Structure and dynamics of calmodulin (CaM) bound to nitric oxide synthase peptides: effects of a phosphomimetic CaM mutation

Insights into human eNOS, nNOS and iNOS structures and medicinal indications from statistical analyses of their interactions with bound compounds

83 Structures of human nNOS, 55 structures of human eNOS, 13 structures of iNOS, and about 126 reported NOS-bound compounds are summarized and analyzed. Structural and statistical analysis show that, at least one copy of each analyzed compound binds to the active site (the substrate arginine binding site) of human NOS. And binding features of the three isoforms show differences, but the binding preference of compounds is not in the way helpful for inhibitor design targeting nNOS and iNOS, or for activator design targeting eNOS. This research shows that there is a strong structural and functional similarity between oxygenase domains of human NOS isoforms, especially the architecture, residue composition, size, shape, and distribution profile of hydrophobicity, polarity and charge of the active site. The selectivity and efficacy of inhibitors over the rest of isoforms rely a lot on chance and randomness. Further increase of selectivity via rational improvement is uncertain, unpredictable and unreliable, therefore, to achieve high selectivity through targeting this site is complicated and requires combinative investigation. After analysis on the current two targeting sites in NOS, the highly conserved arginine binding pocket and H4B binding pocket, new potential drug-targeting sites are proposed based on structure and sequence profiling. This comprehensive analysis on the structure and interaction profiles of human NOS and bound compounds provides fresh insights for drug discovery and pharmacological research, and the new discovery here is practically applied to guide protein-structure based drug discovery.

Creating a Selective Nanobody Against 3-Nitrotyrosine Containing Proteins

A critical step in developing therapeutics for oxidative stress-related pathologies is the ability to determine which specific modified protein species are innocuous by-products of pathology and which are causative agents. To achieve this goal, technologies are needed that can identify, characterize and quantify oxidative post translational modifications (oxPTMs). Nanobodies (Nbs) represent exquisite tools for intracellular tracking of molecules due to their small size, stability and engineerability. Here, we demonstrate that it is possible to develop a selective Nb against an oxPTM protein, with the key advance being the use of genetic code expansion (GCE) to provide an efficient source of the large quantities of high-quality, homogenous and site-specific oxPTM-containing protein needed for the Nb selection process. In this proof-of-concept study, we produce a Nb selective for a 3-nitrotyrosine (nitroTyr) modified form of the 14-3-3 signaling protein with a lesser recognition of nitroTyr in other protein contexts. This advance opens the door to the GCE-facilitated development of other anti-PTM Nbs.

A Free-Energy Landscape Analysis of Calmodulin Obtained from an NMR Data-Utilized Multi-Scale Divide-and-Conquer Molecular Dynamics Simulation

Calmodulin (CaM) is a multifunctional calcium-binding protein, which regulates a variety of biochemical processes. CaM acts through its conformational changes and complex formation with its target enzymes. CaM consists of two globular domains (N-lobe and C-lobe) linked by an extended linker region. Upon calcium binding, the N-lobe and C-lobe undergo local conformational changes, followed by a major conformational change of the entire CaM to wrap the target enzyme. However, the regulation mechanisms, such as allosteric interactions, which regulate the large structural changes, are still unclear. In order to investigate the series of structural changes, the free-energy landscape of CaM was obtained by multi-scale divide-and-conquer molecular dynamics (MSDC-MD). The resultant free-energy landscape (FEL) shows that the Ca²⁺ bound CaM (holo-CaM) would take an experimentally famous elongated structure, which can be formed in the early stage of structural change, by breaking the inter-domain interactions. The FEL also shows that important interactions complete the structural change from the elongated structure to the ring-like structure. In addition, the FEL might give a guiding principle to predict mutational sites in CaM. In this study, it was demonstrated that the movement process of macroscopic variables on the FEL may be diffusive to some extent, and then, the MSDC-MD is suitable to the parallel computation.

The Molecular Simulation Study of nNOS Activation Induced by the Interaction Between Its Calmodulin-Binding Domain and SUMO1

Neuronal nitric oxide synthase (nNOS), an enzyme required for learning and memory, catalyzes L-arginine decomposition during nitric oxide production in mammalian neurons. Over-activation of nNOS leads to oxidative/nitrosative stress, which is part of the pathophysiological process of various neuropsychiatric disorders. Previous experimental studies suggest that nNOS is a target for small ubiquitin-like modifier 1 (SUMO1), and that SUMO1-ylation upregulates nNOS catalytic activity in hippocampal neurons. To date, a comprehensive structural model has not been proposed for nNOS SUMO1-ylation. In this study, our aim was to build in silico models to identify the non-bonded interactions between SUMO1 and the calmodulin binding domain (CaMBD) of nNOS. Using molecular docking and molecular dynamics simulation, we found that SUMO1 modification stabilizes the conformation of nNOS CaMBD, and helps maintain a conformation beneficial for nNOS catalysis. Analysis of the polar contacts and hydrogen bonds, and the root mean square derivation results showed that R726 and R727 of CaMBD formed polar contacts or high occupancy hydrogen bonds with SUMO1. Correlation factor analysis and free energy calculations showed that the W716, L734, F740, M745, and F781 residues were also involved in the SUMO1/CaMBD interaction in an orientation-dependent manner. The potential inhibitor binding pocket of SUMO1, aimed at disrupting SUMO1/CaMBD binding, was detected from the virtual screening results. Our in silico studies revealed that interfering with the non-bonded interactions of SUMO1/CaMBD would blocked nNOS SUMO-ylation and subsequent hyperactivation. This work provides novel structural insight into the functional regulation of nNOS by post-translational SUMO1 modification, and provides suggestions for the design of drugs targeting nNOS hyperactivation.

Tyrosine nitration on calmodulin enhances calcium-dependent association and activation of nitric-oxide synthase

Production of reactive oxygen species caused by dysregulated endothelial nitric-oxide synthase (eNOS) activity is linked to vascular dysfunction. eNOS is a major target protein of the primary calcium-sensing protein calmodulin. Calmodulin is often modified by the main biomarker of nitroxidative stress, 3-nitrotyrosine (nitroTyr). Despite nitroTyr being an abundant post-translational modification on calmodulin, the mechanistic role of this modification in altering calmodulin function and eNOS activation has not been investigated. Here, using genetic code expansion to site-specifically nitrate calmodulin at its two tyrosine residues, we assessed the effects of these alterations on calcium binding by calmodulin and on binding and activation of eNOS. We found that nitroTyr-calmodulin retains affinity for eNOS under resting physiological calcium concentrations. Results from in vitro eNOS assays with calmodulin nitrated at Tyr-99 revealed that this nitration reduces nitric-oxide production and increases eNOS decoupling compared with WT calmodulin. In contrast, calmodulin nitrated at Tyr-138 produced more nitric oxide and did so more efficiently than WT calmodulin. These results indicate that the nitroTyr post-translational modification, like tyrosine phosphorylation, can impact calmodulin sensitivity for calcium and reveal Tyr site-specific gain or loss of functions for calmodulin-induced eNOS activation.

The multifunctional role of phospho-calmodulin in pathophysiological processes

Calmodulin (CaM) is a versatile Ca²⁺-sensor/transducer protein that modulates hundreds of enzymes, channels, transport systems, transcription factors, adaptors and other structural proteins, controlling in this manner multiple cellular functions. In addition to its capacity to regulate target proteins in a Ca²⁺-dependent and Ca²⁺-independent manner, the posttranslational phosphorylation of CaM by diverse Ser/Thr- and Tyr-protein kinases has been recognized as an important additional manner to regulate this protein by fine-tuning its functionality. In this review, we shall cover developments done in recent years in which phospho-CaM has been implicated in signalling pathways that are relevant for the onset and progression of diverse pathophysiological processes. These include diverse systems playing a major role in carcinogenesis and tumour development, prion-induced encephalopathies and brain hypoxia, melatonin-regulated neuroendocrine disorders, hypertension, and heavy metal-induced cell toxicity.

TRPC1 Deficiency Impairs the Endothelial Progenitor Cell Function via Inhibition of Calmodulin/eNOS Pathway

Endothelial progenitor cells (EPCs) promote angiogenesis and play a pivotal role in endothelial repair and re-endothelialization after vascular injury. Transient receptor potential-canonical1 (TRPC1) has been recently implied to play important roles on EPC function. Here, we studied the role of TRPC1 in regulating EPC function in vivo and in vitro. EPCs were cultured from TRPC1-knockout mice and their controls. In vitro, TRPC1 knockout reduced EPC functional activities, including migration and tube formation. Additionally, calmodulin (CaM)/endothelial nitric oxide synthase (eNOS) signaling activity were downregulated after TRPC1 knockout. Administration of CaM or eNOS inhibitor ameliorated TRPC1 knockout-reduced EPC migration and tube formation. In vivo Matrigel plug assay confirmed that TRPC1 knockout decreased formation of functional blood vessels of EPCs compared with wild-type EPCs. Taken together, these data suggest that TRPC1 is a critical regulator of angiogenesis.

Structural Consequences of Calmodulin EF Hand Mutations

Calmodulin (CaM) is a cytosolic Ca²⁺-binding protein that serves as a control element for many enzymes. It consists of two globular domains, each containing two EF hand pairs capable of binding Ca²⁺, joined by a flexible central linker region. CaM is able to bind and activate its target proteins in the Ca²⁺-replete and Ca²⁺-deplete forms. To study the Ca²⁺-dependent/independent properties of binding and activation of target proteins by CaM, CaM constructs with Ca²⁺-binding disrupting mutations of Asp to Ala at position one of each EF hand have been used. These CaM mutant proteins are deficient in binding Ca²⁺ in either the N-lobe EF hands (CaM₁₂), C-lobe EF hands (CaM₃₄), or all four EF hands (CaM₁₂₃₄). To investigate potential structural changes these mutations may cause, we performed detailed NMR studies of CaM₁₂, CaM₃₄, and CaM₁₂₃₄ including determining the solution structure of CaM₁₂₃₄. We then investigated if these CaM mutants affected the interaction of CaM with a target protein known to interact with apoCaM by determining the solution structure of CaM₃₄ bound to the iNOS CaM binding domain peptide. The structures provide direct structural evidence of changes that are present in these Ca²⁺-deficient CaM mutants and show these mutations increase the hydrophobic exposed surface and decrease the electronegative surface potential throughout each lobe of CaM. These Ca²⁺-deficient CaM mutants may not be a true representation of apoCaM and may not allow for native-like interactions of apoCaM with its target proteins.

Correlating Calmodulin Landscapes with Chemical Catalysis in Neuronal Nitric Oxide Synthase using Time-Resolved FRET and a 5-Deazaflavin Thermodynamic Trap

A major challenge in enzymology is the need to correlate the dynamic properties of enzymes with, and understand the impact on, their catalytic cycles. This is especially the case with large, multicenter enzymes such as the nitric oxide synthases (NOSs), where the importance of dynamics has been inferred from a variety of structural, single-molecule, and ensemble spectroscopic approaches but where motions have not been correlated experimentally with mechanistic steps in the reaction cycle. Here we take such an approach. Using time-resolved spectroscopy employing absorbance and Förster resonance energy transfer (FRET) and exploiting the properties of a flavin analogue (5-deazaflavin mononucleotide (5-dFMN)) and isotopically labeled nicotinamide coenzymes, we correlate the timing of CaM structural changes when bound to neuronal nitric oxide synthase (nNOS) with the nNOS catalytic cycle. We show that remodeling of CaM occurs early in the electron transfer sequence (FAD reduction), not at later points in the reaction cycle (e.g., FMN reduction). Conformational changes are tightly correlated with FAD reduction kinetics and reflect a transient “opening” and then “closure” of the bound CaM molecule. We infer that displacement of the C-terminal tail on binding NADPH and subsequent FAD reduction are the likely triggers of conformational change. By combining the use of cofactor/coenzyme analogues and time-resolved FRET/absorbance spectrophotometry, we show how the reaction cycles of complex enzymes can be simplified, enabling a detailed study of the relationship between protein dynamics and reaction cycle chemistry—an approach that can also be used with other complex multicenter enzymes.

Chemical shift assignments of calmodulin constructs with EF hand mutations

Calmodulin (CaM) is a ubiquitous cytosolic Ca(2+)-binding protein able to bind and regulate hundreds of different proteins. It consists of two globular domains joined by a flexible central linker region. Each one of these domains contains two EF hand pairs capable of binding to Ca(2+). Upon Ca(2+) binding CaM undergoes a conformational change exposing hydrophobic patches that interact with its intracellular target proteins. CaM is able to bind to target proteins in the Ca(2+)-replete and Ca(2+)-deplete forms. To study the Ca(2+)-dependent/independent properties of binding and activation of target proteins by CaM, CaM constructs with Ca(2+) binding disrupting mutations of Asp to Ala at position one of each EF hand have been used. One target protein of CaM is nitric oxide synthase, which catalyzes the production of nitric oxide. At elevated Ca(2+) concentrations, CaM binds to neuronal NOS and endothelial NOS, making them the Ca(2+)-dependent NOS enzymes. In contrast, inducible NOS is transcriptionally regulated in vivo and binds to CaM at basal levels of Ca(2+). Here we report the NMR backbone and sidechain resonance assignments of C-lobe Ca(2+)-replete and deplete CaM12, N-lobe Ca(2+)-replete and deplete CaM34, CaM1234 in the absence of Ca(2+) and N-lobe Ca(2+)-replete CaM34 with the iNOS CaM-binding domain peptide.

Clinical Significance of Endothelial Dysfunction in Essential Hypertension

The endothelium is recognized as a major determinant of vascular physiology and pathophysiology. Over the last few decades, a plethora of studies have implicated endothelial dysfunction in the progression of atherosclerosis and the subclinical target organ damage observed in essential hypertension. However, the clinical significance of diagnosing endothelial dysfunction in patients with essential hypertension remains under investigation. Although a number of vascular and non-vascular markers of endothelial dysfunction have been proposed, there is an ongoing quest for a marker in the clinical setting that is optimal, inexpensive, and reproducible. In addition, endothelial dysfunction emerges as a promising therapeutic target of agents that are readily available in clinical practice. In this context, a better understanding of its role in essential hypertension becomes of great importance. Here, we aim to investigate the clinical significance of endothelial dysfunction in essential hypertension by accumulating novel data on (a) early diagnosis using robust markers with prognostic value in cardiovascular risk prediction, (b) the association of endothelial dysfunction with subclinical vascular organ damage, and (c) potential therapeutic targets.

Endothelial nitric oxide synthase: From biochemistry and gene structure to clinical implications of NOS3 polymorphisms

Nitric oxide (NO) is an important vasodilator with a well-established role in cardiovascular homeostasis. While mediator is synthesized from L-arginine by neuronal, endothelial, and inducible nitric oxide synthases (NOS1,NOS3 and NOS2 respectively), NOS3 is the most important isoform for NO formation in the cardiovascular system. NOS3 is a dimeric enzyme whose expression and activity are regulated at transcriptional, posttranscriptional,and posttranslational levels. The NOS3 gene, which encodes NOS3, exhibits a number of polymorphic sites including single nucleotide polymorphisms (SNPs), variable number of tandem repeats (VNTRs), microsatellites, and insertions/deletions. Some NOS3 polymorphisms show functional effects on NOS3 expression or activity, thereby affecting NO formation. Interestingly, many studies have evaluated the effects of functional NOS3 polymorphisms on disease susceptibility and drug responses. Moreover, some studies have investigated how NOS3 haplotypes may impact endogenous NO formation and disease susceptibility. In this article,we carried out a comprehensive review to provide a basic understanding of biochemical mechanisms involved in NOS3 regulation and how genetic variations in NOS3 may translate into relevant clinical and pharmacogenetic implications.

Insight into structural rearrangements and interdomain interactions related to electron transfer between flavin mononucleotide and heme in nitric oxide synthase: A molecular dynamics study

Calmodulin (CaM) binding to nitric oxide synthase (NOS) enables a conformational change, in which the FMN domain shuttles between the FAD and heme domains to deliver electrons to the active site heme center. A clear understanding of this large conformational change is critical, since this step is the rate-limiting in NOS catalysis. Herein molecular dynamics simulations were conducted on a model of an oxygenase/FMN (oxyFMN) construct of human inducible NOS (iNOS). This is to investigate the structural rearrangements and the domain interactions related to the FMN-heme interdomain electron transfer (IET). We carried out simulations on the iNOS oxyFMN·CaM complex models in [Fe(III)][FMNH(-)] and [Fe(II)][FMNH] oxidation states, the pre- and post-IET states. The comparison of the dynamics and conformations of the iNOS construct at the two oxidation states has allowed us to identify key factors related to facilitating the FMN-heme IET process. The computational results demonstrated, for the first time, that the conformational change is redox-dependent. Predictions of the key interacting sites in optimal interdomain FMN/heme docking are well supported by experimental data in the literature. An intra-subunit pivot region is predicted to modulate the FMN domain motion and correlate with existence of a bottleneck in the conformational sampling that leads to the electron transfer-competent state. Interactions of the residues identified in this work are proposed to ensure that the FMN domain moves with appropriate degrees of freedom and docks to proper positions at the heme domain, resulting in efficient IET and nitric oxide production.

Characterization of phospho-(tyrosine)-mimetic calmodulin mutants

Calmodulin (CaM) phosphorylated at different serine/threonine and tyrosine residues is known to exert differential regulatory effects on a variety of CaM-binding enzymes as compared to non-phosphorylated CaM. In this report we describe the preparation and characterization of a series of phospho-(Y)-mimetic CaM mutants in which either one or the two tyrosine residues present in CaM (Y99 and Y138) were substituted to aspartic acid or glutamic acid. It was expected that the negative charge of the respective carboxyl group of these amino acids mimics the negative charge of phosphate and reproduce the effects that distinct phospho-(Y)-CaM species may have on target proteins. We describe some physicochemical properties of these CaM mutants as compared to wild type CaM, after their expression in Escherichia coli and purification to homogeneity, including: i) changes in their electrophoretic mobility in the absence and presence of Ca2+; ii) ultraviolet (UV) light absorption spectra, far- and near-UV circular dichroism data; iii) thermal stability in the absence and presence of Ca2+; and iv) Tb3+-emitted fluorescence upon tyrosine excitation. We also describe some biochemical properties of these CaM mutants, such as their differential phosphorylation by the tyrosine kinase c-Src, and their action as compared to wild type CaM, on the activity of two CaM-dependent enzymes: cyclic nucleotide phosphodiesterase 1 (PDE1) and endothelial nitric oxide synthase (eNOS) assayed in vitro.

Dynamics of nitric oxide synthase-calmodulin interactions at physiological calcium concentrations

The intracellular Ca²⁺ concentration is an important regulator of many cellular functions. The small acidic protein calmodulin (CaM) serves as a Ca²⁺ sensor and control element for many enzymes. Nitric oxide synthase (NOS) is one of the proteins that is activated by CaM and plays a major role in a number of key physiological and pathological processes. Previous studies have shown CaM to act like a switch that causes a conformational change in NOS to allow for the electron transfer between the reductase and oxygenase domains through a process that is thought to be highly dynamic. We have analyzed the structure and dynamics of complexes formed by peptides based on inducible NOS (iNOS) and endothelial NOS (eNOS) with CaM at Ca²⁺ concentrations that mimic the physiological basal (17 and 100 nM) and elevated levels (225 nM) found in mammalian cells using fluorescence techniques and nuclear magnetic resonance spectroscopy. The results show the CaM-NOS complexes have similar structures at physiological and fully saturated Ca²⁺ levels; however, their dynamics are remarkably different. At 225 nM Ca²⁺, the CaM-NOS complexes show overall an increase in backbone dynamics, when compared to the dynamics of the complexes at saturating Ca²⁺ concentrations. Specifically, the N-lobe of CaM in the CaM-iNOS complex displays a lower internal mobility (higher S²) and higher exchange protection compared to those of the CaM-eNOS complex. In contrast, the C-lobe of CaM in the CaM-eNOS complex is less dynamic. These results illustrate that structures of CaM-NOS complexes determined at saturated Ca²⁺ concentrations cannot provide a complete picture because the differences in intramolecular dynamics become visible only at physiological Ca²⁺ levels.

Insights on the vasorelaxant mode of action of malbrancheamide

Objectives

This study was conducted to evaluate the vasorelaxant effect of the fungal alkaloids malbrancheamides on pre-contracted rat aorta rings. Also, we explored the probable mode of action using experimental and theoretical docking studies.

Methods

The vasorelaxant effect was assessed on rat aorta rings pre-contracted with noradrenaline (0.1 μm). The mechanism of action was evaluated using different inhibitors of the pathways involved in the vasorelaxation process, such as l-NAME, indomethacin, tetraethylammonium and atropine. The docking analyses were carried out with AutoDock 4.2 software using the crystallized structure of the cyclooxygenase domain of eNOS.

Key findings

Malbrancheamides (1–3) induced a significant vasorelaxant activity in a concentration- and endothelium-intact model in rat aorta rings, and a lesser effect in an endothelium-denuded model. Malbrancheamide-induced vasorelaxation was significantly weakened by pretreatment of endothelium-intact aortic rings with L-NAME (10 μm), indicating a nitrergic relaxant mechanism. Docking analysis predicted that 1–3 could activate eNOS throughout an allosteric fashion at C1 and C2 pockets.

Conclusions

Experimental evidence revealed that malbrancheamides induced both endothelium-independent and endothelium-dependent relaxant effects. According to theoretical studies, it is feasible that the endothelium-independent relaxation exerted by malbrancheamide could be mediated by its calmodulin inhibitory properties throughout an interference with myosin light chain phosphorylation and a positive modulation of eNOS.

Structural and thermodynamic characterization of the recognition of the S100-binding peptides TRTK12 and p53 by calmodulin

Calmodulin (CaM) is a multifunctional messenger protein that activates a wide variety of signaling pathways in eukaryotic cells in a calcium-dependent manner. CaM has been proposed to be functionally distinct from the S100 proteins, a related family of eukaryotic calcium-binding proteins. Previously, it was demonstrated that peptides derived from the actin-capping protein, TRTK12, and the tumor-suppressor protein, p53, interact with multiple members of the S100 proteins. To test the specificity of these peptides, they were screened using isothermal titration calorimetry against 16 members of the human S100 protein family, as well as CaM, which served as a negative control. Interestingly, both the TRTK12 and p53 peptides were found to interact with CaM. These interactions were further confirmed by both fluorescence and nuclear magnetic resonance spectroscopies. These peptides have distinct sequences from the known CaM target sequences. The TRTK12 peptide was found to independently interact with both CaM domains and bind with a stoichiometry of 2:1 and dissociations constants Kd,C-term  = 2 ± 1 µM and Kd,N-term  = 14 ± 1 µM. In contrast, the p53 peptide was found to interact only with the C-terminal domain of CaM, Kd,C-term = 2 ± 1 µM, 25°C. Using NMR spectroscopy, the locations of the peptide binding sites were mapped onto the structure of CaM. The binding sites for both peptides were found to overlap with the binding interface for previously identified targets on both domains of CaM. This study demonstrates the plasticity of CaM in target binding and may suggest a possible overlap in target specificity between CaM and the S100 proteins.

Solution structure of calmodulin bound to the target peptide of endothelial nitric oxide synthase phosphorylated at Thr495

Nitric oxide synthase (NOS) plays a major role in a number of key physiological and pathological processes, and it is important to understand how this enzyme is regulated. The small acidic calcium binding protein, calmodulin (CaM), is required to fully activate the enzyme. The exact mechanism of how CaM activates NOS is not fully understood at this time. Studies have shown CaM to act like a switch that causes a conformational change in NOS to allow for the transfer of an electron between the reductase and oxygenase domains through a process that is thought to be highly dynamic and at least in part controlled by several possible phosphorylation sites. We have determined the solution structure of CaM bound to a peptide that contains a phosphorylated threonine corresponding to Thr495 in full size endothelial NOS (eNOS) to investigate the structural and functional effects that the phosphorylation of this residue may have on nitric oxide production. Our biophysical studies show that phosphorylation of Thr495 introduces electrostatic repulsions between the target sequence and CaM as well as a diminished propensity for the peptide to form an α-helix. The calcium affinity of the CaM-target peptide complex is reduced because of phosphorylation, and this leads to weaker binding at low physiological calcium concentrations. This study provides an explanation for the reduced level of NO production by eNOS carrying a phosphorylated Thr495 residue.

Dissecting regulation mechanism of the FMN to heme interdomain electron transfer in nitric oxide synthases

Nitric oxide synthase (NOS), a flavo-hemoprotein, is responsible for biosynthesis of nitric oxide (NO) in mammals. Three NOS isoforms, iNOS, eNOS and nNOS (inducible, endothelial, and neuronal NOS), achieve their biological functions by tight control of interdomain electron transfer (IET) process through interdomain interactions. In particular, the FMN-heme IET is essential in coupling electron transfer in the reductase domain with NO synthesis in the heme domain by delivery of electrons required for O2 activation at the catalytic heme site. Emerging evidence indicates that calmodulin (CaM) activates NO synthesis in eNOS and nNOS by a conformational change of the FMN domain from its shielded electron-accepting (input) state to a new electron-donating (output) state, and that CaM is also required for proper alignment of the FMN and heme domains in the three NOS isoforms. In the absence of a structure of full-length NOS, an integrated approach of spectroscopic, rapid kinetic and mutagenesis methods is required to unravel regulation mechanism of the FMN-heme IET process. This is to investigate the roles of the FMN domain motions and the docking between the primary functional FMN and heme domains in regulating NOS activity. The recent developments in this area that are driven by the combined approach are the focuses of this review. A better understanding of the roles of interdomain FMN/heme interactions and CaM binding may serve as a basis for the rational design of new selective modulators of the NOS enzymes.

Nitric oxide synthase domain interfaces regulate electron transfer and calmodulin activation

Nitric oxide (NO) produced by NO synthase (NOS) participates in diverse physiological processes such as vasodilation, neurotransmission, and the innate immune response. Mammalian NOS isoforms are homodimers composed of two domains connected by an intervening calmodulin-binding region. The N-terminal oxidase domain binds heme and tetrahydrobiopterin and the arginine substrate. The C-terminal reductase domain binds FAD and FMN and the cosubstrate NADPH. Although several high-resolution structures of individual NOS domains have been reported, a structure of a NOS holoenzyme has remained elusive. Determination of the higher-order domain architecture of NOS is essential to elucidate the molecular underpinnings of NO formation. In particular, the pathway of electron transfer from FMN to heme, and the mechanism through which calmodulin activates this electron transfer, are largely unknown. In this report, hydrogen-deuterium exchange mass spectrometry was used to map critical NOS interaction surfaces. Direct interactions between the heme domain, the FMN subdomain, and calmodulin were observed. These interaction surfaces were confirmed by kinetic studies of site-specific interface mutants. Integration of the hydrogen-deuterium exchange mass spectrometry results with computational docking resulted in models of the NOS heme and FMN subdomain bound to calmodulin. These models suggest a pathway for electron transfer from FMN to heme and a mechanism for calmodulin activation of this critical step.

Structural diversity of calmodulin binding to its target sites

Calmodulin (CaM) is a ubiquitous, highly conserved, eukaryotic protein that binds to and regulates a number of diverse target proteins involved in different functions such as metabolism, muscle contraction, apoptosis, memory, inflammation and the immune response. In this minireview, we analyze the large number of CaM-complex structures deposited in the Protein Data Bank (i.e. crystal and nuclear magnetic resonance structures) to gain insight into the structural diversity of CaM-binding sites and mechanisms, such as those for CaM-activated protein kinases and phosphatases, voltage-gated Ca(2+)-channels and the plasma membrane Ca(2+)-ATPase.