Antiaging Potential of Fucoxanthin Concentrate Derived from Phaeodactylum tricornutum.

The aim of the study was to investigate the potential of a fucoxanthin concentrate prepared from Phaeodactylum tricornutum as a wrinkle care cosmetic agent. The concentrate (up to 25 µg/ml) did not affect the proliferation of human fibroblasts. In addition, the concentrate significantly increased procollagen synthesis in the fibroblasts at 12.5 and 25 µg/ml; however, it significantly decreased the expression of matrix metalloproteinase (MMP)-1, MMP-2, and MMP-9 at 25 µg/ml. In a follow-up study, a wrinkle care cream containing 0.03% of fucoxanthin concentrate was prepared and tested in women (aged 35-50 years, n = 21) for 8 weeks. The cream was applied twice daily. Safety assessment of the cream was carried out visually. In addition, interviews were conducted to investigate if adverse events such as erythema, edema, scaling, itching, stinging, burning, tightness, or prickling had occurred. No symptoms that threaten skin safety were reported. Evaluation of wrinkles around the eyes using the replica method showed a statistically significant decrease in wrinkles at week 8. Moreover, skin moisture and elasticity increased significantly from week 4. These results suggest that the fucoxanthin concentrate has no adverse effects on the skin and can be used as an active ingredient in wrinkle care cosmetics.

Structural and functional study of ChuY from Escherichia coli strain CFT073.

The uropathogenic Escherichia coli strain CFT073 contains multiple iron and heme transport systems, which facilitate infection of the host urinary tract. To elucidate the molecular and cellular function of ChuY, a hypothetical gene in the heme degradation/utilization pathway, we solved the crystal structure of ChuY at 2.4 Å resolution. ChuY has high structural homology with human biliverdin and flavin reductase. We confirmed that ChuY has flavin mononucleotide (FMN) reductase activity, using NAD(P)H as a cofactor, and shows porphyrin ring binding affinity. A chuY deletion-insertion strain showed reduced survival potential compared to wild-type and complemented strains in mammalian cells. Current results suggest ChuY acts as a reductase in heme homeostasis to maintain the virulence potential of E. coli CFT073.

KOTMIN13, a Korean herbal medicine alleviates allergic inflammation in vivo and in vitro.

BACKGROUND: The ethanol extract of KOTMIN13, composed of Inula japonica Flowers, Trichosanthes kirilowii Semen, Peucedanum praeruptorum Radix, and Allium macrostemon Bulbs, was investigated for its anti-asthmatic and anti-allergic activities. METHODS: The anti-asthmatic effects of KOTMIN13 were evaluated on ovalbumin (OVA)-induced murine asthma model. Anti-allergic properties of KOTMIN13 in bone-marrow derived mast cells (BMMC) and passive cutaneous anaphylaxis (PCA) in vivo were also examined. RESULTS: In asthma model, KOTMIN13 effectively suppressed airway hyperresponsiveness induced by aerosolized methacholine when compared to the levels of OVA-induced mice. KOTMIN13 treatment reduced the total leukocytes, eosinophil percentage, and Th2 cytokines in the bronchoalveolar lavage fluids in OVA-induced mice. The increased levels of eotaxin and Th2 cytokines in the lung as well as serum IgE were decreased by KOTMIN13. The histological analysis shows that the increased inflammatory cell infiltration and mucus secretion were also reduced. In addition, the degranulation and leukotriene C4 production were inhibited in BMMC with IC50 values of 3.9 µg/ml and 1.7 µg/ml, respectively. Furthermore, KOTMIN13 treatment attenuated mast-mediated PCA reaction. CONCLUSIONS: These results demonstrate that KOTMIN13 has anti-asthmatic and anti-allergic effects in vivo and in vitro models.

Structural insights into the molecular mechanism of Escherichia coli SdiA, a quorum-sensing receptor.

Escherichia coli SdiA is a quorum-sensing (QS) receptor that responds to autoinducers produced by other bacterial species to control cell division and virulence. Crystal structures reveal that E. coli SdiA, which is composed of an N-terminal ligand-binding domain and a C-terminal DNA-binding domain (DBD), forms a symmetrical dimer. Although each domain shows structural similarity to other QS receptors, SdiA differs from them in the relative orientation of the two domains, suggesting that its ligand-binding and DNA-binding functions are independent. Consistently, in DNA gel-shift assays the binding affinity of SdiA for the ftsQP2 promoter appeared to be insensitive to the presence of autoinducers. These results suggest that autoinducers increase the functionality of SdiA by enhancing the protein stability rather than by directly affecting the DNA-binding affinity. Structural analyses of the ligand-binding pocket showed that SdiA cannot accommodate ligands with long acyl chains, which was corroborated by isothermal titration calorimetry and thermal stability analyses. The formation of an intersubunit disulfide bond that might be relevant to modulation of the DNA-binding activity was predicted from the proximal position of two Cys residues in the DBDs of dimeric SdiA. It was confirmed that the binding affinity of SdiA for the uvrY promoter was reduced under oxidizing conditions, which suggested the possibility of regulation of SdiA by multiple independent signals such as quorum-sensing inducers and the oxidation state of the cell.

Structure-based development of a receptor activator of nuclear factor-kappaB ligand (RANKL) inhibitor peptide and molecular basis for osteopetrosis.

The receptor activator of nuclear factor-κB (RANK) and its ligand RANKL, which belong to the tumor necrosis factor (TNF) receptor-ligand family, mediate osteoclastogenesis. The crystal structure of the RANKL ectodomain (eRANKL) in complex with the RANK ectodomain (eRANK) combined with biochemical assays of RANK mutants indicated that three RANK loops (Loop1, Loop2, and Loop3) bind to the interface of a trimeric eRANKL. Loop3 is particularly notable in that it is structurally distinctive from other TNF-family receptors and forms extensive contacts with RANKL. The disulfide bond (C125-C127) at the tip of Loop3 is important for determining the unique topology of Loop3, and docking E126 close to RANKL, which was supported by the inability of C127A or E126A mutants of RANK to bind to RANKL. Inhibitory activity of RANK mutants, which contain loops of osteoprotegerin (OPG), a soluble decoy receptor to RANKL, confirmed that OPG shares the similar binding mode with RANK and OPG. Loop3 plays a key role in RANKL binding. Peptide inhibitors designed to mimic Loop3 blocked the RANKL-induced differentiation of osteoclast precursors, suggesting that they could be developed as therapeutic agents for the treatment of osteoporosis and bone-related diseases. Furthermore, some of the RANK mutations associated with autosomal recessive osteopetrosis (ARO) resulted in reduced RANKL-binding activity and failure to induce osteoclastogenesis. These results, together with structural interpretation of eRANK-eRANKL interaction, provided molecular understanding for pathogenesis of ARO.

Theoretical study on the glycosidic C-C bond cleavage of 3''-oxo-puerarin.

Puerarin, daidzein C-glucoside, was known to be biotransformed to daidzein by human intestinal bacteria, which is eventually converted to (S)-equol. The metabolic pathway of puerarin to daidzein by DgpABC of Dorea sp. PUE strain was reported as puerarin (1) → 3''-oxo-puerarin (2) → daidzein (3) + hexose enediolone (C). The second reaction is the cleavage of the glycosidic C-C bond, supposedly through the quinoid intermediate (4). In this work, the glycosidic C-C bond cleavage reaction of 3''-oxo-puerarin (2) was theoretically studied by means of DFT calculation to elucidate chemical reaction mechanism, along with biochemical energetics of puerarin metabolism. It was found that bioenergetics of puerarin metabolism is slightly endergonic by 4.99 kcal/mol, mainly due to the reaction step of hexose enediolone (C) to 3''-oxo-glucose (A). The result implied that there could be additional biochemical reactions for the metabolism of hexose enediolone (C) to overcome the thermodynamic energy barrier of 4.59 kcal/mol. The computational study focused on the C-C bond cleavage of 3''-oxo-puerarin (2) found that formation of the quinoid intermediate (4) was not accessible thermodynamically, rather the reaction was initiated by the deprotonation of 2''C-H proton of 3''-oxo-puerarin (2). The 2''C-dehydro-3''-oxo-puerarin (2a2C) anionic species produced hexose enediolone (C) and 8-dehydro-daidzein anion (3a8), and the latter quickly converted to daidzein through the daidzein anion (3a7). Our study also explains why the reverse reaction of C-glycoside formation from daidzein (3) and hexose enediolone (C) is not feasible.

The structures of non-CG-repeat Z-DNAs co-crystallized with the Z-DNA-binding domain, hZ alpha(ADAR1).

The Z-DNA conformation preferentially occurs at alternating purine-pyrimidine repeats, and is specifically recognized by Z alpha domains identified in several Z-DNA-binding proteins. The binding of Z alpha to foreign or chromosomal DNA in various sequence contexts is known to influence various biological functions, including the DNA-mediated innate immune response and transcriptional modulation of gene expression. For these reasons, understanding its binding mode and the conformational diversity of Z alpha bound Z-DNAs is of considerable importance. However, structural studies of Z alpha bound Z-DNA have been mostly limited to standard CG-repeat DNAs. Here, we have solved the crystal structures of three representative non-CG repeat DNAs, d(CACGTG)(2), d(CGTACG)(2) and d(CGGCCG)(2) complexed to hZ alpha(ADAR1) and compared those structures with that of hZ alpha(ADAR1)/d(CGCGCG)(2) and the Z alpha-free Z-DNAs. hZ alpha(ADAR1) bound to each of the three Z-DNAs showed a well conserved binding mode with very limited structural deviation irrespective of the DNA sequence, although varying numbers of residues were in contact with Z-DNA. Z-DNAs display less structural alterations in the Z alpha-bound state than in their free form, thereby suggesting that conformational diversities of Z-DNAs are restrained by the binding pocket of Z alpha. These data suggest that Z-DNAs are recognized by Z alpha through common conformational features regardless of the sequence and structural alterations.

Targeting Mannitol Metabolism as an Alternative Antimicrobial Strategy Based on the Structure-Function Study of Mannitol-1-Phosphate Dehydrogenase in Staphylococcus aureus.

Mannitol-1-phosphate dehydrogenase (M1PDH) is a key enzyme in Staphylococcus aureus mannitol metabolism, but its roles in pathophysiological settings have not been established. We performed comprehensive structure-function analysis of M1PDH from S. aureus USA300, a strain of community-associated methicillin-resistant S. aureus, to evaluate its roles in cell viability and virulence under pathophysiological conditions. On the basis of our results, we propose M1PDH as a potential antibacterial target. In vitro cell viability assessment of ΔmtlD knockout and complemented strains confirmed that M1PDH is essential to endure pH, high-salt, and oxidative stress and thus that M1PDH is required for preventing osmotic burst by regulating pressure potential imposed by mannitol. The mouse infection model also verified that M1PDH is essential for bacterial survival during infection. To further support the use of M1PDH as an antibacterial target, we identified dihydrocelastrol (DHCL) as a competitive inhibitor of S. aureus M1PDH (SaM1PDH) and confirmed that DHCL effectively reduces bacterial cell viability during host infection. To explain physiological functions of SaM1PDH at the atomic level, the crystal structure of SaM1PDH was determined at 1.7-Å resolution. Structure-based mutation analyses and DHCL molecular docking to the SaM1PDH active site followed by functional assay identified key residues in the active site and provided the action mechanism of DHCL. Collectively, we propose SaM1PDH as a target for antibiotic development based on its physiological roles with the goals of expanding the repertory of antibiotic targets to fight antimicrobial resistance and providing essential knowledge for developing potent inhibitors of SaM1PDH based on structure-function studies.IMPORTANCE Due to the shortage of effective antibiotics against drug-resistant Staphylococcus aureus, new targets are urgently required to develop next-generation antibiotics. We investigated mannitol-1-phosphate dehydrogenase of S. aureus USA300 (SaM1PDH), a key enzyme regulating intracellular mannitol levels, and explored the possibility of using SaM1PDH as a target for developing antibiotic. Since mannitol is necessary for maintaining the cellular redox and osmotic potential, the homeostatic imbalance caused by treatment with a SaM1PDH inhibitor or knockout of the gene encoding SaM1PDH results in bacterial cell death through oxidative and/or mannitol-dependent cytolysis. We elucidated the molecular mechanism of SaM1PDH and the structural basis of substrate and inhibitor recognition by enzymatic and structural analyses of SaM1PDH. Our results strongly support the concept that targeting of SaM1PDH represents an alternative strategy for developing a new class of antibiotics that cause bacterial cell death not by blocking key cellular machinery but by inducing cytolysis and reducing stress tolerance through inhibition of the mannitol pathway.

Conformational flexibility in mammalian 15S-lipoxygenase: Reinterpretation of the crystallographic data.

Lipoxygenases (LOXs) are a family of nonheme iron dioxygenases that catalyze the regioselective and stereospecific hydroperoxidation of polyunsaturated fatty acids, and are involved in a variety of inflammatory diseases and cancers. The crystal structure of rabbit 15S-LOX1 that was reported by Gillmor et al. in 1997 has played key roles for understanding the properties of mammalian LOXs. In this structure, three segments, including 12 residues in the superficial alpha2 helix, are absent and have usually been described as "disordered." By reinterpreting the original crystallographic data we were able to elucidate two different conformations of the molecule, both having well ordered alpha2 helices. Surprisingly, one molecule contained an inhibitor and the other did not, thereby adopting a closed and an open form, respectively. They differed in the conformation of the segments that were absent in the original structure, which is highlighted by a 12 A movement of alpha2. Consequently, they showed a difference in the size and shape of the substrate-binding cavity. The new model should provide new insight into the catalytic mechanism involving induced conformational change of the binding pocket. It may also be helpful for the structure-based design of LOX inhibitors.

Anti-inflammatory Effects of KOTMIN13: A Mixed Herbal Medicine in LPS-stimulated RAW 264.7 Cells and Mouse Edema Models.

BACKGROUND: A Korean herbal medicine, KOTMIN13, composed of Inula japonica Thunberg, Trichosanthes kirilowii Maximowicz var. japonica kitamura, Peucedanum praeruptorum Dunn, and Allium macrostemon Bge, has been used for anti-allergic and anti-asthmatic treatment in oriental clinics, but its activity has not been investigated. MATERIALS AND METHODS: To evaluate the anti-inflammatory activity of KOTMIN13 for in vitro study, LPS-stimulated RAW 264.7 cells were used to induce the production and expression of inflammatory mediators and its mechanisms. 12-O-Tetradecanoylphorobol-13 aceate (TPA)-induced ear edema and carrageenan-induced paw edema models were also used to evaluate the effect of KOTMIN13 on acute inflammation in vivo. RESULTS: KOTMIN13 reduced the release of inflammatory mediators [nitric oxide, prostaglandin E2, interleukin (IL)-1ß, and IL-6] and the protein expression of inducible nitric oxide synthase and cyclooxygenase-2 in LPS-stimulated RAW 264.7 cells. Mechanism studies showed the attenuation of LPS-induced NF-κB activation by KOTMIN13 via IκBα degradation abrogation and a subsequent decrease in nuclear p65 levels. Activation of mitogen-activated protein kinases (ERK, JNK, and p38) was also suppressed. Furthermore, KOTMIN13 ameliorated the development of TPA-induced ear edema and carrageenan-induced paw edema in acute inflammatory edema mouse models. CONCLUSION: Our study demonstrates that KOTMIN13 inhibits inflammatory mediators through the inhibitions of NF-κB and MAPK activities in LPS-induced RAW 264.7 cells, as well as acute inflammation in edema models, indicating that KOTMIN13 is an effective suppressor for anti-inflammatory activities. SUMMARY: KOTMIN13 decrease the production of No, PGE2, and proinflammatory cytokine (TNF-∝, IL-1ß,IL-6).KOTMIN13 Suppressed the degradation of NF-kß and IKßα and the phosorylation of MAP Kinases.Topical application of KOTMIN13 reduced mouse ear edema.Oral administration of KOTMIN13 decreased carrageenan-induced paw edema. Abbreviations used: NO: nitric oxide; PGE2: prostaglandin E2; iNOS: inducible NO synthase; COX-2: cyclooxygenase-2; TNF-α: tumor necrosis factor-α; IL: interleukin; NF-κB: nuclear factor kappaB; MAPK: mitogen-activated protein kinases; ERK: extracellular signal regulated kinase; JNK: c-jun N terminal kinase; TPA: 12-O-tetradecanoylphorbol-13-acetate.

Structural Studies on the Extracellular Domain of Sensor Histidine Kinase YycG from Staphylococcus aureus and Its Functional Implications.

Bacterial two-component signal transduction systems are used to adapt to fluctuations in the environment. YycG, a key two-component histidine kinase in Staphylococcus aureus, plays an essential role in cell viability and regulates cell wall metabolism, biofilm formation, virulence, and antibiotic resistance. For these reasons, YycG is considered a compelling target for the development of novel antibiotics. However, to date, the signaling mechanism of YycG and its stimulus are poorly understood mainly because of a lack of structural information on YycG. To address this deficiency, we determined the crystal structure of the extracellular domain of S. aureus YycG (YycGex) at 2.0-Å resolution. The crystal structure indicated two subunits with an extracellular Per-Arnt-Sim (PAS) topology packed into a dimer with interloop interactions. Disulfide scanning using cysteine-substituted mutants revealed that YycGex possessed dimeric interfaces not only in the loop but also in the helix α1. Cross-linking studies using intact YycG demonstrated that it was capable of forming high molecular weight oligomers on the cell membrane. Furthermore, we also observed that two auxiliary proteins of YycG, YycH and YycI, cooperatively interfered with the multimerization of YycG. From these results, we propose that signaling through YycG is regulated by multimerization and binding of YycH and YycI. These structural studies, combined with biochemical analyses, provide a better understanding of the signaling mechanism of YycG, which is necessary for developing novel antibacterial drugs targeting S. aureus.

Crystallization and preliminary X-ray analysis of a truncated mutant of yeast nuclear thiol peroxidase, a novel atypical 2-Cys peroxiredoxin.

Saccharomyces cerevisiae nTPx is a thioredoxin-dependent thiol peroxidase that is localized in the nucleus. nTPx belongs to the C-type atypical 2-Cys peroxiredoxin family members, which are frequently called BCPs or PrxQs. A double mutant (C107S/C112S) of nTPx overexpressed in Escherichia coli was spontaneously degraded upon freezing and thawing and its truncated form (residues 57-215; MW = 17837 Da) was crystallized with PEG 3350 and mercury(II) acetate as precipitants using the hanging-drop vapour-diffusion method. Diffraction data were collected to 1.8 A resolution using X-ray synchrotron radiation. The crystals belong to the trigonal space group P3(2), with unit-cell parameters a = b = 37.54, c = 83.26 A. The asymmetric unit contains one molecule of truncated mutant nTPx, with a corresponding VM of 1.91 A3 Da(-1) and a solvent content of 35.5%.

Crystal structure analysis of c4763, a uropathogenic Escherichia coli-specific protein.

Urinary-tract infections (UTIs), which are some of the most common infectious diseases in humans, can cause sepsis and death without proper treatment. Therefore, it is necessary to understand their pathogenicity for proper diagnosis and therapeutics. Uropathogenic Escherichia coli, the major causative agents of UTIs, contain several genes that are absent in nonpathogenic strains and are therefore considered to be relevant to UTI pathogenicity. c4763 is one of the uropathogenic E. coli-specific proteins, but its function is unknown. To investigate the function of c4763 and its possible role in UTI pathogenicity, its crystal structure was determined at a resolution of 1.45 Šby a multiple-wavelength anomalous diffraction method. c4763 is a homodimer with 129 residues in one subunit that contains a GGCT-like domain with five α-helices and seven ß-strands. c4763 shows structural similarity to the C-terminal domain of allophanate hydrolase from Kluyveromyces lactis, which is involved in the degradation of urea. These results suggest that c4763 might be involved in the utilization of urea, which is necessary for bacterial survival in the urinary tract. Further biochemical and physiological investigation will elucidate its functional relevance in UTIs.

Discovery and characterization of the tuberculosis drug lead ecumicin.

The new tuberculosis (TB) lead ecumicin (1), a cyclic tridecapeptide, was isolated from Nonomuraea sp. MJM5123, following a high-throughput campaign for anti-TB activity. The large molecular weight of 1599 amu detected by LC-HR-MS precluded the initial inference of its molecular formula. The individual building blocks were identified by extensive NMR experiments. The resulting two possible planar structures were distinguished by LC-MS(2). Determination of absolute configuration and unambiguous structural confirmation were carried out by X-ray crystallography and Marfey's analysis.

Ameliorative effects of Monascus pilosus-fermented black soybean (Glycine max L. Merrill) on high-fat diet-induced obesity.

The purpose of this study was to examine the antiobesity effects of Monascus pilosus-fermented black soybean (F-BS) in C57BL/6 mice with high-fat diet (HFD)-induced obesity. F-BS (oral, 0.5 and 1.0 g/kg per body weight, twice per day) ameliorated obesity by reducing body and liver weight increases, and regulating blood glucose and cholesterol levels in C57BL/6 mice fed a control or HFD with oral administration of F-BS for 12 weeks. F-BS suppressed the growth of epididymal, retroperitoneal, and perirenal fat pads by preventing increases in the adipocyte size. Moreover, the levels of blood glucose, total cholesterol, and leptin were significantly lowered by F-BS administration in a dose-dependent manner. These results indicated that F-BS is a beneficial food supplement for preventing obesity, controlling blood glucose, and lowering cholesterol. Future research strategies should address the mechanisms that selectively regulate obesity, including hyperglycemia and hypercholesterolemia.

Crystal structure of the LG3 domain of endorepellin, an angiogenesis inhibitor.

Endorepellin, the C-terminal region of perlecan, inhibits angiogenesis by disrupting actin cytoskeleton and focal adhesions. The C-terminal laminin-like globular domain (LG3) of endorepellin directs most of this antiangiogenic activity. To investigate the angiostatic mechanism and to identify structural determinants, we have solved crystal structures of the LG3 domain in both apo- and calcium-bound forms at resolutions of 1.5 Å and 2.8 Å, respectively. The conserved core has the jellyroll fold characteristic of LG domains. The calcium-induced structural changes seem very restricted, and the calcium binding site appears to be preformed, suggesting that the bound calcium ion, rather than structural rearrangements, contributes to antiangiogenesis. We have identified H4268 on the EF loop as a key residue for the biochemical function of LG3, since its mutation abolishes antiangiogenic activity, and mutant LG3 can no longer form a direct interaction with integrin. Taken together, we propose that these two distinct structural elements contribute to the angiostatic effect of endorepellin.

Structural basis for the negative regulation of bacterial stress response by RseB.

The sigmaE-dependent stress response in bacterial cells is initiated by the DegS- and RseP-regulated intramembrane proteolysis of a membrane-spanning antisigma factor, RseA. RseB binds to RseA and inhibits its sequential cleavage, thereby functioning as a negative modulator of this response. In the crystal structure of the periplasmic domain of RseA bound to RseB, the DegS cleavage site of RseA is unstructured, however, its P1 residue is buried in the hydrophobic pocket of RseB, which suggests that RseB binding blocks the access of DegS to the cleavage site.

Structural basis for the reaction mechanism of UDP-glucose pyrophosphorylase.

UDP-glucose pyrophosphorylases (UGPase; EC 2.7.7.9) catalyze the conversion of UTP and glucose-1-phosphate to UDP-glucose and pyrophosphate and vice versa. Prokaryotic UGPases are distinct from their eukaryotic counterparts and are considered appropriate targets for the development of novel antibacterial agents since their product, UDP-glucose, is indispensable for the biosynthesis of virulence factors such as lipopolysaccharides and capsular polysaccharides. In this study, the crystal structures of UGPase from Helicobacter pylori (HpUGPase) were determined in apo- and UDP-glucose/Mg(2+)-bound forms at 2.9 A and 2.3 A resolutions, respectively. HpUGPase is a homotetramer and its active site is located in a deep pocket of each subunit. Magnesium ion is coordinated by Asp130, two oxygen atoms of phosphoryl groups, and three water molecules with octahedral geometry. Isothermal titration calorimetry analyses demonstrated that Mg(2+) ion plays a key role in the enzymatic activity of UGPase by enhancing the binding of UGPase to UTP or UDP-glucose, suggesting that this reaction is catalyzed by an ordered sequential Bi Bi mechanism. Furthermore, the crystal structure explains the specificity for uracil bases. The current structural study combined with functional analyses provides essential information for understanding the reaction mechanism of bacterial UGPases, as well as a platform for the development of novel antibacterial agents.

The crystal structure of guamerin in complex with chymotrypsin and the development of an elastase-specific inhibitor.

Guamerin, a canonical serine protease inhibitor from Hirudo nipponia, was identified as an elastase-specific inhibitor and has potential application in various diseases caused by elevated elastase concentration. However, the application of guamerin is limited because it also shows inhibitory activity against other proteases. To improve the selectivity of guamerin as an elastase inhibitor, it is essential to understand the binding mode of the inhibitor to elastase and to other proteases. For this purpose, we determined the crystal structure of guamerin in complex with chymotrypsin at 2.5 A resolution. The binding mode of guamerin on elastase was explored from the model structure of guamerin/elastase. Guamerin binds to the hydrophobic pocket of the protease in a substrate-like manner using its binding loop. In order to improve the binding selectivity of guamerin to elastase, several residues in the binding loop were mutated and the inhibitory activities of the mutants against elastase and chymotrypsin were monitored. The substitution of the Met36 residue for Ala in the P1 site increased the inhibitory activity against elastase up to 14-fold, while the same mutant showed 7-fold decreased activity against chymotrypsin compared to the wild-type guamerin. Furthermore, the M36A guamerin mutant more effectively protected endothelial cells against cell damage caused by elastase than the wild-type guamerin.