The structure and orientation of the C-terminus of LRAP

Three Decades of REDOR in Protein Science: A Solid-State NMR Technique for Distance Measurement and Spectral Editing

Solid-state NMR (ss-NMR) is a powerful tool to investigate noncrystallizable, poorly soluble molecular systems, such as membrane proteins, amyloids, and cell walls, in environments that closely resemble their physical sites of action. Rotational-echo double resonance (REDOR) is an ss-NMR methodology, which by reintroducing heteronuclear dipolar coupling under magic angle spinning conditions provides intramolecular and intermolecular distance restraints at the atomic level. In addition, REDOR can be exploited as a selection tool to filter spectra based on dipolar couplings. Used extensively as a spectroscopic ruler between isolated spins in site-specifically labeled systems and more recently as a building block in multidimensional ss-NMR pulse sequences allowing the simultaneous measurement of multiple distances, REDOR yields atomic-scale information on the structure and interaction of proteins. By extending REDOR to the determination of 1H-X dipolar couplings in recent years, the limit of measurable distances has reached ~15-20 Å, making it an attractive method of choice for the study of complex biomolecular assemblies. Following a methodological introduction including the most recent implementations, examples are discussed to illustrate the versatility of REDOR in the study of biological systems.

Biomedical applications of solid-binding peptides and proteins

Over the past decades, solid-binding peptides (SBPs) have found multiple applications in materials science. In non-covalent surface modification strategies, solid-binding peptides are a simple and versatile tool for the immobilization of biomolecules on a vast variety of solid surfaces. Especially in physiological environments, SBPs can increase the biocompatibility of hybrid materials and offer tunable properties for the display of biomolecules with minimal impact on their functionality. All these features make SBPs attractive for the manufacturing of bioinspired materials in diagnostic and therapeutic applications. In particular, biomedical applications such as drug delivery, biosensing, and regenerative therapies have benefited from the introduction of SBPs. Here, we review recent literature on the use of solid-binding peptides and solid-binding proteins in biomedical applications. We focus on applications where modulating the interactions between solid materials and biomolecules is crucial. In this review, we describe solid-binding peptides and proteins, providing background on sequence design and binding mechanism. We then discuss their application on materials relevant for biomedicine (calcium phosphates, silicates, ice crystals, metals, plastics, and graphene). Although the limited characterization of SBPs still represents a challenge for their design and widespread application, our review shows that SBP-mediated bioconjugation can be easily introduced into complex designs and on nanomaterials with very different surface chemistries.

Amelogenin-Derived Peptides in Bone Regeneration: A Systematic Review

Amelogenins are enamel matrix proteins currently used to treat bone defects in periodontal surgery. Recent studies have highlighted the relevance of amelogenin-derived peptides, named LRAP, TRAP, SP, and C11, in bone tissue engineering. Interestingly, these peptides seem to maintain or even improve the biological activity of the full-length protein, which has received attention in the field of bone regeneration. In this article, the authors combined a systematic and a narrative review. The former is focused on the existing scientific evidence on LRAP, TRAP, SP, and C11's ability to induce the production of mineralized extracellular matrix, while the latter is concentrated on the structure and function of amelogenin and amelogenin-derived peptides. Overall, the collected data suggest that LRAP and SP are able to induce stromal stem cell differentiation towards osteoblastic phenotypes; specifically, SP seems to be more reliable in bone regenerative approaches due to its osteoinduction and the absence of immunogenicity. However, even if some evidence is convincing, the limited number of studies and the scarcity of in vivo studies force us to wait for further investigations before drawing a solid final statement on the real potential of amelogenin-derived peptides in bone tissue engineering.

Binding Mode Characterization of Osteopontin on Hydroxyapatite by Solution NMR Spectroscopy

Extracellular matrix glycoproteins play a major role in bone mineralization and modulation of osteogenesis. Among these, the intrinsically disordered protein osteopontin (OPN) is associated with the inhibition of formation, growth and proliferation of the bone mineral hydroxyapatite (HAP). Furthermore, post-translational modifications like phosphorylation can alter conformations and interaction properties of intrinsically disordered proteins (IDPs). Therefore, the actual interaction of OPN with a HAP surface on an atomic level and how this interaction is affected by phosphorylation is of great interest. Here, we study the interaction of full-length OPN on the surface of suspended HAP nanoparticles by solution NMR spectroscopy. We report the binding modes of this IDP and provide evidence for the influence of hyperphosphorylation on the binding character and an explanation for the differing roles in biomineralization. Our study moreover presents an easy and suitable option to measure interaction of nanoparticles in a stable suspension with full-length proteins.

An oligopeptide/aptamer-conjugated dendrimer-based nanocarrier for dual-targeting delivery to bone

Bone targeting is one of the most potentially valuable therapeutic methods for medically treating bone diseases, such as osteoarthritis, osteoporosis, nonunion bone defects, bone cancer, and myeloma-related bone disease, but its efficacy remains a challenge due to unfavorable bone biodistribution, off-target effects, and the lack of cell specificity. To address these problems, we synthesized a new dual-targeting nanocarrier for delivery to bone by covalently modifying the G4.0 PAMAM dendrimer with the C11 peptide and the CH6 aptamer (CH6-PAMAM-C11). The molecular structure was confirmed using ¹H-NMR and FT-IR spectroscopy. CLSM results showed that the novel nanocarrier could successfully accumulate in the targeted cells, mineralized areas and tissues. DLS and TEM demonstrated that CH6-PAMAM-C11 was approximately 40-50 nm in diameter. In vitro targeting experiments confirmed that the C11 ligand had a high affinity for HAP, while the CH6 aptamer had a high affinity for osteoblasts. The in vivo biodistribution analysis showed that CH6-PAMAM-C11 could rapidly accumulate in bone within 4 h and 12 h and then deliver drugs to sites of osteoblast activity. The components of CH6-PAMAM-C11 were well excreted via the kidneys. The accumulation of many more CH6-PAMAM-C11 dual-targeting nanocarriers than single-targeting nanocarriers was observed in the periosteal layer of the rat skull, along with aggregation at sites of osteoblast activity. All of these results indicate that CH6-PAMAM-C11 may be a promising nanocarrier for the delivery of drugs to bone, particularly for the treatment of osteoporosis, and our research strategy may serve as a reference for research in targeted drug, small molecule drug and nucleic acid delivery.

Unraveling the mechanism for an amelogenin-derived peptide regulated hydroxyapatite mineralization via specific functional domain identification

Amelogenin and its various derived peptides play important roles in promoting biomimetic mineralization of enamel. Previously, an amelogenin-derived peptide named QP5 was proved to be able to repair demineralized enamel. The objective here was to interpret the mechanism of QP5 by elucidating the specific function of each domain for further sequence and efficacy improvement. Peptide QP5 was separated into domains (QPX)5 and C-tail. (QPX)3 was also synthesized to investigate how QPX repeats affect the mineralization process. Circular dichroism spectroscopy showed that two (QPX) repeats adopted a β-sheet structure, while C-tail exhibited a disordered structure. (QPX)5 showed more absorption in confocal laser scanning microscopy observation and a higher K value in Langmuir adsorption isotherms compared to C-tail, while (QPX)3 with better hydropathy had greater adsorption capability than (QPX)5. Meanwhile, calcium consumption kinetics, transmission electron microscopy and selected area electron diffraction indicated that (QPX)5, C-tail and (QPX)3 had similar inhibitory effects on the spontaneous calcium consumption and the morphology of their nucleation products were alike, while QP5 had a greater inhibitory effect than them and induced elongated plate-like crystals. X-Ray diffraction further showed that both C-tail and (QPX)3 had greater potential in improving the apatite crystal orientation degree. In conclusion, (QPX)5 was the major adsorption region, both (QPX)5 and C-tail inhibited the nucleation, and C-tail contributed more to improve the HAP orientation degree, so QP5 could exert a significant remineralization effect. By reducing two repeats, (QPX)3 showed higher hydropathicity than (QPX)5 and achieved higher binding affinity, and it was more potential in improving the HAP orientation degree with lower economic cost.

Solid-State NMR and MD Study of the Structure of the Statherin Mutant SNa15 on Mineral Surfaces

Elucidation of the structure and interactions of proteins at native mineral interfaces is key to understanding how biological systems regulate the formation of hard tissue structures. In addition, understanding how these same proteins interact with non-native mineral surfaces has important implications for the design of medical and dental implants, chromatographic supports, diagnostic tools, and a host of other applications. Here, we combine solid-state NMR spectroscopy, isotherm measurements, and molecular dynamics simulations to study how SNa15, a peptide derived from the hydroxyapatite (HAP) recognition domain of the biomineralization protein statherin, interacts with HAP, silica (SiO₂), and titania (TiO₂) mineral surfaces. Adsorption isotherms are used to characterize the binding affinity of SNa15 to HAP, SiO₂, and TiO₂. We also apply 1D ¹³C CP MAS, 1D ¹⁵N CP MAS, and 2D ¹³C-¹³C DARR experiments to SNa15 samples with uniformly ¹³C- and ¹⁵N-enriched residues to determine backbone and side-chain chemical shifts. Different computational tools, namely TALOS-N and molecular dynamics simulations, are used to deduce secondary structure from backbone and side-chain chemical shift data. Our results show that SNa15 adopts an α-helical conformation when adsorbed to HAP and TiO₂, but the helix largely unravels upon adsorption to SiO₂. Interactions with HAP are mediated in general by acidic and some basic amino acids, although the specific amino acids involved in direct surface interaction vary with surface. The integrated experimental and computational approach used in this study is able to provide high-resolution insights into adsorption of proteins on interfaces.

Bilirubin Oxidase Adsorption onto Charged Self-Assembled Monolayers: Insights from Multiscale Simulations

The efficient immobilization and orientation of bilirubin oxidase (BOx) on different solid substrates are essential for its application in biotechnology. The T1 copper site within BOx is responsible for the electron transfer. In order to obtain quick direct electron transfer (DET), it is important to keep the distance between the T1 copper site and electrode surface small and to maintain the natural structure of BOx at the same time. In this work, the combined parallel tempering Monte Carlo simulation with the all-atom molecular dynamics simulation approach was adopted to reveal the adsorption mechanism, orientation, and conformational changes of BOx from Myrothecium verrucaria (MvBOx) adsorbed on charged self-assembled monolayers (SAMs), including COOH-SAM and NH₂-SAM with different surface charge densities (±0.05 and ±0.19 C·m^-2). The results show that MvBOx adsorbs on negatively charged surfaces with a "back-on" orientation, whereas on positively charged surfaces, MvBOx binds with a "lying-on" orientation. The locations of the T1 copper site are closer to negatively charged surfaces. Furthermore, for negatively charged surfaces, the T1 copper site prefers to orient closer to the surface with lower surface charge density. Therefore, the negatively charged surface with low surface charge density is more suitable for the DET of MvBOx on electrodes. Besides, the structural changes primarily take place on the relatively flexible turns, coils, and α-helix. The native structure of MvBOx is well preserved when it adsorbs on both charged surfaces. This work sheds light on the controlling orientation and conformational information on MvBOx on charged surfaces at the atomistic level. This understanding would certainly promote our understanding of the mechanism of MvBOx immobilization and provide theoretical support for BOx-based bioelectrode design.

Bioinspired heptapeptides as functionalized mineralization inducers with enhanced hydroxyapatite affinity

The regeneration of mineral crystals under physiological conditions is an efficient way to repair defects in hard tissues. To achieve robust mineralization on surfaces such as the tooth enamel, an inducer requires strong affinity with the substrates and should be able to induce mineralization. Thus far, most studies used a single molecule containing two components to realize the above functions separately, which might be troublesome to synthesize and purify. In this work, inspired by the statherin in the salivary acquired pellicle, we designed a simple peptide sequence, Asp-Asp-Asp-Glu-Glu-Lys-Cys (peptide-7), to accomplish the dual tasks of adsorption and mineralization on enamel surfaces. We speculate the calcium binding ability of the negatively charged carboxylic acid groups in the peptide itself contributes to the dual functions of peptide-7. In vitro and in vivo experiments demonstrated its excellent repair effect on enamel as compared to fluoride. More importantly, due to the strong affinity between peptides and hydroxyapatite, a compact mineralized crystal layer and a strong adhesion between the regenerated minerals and the bottom substrates were observed, similar to the effect induced by fluoride. This work sheds light on the interaction mechanism between peptide-7 and minerals. In addition, since it is safer than fluoride, peptide-7 may have potential applications in the repair of other hard tissues and the functionalization of biomaterials.

Meeting report: a hard look at the state of enamel research

The Encouraging Novel Amelogenesis Models and Ex vivo cell Lines (ENAMEL) Development workshop was held on 23 June 2017 at the Bethesda headquarters of the National Institute of Dental and Craniofacial Research (NIDCR). Discussion topics included model organisms, stem cells/cell lines, and tissues/3D cell culture/organoids. Scientists from a number of disciplines, representing institutions from across the United States, gathered to discuss advances in our understanding of enamel, as well as future directions for the field.

Single Molecule Force Spectroscopy and Molecular Dynamics Simulations as a Combined Platform for Probing Protein Face-Specific Binding

Biomolecular interactions frequently occur in orientation-specific manner. For example, prior nuclear magnetic resonance spectroscopy experiments in our lab have suggested the presence of a group of strongly binding residues on a particular face of the protein ubiquitin for interactions with Capto MMC multimodal ligands ("Capto" ligands) (Srinivasan, K.; et al. Langmuir 2014, 30 (44), 13205-13216). We present a clear confirmation of those studies by performing single-molecule force spectroscopy (SMFS) measurements of unbinding complemented with molecular dynamics (MD) calculations of the adsorption free energy of ubiquitin in two distinct orientations with self-assembled monolayers (SAMs) functionalized with "Capto" ligands. These orientations were maintained in the SMFS experiments by tethering ubiquitin mutants to SAM surfaces through strategically located cysteines, thus exposing the desired faces of the protein. Analogous orientations were maintained in MD simulations using suitable constraining methods. Remarkably, despite differences between the finer details of experimental and simulation methodologies, they confirm a clear preference for the previously hypothesized binding face of ubiquitin. Furthermore, MD simulations provided significant insights into the mechanism of protein binding onto this multimodal surface. Because SMFS and MD simulations both directly probe protein-surface interactions, this work establishes a key link between experiments and simulations at molecular scale through the determination of protein face-specific binding energetics. Our approach may have direct applications in biophysical systems where face- or orientation-specific interactions are important, such as biomaterials, sensors, and biomanufacturing.

Protein Phosphorylation and Mineral Binding Affect the Secondary Structure of the Leucine-Rich Amelogenin Peptide

Previously, we have shown that serine-16 phosphorylation in native full-length porcine amelogenin (P173) and the Leucine-Rich Amelogenin Peptide (LRAP(+P)), an alternative amelogenin splice product, affects protein assembly and mineralization in vitro. Notably, P173 and LRAP(+P) stabilize amorphous calcium phosphate (ACP) and inhibit hydroxyapatite (HA) formation, while non-phosphorylated counterparts (rP172, LRAP(-P)) guide the growth of ordered bundles of HA crystals. Based on these findings, we hypothesize that the phosphorylation of full-length amelogenin and LRAP induces conformational changes that critically affect its capacity to interact with forming calcium phosphate mineral phases. To test this hypothesis, we have utilized Fourier transform infrared spectroscopy (FTIR) to determine the secondary structure of LRAP(-P) and LRAP(+P) in the absence/presence of calcium and selected mineral phases relevant to amelogenesis; i.e., hydroxyapatite (HA: an enamel crystal prototype) and (ACP: an enamel crystal precursor phase). Aqueous solutions of LRAP(-P) or LRAP(+P) were prepared with or without 7.5 mM of CaCl₂ at pH 7.4. FTIR spectra of each solution were obtained using attenuated total reflectance, and amide-I peaks were analyzed to provide secondary structure information. Secondary structures of LRAP(+P) and LRAP(-P) were similarly assessed following incubation with suspensions of HA and pyrophosphate-stabilized ACP. Amide I spectra of LRAP(-P) and LRAP(+P) were found to be distinct from each other in all cases. Spectra analyses showed that LRAP(-P) is comprised mostly of random coil and β-sheet, while LRAP(+P) exhibits more β-sheet and α-helix with little random coil. With added Ca, the random coil content increased in LRAP(-P), while LRAP(+P) exhibited a decrease in α-helix components. Incubation of LRAP(-P) with HA or ACP resulted in comparable increases in β-sheet structure. Notably, however, LRAP(+P) secondary structure was more affected by ACP, primarily showing an increase in β-sheet structure, compared to that observed with added HA. These collective findings indicate that phosphorylation induces unique secondary structural changes that may enhance the functional capacity of native phosphorylated amelogenins like LRAP to stabilize an ACP precursor phase during early stages of enamel mineral formation.

STEM and HRTEM studies of accumulated deposits on human tooth surface

The aim of this study was to clarify the fine structure of accumulated deposits on the surface of teeth that are considered to affect the gloss of teeth. The study was carried out using, as specimens, human incisor teeth having gloss, which were extracted from teenage donors and those incapable of showing gloss even by brushing which were extracted from donors in their 50s. Thin longitudinal sections of tooth enamel with accumulated deposits on the surface were prepared by focused ion beam (FIB) milling, and the fine structure was analyzed using a scanning transmission electron microscope (STEM) and a high resolution transmission electron microscope (HRTEM). By FIB, thin longitudinal sections could be prepared from tooth enamel together with organic and inorganic substances accumulated on the surface without artifacts. The accumulated deposits on the surface of teeth having gloss were composed of organic substances. However, it was first revealed by STEM observation that the accumulated solid deposits on the surface of teeth having no gloss had a complicated structure wherein inorganic and organic substances coexisted. It is suggested that the organic substances contain proteins derived from saliva. The inorganic substances were spherical and needle-like hydroxyapatites (HAs). It is considered that amino acids constituting the proteins affected the nucleus formation and the crystal formation of HA. It is considered that the unevenness of the accumulated deposits existing on the surface of tooth enamel having no gloss causes the decrease in gloss of teeth due to diffuse reflection of light.

Solid-state NMR studies of proteins immobilized on inorganic surfaces

Solid state NMR is the primary tool for studying the quantitative, site-specific structure, orientation, and dynamics of biomineralization proteins under biologically relevant conditions. Two calcium phosphate proteins, statherin (43 amino acids) and leucine rich amelogenin protein (LRAP; 59 amino acids), have been studied in depth and have different dynamic properties and 2D- and 3D-structural features. These differences make it difficult to extract design principles used in nature for building materials with properties such as high strength, unusual morphologies, or uncommon phases. Consequently, design principles needed for developing synthetic materials controlled by proteins are not clear. Many biomineralization proteins are much larger than statherin and LRAP, necessitating the study of larger biomineralization proteins. More recent studies of the significantly larger full-length amelogenin (180 residues) represent a significant step forward to ultimately investigate the full diversity of biomineralization proteins. Interactions of amino acids, a silaffin derived peptide, and the model LK peptide with silica are also being studied, along with qualitative studies of the organic matrices interacting with calcium carbonate. Dipolar recoupling techniques have formed the core of the quantitative studies, yet the need for isolated spin pairs makes this approach costly and time intensive. The use of multi-dimensional techniques to study biomineralization proteins is becoming more common, methodology which, despite its challenges with these difficult-to-study proteins, will continue to drive future advancements in this area.

Amelogenin and Enamel Biomimetics

Mature tooth enamel is acellular and does not regenerate itself. Developing technologies that rebuild tooth enamel and preserve tooth structure is therefore of great interest. Considering the importance of amelogenin protein in dental enamel formation, its ability to control apatite mineralization in vitro, and its potential to be applied in fabrication of future bio-inspired dental material this review focuses on two major subjects: amelogenin and enamel biomimetics. We review the most recent findings on amelogenin secondary and tertiary structural properties with a focus on its interactions with different targets including other enamel proteins, apatite mineral, and phospholipids. Following a brief overview of enamel hierarchical structure and its mechanical properties we will present the state-of-the-art strategies in the biomimetic reconstruction of human enamel.

Amelogenin and enamel biomimetics

Mature tooth enamel is acellular and does not regenerate itself.

The flexible structure of the K24S28 region of Leucine-Rich Amelogenin Protein (LRAP) bound to apatites as a function of surface type, calcium, mutation, and ionic strength

Leucine-Rich Amelogenin Protein (LRAP) is a member of the amelogenin family of biomineralization proteins, proteins which play a critical role in enamel formation. Recent studies have revealed the structure and orientation of the N- and C-terminus of LRAP bound to hydroxyapatite (HAP), a surface used as an analog of enamel. The structure of one region, K24 to S28, was found to be sensitive to phosphorylation of S16, the only naturally observed site of serine phosphorylation in LRAP, suggesting that K24S28 may sit at a key region of structural flexibility and play a role in the protein's function. In this work, we investigated the sensitivity of the structure and orientation of this region when bound to HAP as a function of several factors which may vary during enamel formation to influence structure: the ionic strength (0.05, 0.15, 0.2 M), the calcium concentration (0.07 and 0.4 mM), and the surface to which it is binding [HAP and carbonated apatite (CAP), a more direct mimic of enamel]. A naturally occurring mutation found in amelogenin (T21I) was also investigated. The structure in the K24S28 region of the protein was found to be sensitive to these conditions, with the CAP surface and excess Ca(2+) (8:1 [Ca(2+)]:[LRAP-K24S28(+P)]) resulting in a tighter helix, while low ionic strength relaxed the helical structure. Higher ionic strength and the point mutation did not result in any structural change in this region. The distance of the backbone of K24 from the surface was most sensitive to excess Ca(2+) and in the T21I-mutation. Collectively, these data suggest that phosphorylated LRAP is able to accommodate structural changes while maintaining its interaction with the surface, and provides further evidence of the structural sensitivity of the K24S28 region, a sensitivity that may contribute to function in biomineralization.

Recent NMR developments applied to organic-inorganic materials

In this contribution, the latest developments in solid state NMR are presented in the field of organic-inorganic (O/I) materials (or hybrid materials). Such materials involve mineral and organic (including polymeric and biological) components, and can exhibit complex O/I interfaces. Hybrids are currently a major topic of research in nanoscience, and solid state NMR is obviously a pertinent spectroscopic tool of investigation. Its versatility allows the detailed description of the structure and texture of such complex materials. The article is divided in two main parts: in the first one, recent NMR methodological/instrumental developments are presented in connection with hybrid materials. In the second part, an exhaustive overview of the major classes of O/I materials and their NMR characterization is presented.

Solid-state NMR studies of biomineralization peptides and proteins

Nature has evolved sophisticated strategies for engineering hard tissues through the interaction of proteins, and ultimately cells, with inorganic mineral phases. This process, called biomineralization, is how living organisms transform inorganic materials such as hydroxyapatite, calcite, and silica into highly intricate and organized structures. The remarkable material properties of shell, bone, and teeth come from the activities of proteins that function at the organic-inorganic interface. A better understanding of the biomolecular mechanisms used to promote or retard the formation of mineral-based structures could provide important design principles for the development of calcification inhibitors and promoters in orthopedics, cardiology, urology, and dentistry. With the knowledge of the structural basis for control of hard tissue growth by proteins, scientists could potentially develop materials using biomimetic principles with applications in catalysis, biosensors, electronic devices, and chromatographic separations, to name a few. Additionally, biomineralization also has potential applications in electronics, catalysis, magnetism, sensory devices, and mechanical design. Where man-made hard materials require the use of extreme temperatures, high pressure, and pH, biological organisms can accomplish these feats at ambient temperature and at physiological pH. Despite the fact that many researchers want to identify and control the structure of proteins at material and biomineral interfaces, there is a decided lack of molecular-level structure information available for proteins at biomaterial interfaces in general. In particular, this holds for mammalian proteins that directly control calcification processes in hard tissue. The most fundamental questions regarding the secondary and tertiary structures of proteins adsorbed to material surfaces, how proteins catalyze the formation of biomineral composites, or how proteins interact at biomaterial interfaces remain unanswered. This is largely due to a lack of methods capable of providing high-resolution structural information for proteins adsorbed to material surfaces under physiologically relevant conditions. In this Account, we highlight recent work that is providing insight into the structure and crystal recognition mechanisms of a salivary protein model system, as well as the structure and interactions of a peptide that catalyzes the formation of biosilica composites. To develop a better understanding of the structure and interactions of proteins in biomaterials, we have used solid-state NMR techniques to determine the molecular structure and dynamics of proteins and peptides adsorbed onto inorganic crystal surfaces and embedded within biomineral composites. This work adds to the understanding of the structure and crystal recognition mechanisms of an acidic human salivary phosphoprotein, statherin.

Phosphorylation and ionic strength alter the LRAP-HAP interface in the N-terminus

The conditions present during enamel crystallite development change dramatically as a function of time, including the pH, protein concentration, surface type, and ionic strength. In this work, we investigate the role that two of these changing conditions, pH and ionic strength, have in modulating the interaction of the amelogenin, LRAP, with hydroxyapatite (HAP). Using solid-state NMR dipolar recoupling and chemical shift data, we investigate the structure, orientation, and dynamics of three regions in the N-terminus of the protein: L(15) to V(19), V(19) to L(23), and K(24) to S(28). These regions are also near the only phosphorylated residue in the protein pS(16); therefore, changes in the LRAP-HAP interaction as a function of phosphorylation (LRAP(-P) vs LRAP(+P)) were also investigated. All of the regions and conditions studied for the surface immobilized proteins showed restricted motion, with indications of slightly more mobility under all conditions for L(15)(+P) and K(24)(-P). The structure and orientation of the LRAP-HAP interaction in the N-terminus of the phosphorylated protein is very stable to changing solution conditions. From REDOR dipolar recoupling data, the structure and orientation in the region L(15)V(19)(+P) did not change significantly as a function of pH or ionic strength. The structure and orientation of the region V(19)L(23)(+P) were also stable to changes in pH, with the only significant change observed at high ionic strength, where the region becomes extended, suggesting this may be an important region in regulating mineral development. Chemical shift studies also suggest minimal changes in all three regions studied for both LRAP(-P) and LRAP(+P) as a function of pH or ionic strength, and also reveal that K(24) has multiple resolvable resonances, suggestive of two coexisting structures. Phosphorylation also alters the LRAP-HAP interface. All of the three residues investigated (L(15), V(19), and K(24)) are closer to the surface in LRAP(+P), but only K(24)S(28) changes structure as a result of phosphorylation, from a random coil to a largely helical structure, and V(19)L(23) becomes more extended at high ionic strength when phosphorylated. These observations suggest that ionic strength and dephosphorylation may provide switching mechanisms to trigger a change in the function of the N-terminus during enamel development.

Neutron reflectometry studies of the adsorbed structure of the amelogenin, LRAP

Amelogenins make up over 90% of the protein present during enamel formation and have been demonstrated to be critical in proper enamel development, but the mechanism governing this control is not well understood. Leucine-rich amelogenin peptide (LRAP) is a 59-residue splice variant of amelogenin and contains the charged regions from the full protein thought to control crystal regulation. In this work, we utilized neutron reflectivity (NR) to investigate the structure and orientation of LRAP adsorbed from solutions onto molecularly smooth COOH-terminated self-assembled monolayer (SAM) surfaces. Sedimentation velocity (SV) experiments revealed that LRAP is primarily a monomer in saturated calcium phosphate (SCP) solutions (0.15 M NaCl) at pH 7.4. LRAP adsorbed as ∼32 Å thick layers at ∼70% coverage as determined by NR. Rosetta simulations of the dimensions of LRAP in solution (37 Å diameter) indicate that the NR determined z dimension is consistent with an LRAP monomer. SV experiments and Rosetta simulations show that the LRAP monomer has an extended, asymmetric shape in solution. The NR data suggests that the protein is not completely extended on the surface, having some degree of structure away from the surface. A protein orientation with the C-terminal and inner N-terminal regions (residues ∼8-24) located near the surface is consistent with the higher scattering length density (SLD) found near the surface by NR. This work presents new information on the tertiary and quaternary structure of LRAP in solution and adsorbed onto surfaces. It also presents further evidence that the monomeric species may be an important functional form of amelogenin proteins.

Biophysical characterization of synthetic amelogenin C-terminal peptides

Amelogenin plays a key role in the formation of the highly mineralized structure of tooth enamel. During the secretory stage, amelogenin is cleaved gradually by a protease, matrix metalloproteinase-20 (MMP-20), releasing hydrophilic C-terminal peptides. In this study, the biophysical properties of synthetic C-terminal peptides (of 28, 17, and 11 residues), mimicking native peptides, were explored in vitro. A sudden decrease was observed in the zeta (ζ)-potential upon the addition of calcium or phosphates, which was also accompanied by an increased aggregation propensity of the peptides. Under most of the experimental conditions, the particle size increased at a pH 2-3 units higher than the isoelectric point (pI) of the peptides, while the peptides existed as smaller particles (<2 nm) near their pI values and in the acidic range. They showed poor affinity for calcium and phosphates, comparable to full-length amelogenin and variants. The secondary structure determination showed that the 11-amino-acid peptide contained defined secondary structure comprising beta-sheets and turns. Atomic force microscopy analysis revealed the presence of thin, disk-like nanostructures of 54.4 nm diameter for the 28-amino-acid peptide and 54.9 nm diameter for the 11-amino acid peptide, whereas no definite structures were observed for the 17-amino-acid peptide. It is concluded that the amelogenin C-terminal peptides are capable of interacting with calcium and phosphate ions, of self-assembly into nanostructures, and may have some secondary structure, and hence may have some role in enamel synthesis.

Partial high-resolution structure of phosphorylated and non-phosphorylated leucine-rich amelogenin protein adsorbed to hydroxyapatite

The formation of biogenic materials requires the interaction of organic molecules with the mineral phase. In forming enamel, the amelogenin proteins contribute to the mineralization of hydroxyapatite (HAp). Leucine-rich amelogenin protein (LRAP) is a naturally occurring splice variant of amelogenin that comprises amelogenin's predicted HAp binding domains. We determined the partial structure of phosphorylated and non-phosphorylated LRAP variants bound to HAp using combined solid-state NMR (ssNMR) and ssNMR-biased computational structure prediction. New ssNMR measurements in the N-terminus indicate a largely extended structure for both variants, though some measurements are consistent with a partially helical N-terminal segment. The N-terminus of the phosphorylated variant is found to be consistently closer to the HAp surface than the non-phosphorylated variant. Structure prediction was biased using 21 ssNMR measurements in the N- and C-terminus at five HAp crystal faces. The predicted fold of LRAP is similar at all HAp faces studied, regardless of phosphorylation. Largely consistent with experimental observations, LRAP's predicted structure is relatively extended with a helix-turn-helix motif in the N-terminal domain and some helix in the C-terminal domain, and the N-terminal domain of the phosphorylated variant binds HAp more closely than the N-terminal domain of the non-phosphorylated variant. Predictions for both variants show some potential binding specificity for the {010} HAp crystal face, providing further support that amelogenins block crystal growth on the a and b faces to allow elongated crystals in the c-axis.

Leucine-rich amelogenin peptides regulate mineralization in vitro

Amelogenin's capacity to regulate enamel formation is related to its conserved N- and C-terminal domains, its ability to self-assemble, and its ability to stabilize amorphous calcium phosphate (ACP) - a capacity enhanced by amelogenin phosphorylation. This in vitro study provides further insight into amelogenin function, using variations of the Leucine-Rich Amelogenin Peptide (LRAP), an alternative splice product comprised solely of amelogenin's N- and C-terminal domains. Peptide self-assembly was studied by dynamic light-scattering and transmission electron microscopy (TEM). TEM, selected area electron diffraction, and Fourier transform-infrared spectroscopy were also used to determine the effect of phosphorylated and non-phosphorylated LRAP on calcium phosphate formation. Results show that phosphorylated and non-phosphorylated LRAP can self-assemble into chain-like structures in a fashion dependent on the C-terminal domain. Notably, this capacity was enhanced by added calcium and to a much greater degree for phosphorylated LRAP. Furthermore, phosphorylated LRAP was found to stabilize ACP and prevent its transformation to hydroxyapatite (HA), while aligned HA crystals formed in the presence of non-phosphorylated LRAP. The N- and C-terminal amelogenin domains in non-phosphorylated LRAP are, therefore, sufficient to guide ACP transformation into ordered bundles of apatite crystals, making LRAP an excellent candidate for biomimetic approaches for enamel regeneration.

Potential role of the amelogenin N-terminus in the regulation of calcium phosphate formation in vitro

N-terminal and C-terminal (CT) domains of amelogenin have been shown to be essential for proper enamel formation. Recent studies have also suggested that although the C-terminus plays an apparent role in protein-mineral interactions, other amelogenin structural domains are involved. The objective was to explore the role of the amelogenin N-terminus in the regulation of calcium phosphate formation in vitro. Spontaneous mineralization studies were carried out using the phosphorylated (+P) and nonphosphorylated (-P) N-terminus of the leucine-rich amelogenin peptide (LRAP) that lacks the hydrophilic CT domain. Mineralization progress was monitored via changes in solution pH. Mineral phases formed were characterized using TEM, selected area electron diffraction, and FT-IR. In controls, amorphous calcium phosphate was initially formed and subsequently transformed to randomly oriented hydroxyapatite (HA) plate-like crystals. In contrast to the control, LRAP(+P)-CT stabilized ACP formation for >1 day, while LRAP(-P)-CT accelerated the transformation of ACP to HA but had little effect on crystal shape or orientation. In conclusion, the N-terminal domain found in LRAP, as in amelogenins, appears to have the capacity to interact with forming calcium phosphate mineral phases. Results suggest that the N-terminal domain of amelogenin may play a direct role in early stages of enamel formation.

Toward a structure determination method for biomineral-associated protein using combined solid- state NMR and computational structure prediction

Protein-biomineral interactions are paramount to materials production in biology, including the mineral phase of hard tissue. Unfortunately, the structure of biomineral-associated proteins cannot be determined by X-ray crystallography or solution nuclear magnetic resonance (NMR). Here we report a method for determining the structure of biomineral-associated proteins. The method combines solid-state NMR (ssNMR) and ssNMR-biased computational structure prediction. In addition, the algorithm is able to identify lattice geometries most compatible with ssNMR constraints, representing a quantitative, novel method for investigating crystal-face binding specificity. We use this method to determine most of the structure of human salivary statherin interacting with the mineral phase of tooth enamel. Computation and experiment converge on an ensemble of related structures and identify preferential binding at three crystal surfaces. The work represents a significant advance toward determining structure of biomineral-adsorbed protein using experimentally biased structure prediction. This method is generally applicable to proteins that can be chemically synthesized.

Amelogenin nanoparticles in suspension: deviations from spherical shape and pH-dependent aggregation

It is well-known that amelogenin self-assembles to form nanoparticles, usually referred to as amelogenin nanospheres, despite the fact that not much is known about their actual shape in solution. In the current paper, we combine SAXS and DLS to study the three-dimensional shape of the recombinant amelogenins rP172 and rM179. Our results show for the first time that amelogenins build oblate nanoparticles in suspension using experimental approaches that do not require the proteins to be in contact with a support material surface. The SAXS studies give evidence for the existence of isolated amelogenin nano-oblates with aspect ratios in the range of 0.45-0.5 at pH values higher than pH 7.2 and show an aggregation of these nano-oblates at lower pH values. The role of the observed oblate shape in the formation of chain-like structures at physiological conditions is discussed as a key factor in the biomineralization of dental enamel.

Sum frequency generation and solid-state NMR study of the structure, orientation, and dynamics of polystyrene-adsorbed peptides

The power of combining sum frequency generation (SFG) vibrational spectroscopy and solid-state nuclear magnetic resonance (ssNMR) spectroscopy to quantify, with site specificity and atomic resolution, the orientation and dynamics of side chains in synthetic model peptides adsorbed onto polystyrene (PS) surfaces is demonstrated in this study. Although isotopic labeling has long been used in ssNMR studies to site-specifically probe the structure and dynamics of biomolecules, the potential of SFG to probe side chain orientation in isotopically labeled surface-adsorbed peptides and proteins remains largely unexplored. The 14 amino acid leucine-lysine peptide studied in this work is known to form an alpha-helical secondary structure at liquid-solid interfaces. Selective, individual deuteration of the isopropyl group in each leucine residue was used to probe the orientation and dynamics of each individual leucine side chain of LKalpha14 adsorbed onto PS. The selective isotopic labeling methods allowed SFG analysis to determine the orientations of individual side chains in adsorbed peptides. Side chain dynamics were obtained by fitting the deuterium ssNMR line shape to specific motional models. Through the combined use of SFG and ssNMR, the dynamic trends observed for individual side chains by ssNMR have been correlated with side chain orientation relative to the PS surface as determined by SFG. This combination provides a more complete and quantitative picture of the structure, orientation, and dynamics of these surface-adsorbed peptides than could be obtained if either technique were used separately.

A solution NMR investigation into the murine amelogenin splice-variant LRAP (Leucine-Rich Amelogenin Protein)

Amelogenins are the dominant proteins present in ameloblasts during the early stages of enamel biomineralization, making up >90% of the matrix protein. Along with the full-length protein there are several splice-variant isoforms of amelogenin present including LRAP (Leucine-Rich Amelogenin Protein), a protein that consists of the first 33 and the last 26 residues of full-length amelogenin. Using solution-state NMR spectroscopy we have assigned the (1)H-(15)N HSQC spectrum of murine LRAP (rp(H)LRAP) in 2% acetic acid at pH 3.0 by making extensive use of previous chemical shift assignments for full-length murine amelogenin (rp(H)M180). This correlation was possible because LRAP, like the full-length protein, is intrinsically disordered under these solution conditions. The major difference between the (1)H-(15)N HSQC spectra of rp(H)M180 and rp(H)LRAP was an additional set of amide resonances for each of the seven non-proline residues between S12 and Y12 near the N-terminus of rp(H)LRAP indicating that the N-terminal region of LRAP exists in two different conformations. Analysis of the proline carbon chemical shifts suggests that the molecular basis for the two states is not a cis-trans isomerization of one or more of the proline residues in the N-terminal region. Starting from 2% acetic acid, where rp(H)LRAP was monomeric in solution, NaCl addition effected residue specific changes in molecular dynamics manifested by the reduction in intensity and disappearance of (1)H-(15)N HSQC cross peaks. As observed for the full-length protein, these perturbations may signal early events governing supramolecular self-assembly of rp(H)LRAP into nanospheres. However, the different patterns of (1)H-(15)N HSQC cross peak perturbation between rp(H)LRAP and rp(H)M180 in high salt suggest that the termini may behave differently in their respective nanospheres, and perhaps, these differences contribute to the cell signaling properties attributable to LRAP but not to the full-length protein.

The leucine rich amelogenin protein (LRAP) adsorbs as monomers or dimers onto surfaces

Amelogenin is believed to be involved in controlling the formation of the highly anisotropic and ordered hydroxyapatite crystallites that form enamel. The adsorption behavior of amelogenin proteins onto substrates is very important because protein-surface interactions are critical to its function. We have previously used LRAP, a splice variant of amelogenin, as a model protein for the full-length amelogenin in solid-state NMR and neutron reflectivity studies at interfaces. In this work, we examined the adsorption behavior of LRAP in greater detail using model self-assembled monolayers containing COOH, CH(3), and NH(2) end groups as substrates. Dynamic light scattering (DLS) experiments indicated that LRAP in phosphate buffered saline and solutions containing low concentrations of calcium and phosphate consisted of aggregates of nanospheres. Null ellipsometry and atomic force microscopy (AFM) were used to study protein adsorption amounts and quaternary structures on the surfaces. Relatively high amounts of adsorption occurred onto the CH(3) and NH(2) surfaces from both buffer solutions. Adsorption was also promoted onto COOH surfaces only when calcium was present in the solutions suggesting an interaction that involves calcium bridging with the negatively charged C-terminus. The ellipsometry and AFM studies revealed that LRAP adsorbed onto the surfaces as small subnanosphere-sized structures such as monomers or dimers. We propose that the monomers/dimers were present in solution even though they were not detected by DLS or that they adsorbed onto the surfaces by disassembling or "shedding" from the nanospheres that are present in solution. This work reveals the importance of small subnanosphere-sized structures of LRAP at interfaces.

A (13)C{(31)P} REDOR NMR investigation of the role of glutamic acid residues in statherin- hydroxyapatite recognition

The side chain carboxyl groups of acidic proteins found in the extra-cellular matrix (ECM) of mineralized tissues play a key role in promoting or inhibiting the growth of minerals such as hydroxyapatite (HAP), the principal mineral component of bone and teeth. Among the acidic proteins found in the saliva is statherin, a 43-residue tyrosine-rich peptide that is a potent lubricant in the salivary pellicle and an inhibitor of both HAP crystal nucleation and growth. Three acidic amino acids-D1, E4, and E5-are located in the N-terminal 15 amino acid segment, with a fourth amino acid, E26, located outside the N-terminus. We have utilized (13)C{(31)P} REDOR NMR to analyze the role played by acidic amino acids in the binding mechanism of statherin to the HAP surface by measuring the distance between the delta-carboxyl (13)C spins of the three glutamic acid side chains of statherin (residues E4, E5, E26) and (31)P spins of the phosphate groups at the HAP surface. (13)C{(31)P} REDOR studies of glutamic-5-(13)C acid incorporated at positions E4 and E26 indicate a (13)C-(31)P distance of more than 6.5 A between the side chain carboxyl (13)C spin of E4 and the closest (31)P in the HAP surface. In contrast, the carboxyl (13)C spin at E5 has a much shorter (13)C-(31)P internuclear distance of 4.25 +/- 0.09 A, indicating that the carboxyl group of this side chain interacts directly with the surface. (13)C T(1rho) and slow-spinning MAS studies indicate that the motions of the side chains of E4 and E5 are more restricted than that of E26. Together, these results provide further insight into the molecular interactions of statherin with HAP surfaces.

Analysis of secondary structure and self-assembly of amelogenin by variable temperature circular dichroism and isothermal titration calorimetry

Amelogenin is a proline-rich enamel matrix protein known to play an important role in the oriented growth of enamel crystals. Amelogenin self-assembles to form nanospheres and higher order structures mediated by hydrophobic interactions. This study aims to obtain a better insight into the relationship between primary-secondary structure and self-assembly of amelogenin by applying computational and biophysical methods. Variable temperature circular dichroism studies indicated that under physiological pH recombinant full-length porcine amelogenin contains unordered structures in equilibrium with polyproline type II (PPII) structure, the latter being more populated at lower temperatures. Increasing the concentration of rP172 resulted in the promotion of folding to an ordered beta-structured assembly. Isothermal titration calorimetry dilution studies revealed that at all temperatures, self-assembly is entropically driven due to the hydrophobic effect and the molar heat of assembly (DeltaH(A)) decreases with temperature. Using a computational approach, a profile of domains in the amino acid sequence that have a high propensity to assemble and to have PPII structures has been identified. We conclude that the assembly properties of amelogenin are due to complementarity between the hydrophobic and PPII helix prone regions.

Adsorption of amelogenin onto self-assembled and fluoroapatite surfaces

The interactions of proteins at surfaces are of great importance to biomineralizaton processes and to the development and function of biomaterials. Amelogenin is a unique biomineralization protein because it self-assembles to form supramolecular structures called "nanospheres", spherical aggregates of monomers that are 20-60 nm in diameter. Although the nanosphere quaternary structure has been observed in solution, the quaternary structure of amelogenin adsorbed onto surfaces is also of great interest because the surface structure is critical to its function. We report studies of the adsorption of the amelogenin onto self-assembled monolayers (SAMs) with COOH and CH(3) end group functionality and single crystal fluoroapatite (FAP). Dynamic light scattering (DLS) experiments showed that the solutions contained nanospheres and aggregates of nanospheres. Protein adsorption onto the various substrates was evidenced by null ellipsometry, X-ray photoelectron spectroscopy (XPS), and external reflectance Fourier transform infrared spectroscopy (ERFTIR). Although only nanospheres were observed in solution, ellipsometry and atomic force microscopy (AFM) indicated that the protein adsorbates were much smaller structures than the original nanospheres, from monomers to small oligomers in size. Monomer adsorption was promoted onto the CH(3) surfaces, and small oligomer adsorption was promoted onto the COOH and FAP substrates. In some cases, remnants of the original nanospheres adsorbed as multilayers on top of the underlying subnanosphere layers. Although the small structures may be present in solution even though they are not detected by DLS, we also propose that amelogenin may adsorb by the "shedding" or disassembling of substructures from the nanospheres onto the substrates. This work suggests that amelogenin may have a range of possible quaternary structures that interact with surfaces.

Solution- and adsorbed-state structural ensembles predicted for the statherin-hydroxyapatite system

We have developed a multiscale structure prediction technique to study solution- and adsorbed-state ensembles of biomineralization proteins. The algorithm employs a Metropolis Monte Carlo-plus-minimization strategy that varies all torsional and rigid-body protein degrees of freedom. We applied the technique to fold statherin, starting from a fully extended peptide chain in solution, in the presence of hydroxyapatite (HAp) (001), (010), and (100) monoclinic crystals. Blind (unbiased) predictions capture experimentally observed macroscopic and high-resolution structural features and show minimal statherin structural change upon adsorption. The dominant structural difference between solution and adsorbed states is an experimentally observed folding event in statherin's helical binding domain. Whereas predicted statherin conformers vary slightly at three different HAp crystal faces, geometric and chemical similarities of the surfaces allow structurally promiscuous binding. Finally, we compare blind predictions with those obtained from simulation biased to satisfy all previously published solid-state NMR (ssNMR) distance and angle measurements (acquired from HAp-adsorbed statherin). Atomic clashes in these structures suggest a plausible, alternative interpretation of some ssNMR measurements as intermolecular rather than intramolecular. This work demonstrates that a combination of ssNMR and structure prediction could effectively determine high-resolution protein structures at biomineral interfaces.

The tooth enamel protein, porcine amelogenin, is an intrinsically disordered protein with an extended molecular configuration in the monomeric form

Amelogenins make up a class of proteins associated with the formation of mineralized enamel in vertebrates, possess highly conserved N- and C-terminal sequence regions, and represent an interesting model protein system for understanding biomineralization and protein assembly. Using bioinformatics, we report here the identification of molecular traits that classify 12 amelogenin proteins as members of the intrinsically disordered or unstructured protein family (IDPs), a group of proteins that normally exist as unfolded species but are capable of transformation to a folded state as part of their overall function. Using biophysical techniques (CD and NMR), we follow up on our bioinformatics studies and confirm that one of the amelogenins, recombinant porcine rP172, exists in an extended, unfolded state in the monomeric form. This protein exhibits evidence of conformational exchange between two states, and this exchange may be mediated by Pro residues in the sequence. Although the protein is globally unfolded, we detect the presence of local residual secondary structure [alpha-helix, extended beta-strand, turn/loop, and polyproline type II (PPII)] that may serve several functional roles within the enamel matrix. The extended, labile conformation of rP172 amelogenin is compatible with the known functions of amelogenin in enamel biomineralization, i.e., self-assembly, associations with other enamel matrix proteins and with calcium phosphate biominerals, and interaction with cell receptors. It is likely that the labile structure of this protein facilitates interactions of amelogenin with other macromolecules or with minerals for achievement of internal protein stabilization.

A solution NMR investigation into the early events of amelogenin nanosphere self-assembly initiated with sodium chloride or calcium chloride

Using solution-state NMR spectroscopy, new insights into the early events governing amelogenin supramolecular self-assembly have been identified using sodium chloride and calcium chloride to trigger the association. Two-dimensional 1H-15N HSQC spectra were recorded for 15N- and 13C-labeled murine amelogenin as a function of increasing NaCl and CaCl2 concentration beginning with solution conditions of 2% acetic acid at pH 3.0, where amelogenin was monomeric. Residue specific changes in molecular dynamics, manifested by the reduction in intensity and disappearance of 1H-15N HSQC cross-peaks, were observed with the addition of either salt to the protein. With increasing NaCl concentrations, residues between T21 and R31 near the N-terminus were affected first, suggesting that these residues may initiate amelogenin dimerization, the first step in nanosphere assembly. At higher NaCl concentrations, more residues near the N-terminus (Y12-I51) were affected, and with further additions of NaCl, residues near the C-terminus (L141-T171) began to show a similar change in molecular dynamics. With increasing CaCl2 concentrations, a similar stepwise change in molecular dynamics involving essentially the same set of amelogenin residues was observed. As the concentration of either salt was increased, a concomitant increase in the estimated overall rotational correlation time (tau(c)) was observed, consistent with assembly. Self-assembly into a dimer or trimer was established with dynamic light scattering studies under similar conditions that showed an increase in diameter of the smallest species from 4.1 nm in the absence of salt to 10 nm in the presence of salt. These results suggest a possible stepwise interaction mechanism, starting with the N-terminus and followed by the C-terminus, leading to amelogenin nanosphere assembly.

Structure, orientation, and dynamics of the C-terminal hexapeptide of LRAP determined using solid-state NMR

Amelogenin is the predominant protein found during enamel development and has been shown to be essential to proper enamel formation. Leucine-rich amelogenin peptide (LRAP) is a naturally occurring splice variant that preserves the charged N- and C-termini of full length amelogenin, regions thought to be crucial in interacting with hydroxaypatite. Particularly, the highly charged C-terminal hexapeptide (KREEVD) is thought to be the region most intimately interacting with hydroxyapatite (HAP). The structure of this charged region was investigated, along with the proximity to the surface and the mobility of two of the residues. The structure was found to be consistent with a random coil or more extended structure, as has been seen for more internalized residues in the C-terminus. The backbone K54(13C'), V58(13C'), and V58(15N) were all found to be close to the surface of HAP, approximately 6.0 angstroms from the nearest 31P atom, suggesting a strong interaction and emphasizing the importance of these residues in interacting with HAP. However, both ends of the hexapeptide at residues K54 and V58 experience significant mobility under hydrated conditions, implying that another portion of the protein helps to stabilize the strong LRAP-HAP interaction. Interestingly, the backbone of the C-terminal third of the protein is consistently 6.0 angstroms from the HAP surface, providing a model in this region of the protein lying flat on the surface with no three-dimensional folding. The combination of these features, that is, a random coil structure, a significant mobility, and a lack of three-dimensional folding in this region of the protein, may have an important functional role, possibly allowing maximum crystal inhibition at low protein concentrations.