulations, we explore Cl- binding to this hydrophobic site and compare the dynamics arising when electronic polarization is neglected (CHARMM36 (c36) fixed-charge force field), included implicitly (via the prosECCo force field), or included explicitly (through the polarizable force field, AMOEBA). Free energy landscapes of Cl- moving out of the binding site and into bulk solution demonstrate that the inclusion of polarization results in stronger ion binding and a second metastable binding site in ClR. Simulations focused on this hydrophobic binding site also indicate longer binding durations and closer ion proximity when polarization is included. Furthermore, simulations reveal that Cl- within this binding site interacts with an adjacent loop to facilitate rebinding events that are not observed when polarization is neglected. These results demonstrate how the inclusion of polarization can influence the behavior of anions within protein binding sites and thereby reveal novel mechanisms. Statement of SignificanceMolecular simulations based on classical (Newtonian) mechanics represent the most common method of visualizing the behavior of water and ions within channels and nanopores. Although computationally efficient, many of the approximations required mean that these simulations often do not fully capture the complex and dynamic interactions involved. Here, we use the prosECCo force field that offers an improved electronic description whilst maintaining computational efficiency. We show that using this method to include the effects of polarization greatly influences the binding dynamics of anions to a protein binding site and yields results similar to more accurate but computationally demanding methods.

biophysics↗

bioRxiv2021

Use of Raman and Raman optical activity to extract atomistic details of saccharides in aqueous solution

Sugars are crucial components in biosystems and industrial applications. In aqueous environments, the natural state of short saccharides or charged glycosaminoglycans is floating and wiggling in solution. Therefore, tools to characterize their structure in a native aqueous environment are crucial but not always available. Here, we show that a combination of Raman/ROA and, on occasions, NMR experiments with Molecular Dynamics (MD) and Quantum Mechanics (QM) is a viable method to gain insights into structural features of sugars in solutions. Combining these methods provides information about accessible ring puckering conformers and their proportions. It also provides information about the conformation of the linkage between the sugar monomers, i.e., glycosidic bonds, allowing for identifying significantly accessible conformers and their relative abundance. For mixtures of sugar moieties, this method enables the deconvolution of the Raman/ROA spectra to find the actual amounts of its molecular constituents, serving as an effective analytical technique. For example, it allows calculating anomeric ratios for reducing sugars and analyzing more complex sugar mixtures to elucidate their real content. Altogether, we show that combining Raman/ROA spectroscopies with simulations is a versatile method applicable to saccharides. It allows for accessing many features with precision comparable to other methods routinely used for this task, making it a viable alternative. Furthermore, we prove that the proposed technique can scale up by studying the complicated Raffinose trisaccharide, and therefore, we expect its wide adoption to characterize sugar structural features in solution.

biophysics↗

bioRxiv2020

N-glycosylation blocks and simultaneously fosters different receptor-ligand binding sites: the chameleonic CD44-hyaluronan interaction

While DNA encodes protein structure, glycans provide a complementary layer of information to protein function. As a prime example of the significance of glycans, the ability of the cell surface receptor CD44 to bind its ligand, hyaluronan, is modulated by N-glycosylation. However, the details of this modulation remain unclear. Based on atomistic simulations and NMR, we provide evidence that CD44 has multiple distinct binding sites for hyaluronan, and that N-glycosylation modulates their respective roles. We find that non-glycosylated CD44 favors the canonical sub-micromolar binding site, while glycosylated CD44 binds hyaluronan with an entirely different micromolar binding site. Our findings show (for the first time) how glycosylation can alter receptor affinity by shielding specific regions of the host protein, thereby promoting weaker binding modes. The mechanism revealed in this work emphasizes the importance of glycosylation in protein function and poses a challenge for protein structure determination where glycosylation is usually neglected.

molecular biology↗