Bacillus pumilus cellulitis with bacteremia in a person who injects drugs, living with HIV-HCV co-infection: a case report.

Bacillus pumilus (B. pumilus) is a ubiquitous spore-forming bacteria that has rarely been implicated in extraintestinal infections, mostly in immunocompromised hosts. The authors report a case of B. pumilus cellulitis with bacteremia in a person who injects drugs living with human immunodeficiency virus-hepatitis C virus (HIV-HCV) co-infection. Although similar cases have been reported for some species of the genus, namely Bacillus anthracis (B. anthracis) and Bacillus cereus (B. cereus), this case reinforces the importance of considering other Bacillus spp. as potential pathogens in skin and soft tissue infections and bloodstream infections related to intravenous drug use.

Beta-Adrenergic Blockade in Advanced Non-Small Cell Lung Cancer Patients Receiving Immunotherapy: A Multicentric Study.

Introduction The standard treatment of cancer has dramatically improved with immune checkpoint inhibitors (ICIs). Despite their proven advantage, many patients fail to exhibit a meaningful and lasting response. The beta-adrenergic signalling pathway may hold significant promise due to its role in promoting an immunosuppressive milieu within the tumour microenvironment. Inhibiting ß-adrenergic signalling could enhance ICI activity; however, blocking this pathway for this purpose has yielded conflicting results. The primary objective of this study was to evaluate the effect of beta-blocker use on overall survival and progression-free survival during ICI therapy. Methods A multicentric, retrospective, observational study was conducted in four Portuguese institutions. Patients with advanced non-small cell lung cancer treated with ICIs between January 2018 and December 2019 were included. Those using beta blockers for non-oncological reasons were compared with non-users. Results Among the 171 patients included, 36 concomitantly received beta blockers and ICIs. No significant increase was found in progression-free survival among patients who took ß-blockers (HR 0.74, 95% confidence interval (CI) 0.48-1.12, p = 0.151), and no statistically significant difference was found in overall survival. An apparent trend was observed towards better outcomes in the beta-blocker group, with a median overall survival of 9.93 months in the group not taking ß-blockers versus 14.90 months in the ß-blocker group (p = 0.291) and a median progression-free survival of 5.37 in the group not taking ß-blockers versus 10.87 months in the ß-blocker group (p = 0.151). Nine (25%) patients in the beta-blocker group and 16 (12%) in the non-beta-blocker group were progressive disease-free at the end of follow-up. This difference between the two groups is statistically significant (p = 0.047). Conclusion Our study found no statistically significant evidence that beta blockers enhance the effectiveness of immunotherapy. Using adrenergic blockade to modulate the immune system shows promise, warranting the need to develop prospective clinical studies.

What's new about the tumor microenvironment of urothelial carcinoma?

Urothelial carcinoma is a significant global health concern that accounts for a substantial part of cancer diagnoses and deaths worldwide. The tumor microenvironment is a complex ecosystem composed of stromal cells, soluble factors, and altered extracellular matrix, that mutually interact in a highly immunomodulated environment, with a prominent role in tumor development, progression, and treatment resistance. This article reviews the current state of knowledge of the different cell populations that compose the tumor microenvironment of urothelial carcinoma, its main functions, and distinct interactions with other cellular and non-cellular components, molecular alterations and aberrant signaling pathways already identified. It also focuses on the clinical implications of these findings, and its potential to translate into improved quality of life and overall survival. Determining new targets or defining prognostic signatures for urothelial carcinoma is an ongoing challenge that could be accelerated through a deeper understanding of the tumor microenvironment.

Catalytically Active Snake Venom PLA2 Enzymes: An Overview of Its Elusive Mechanisms of Reaction.

Snake venom-secreted phospholipase A2 (svPLA2) enzymes, both catalytically active and inactive, are a central component in envenoming. These are responsible for disrupting the cell membrane's integrity, inducing a wide range of pharmacological effects, such as the necrosis of the bitten limb, cardiorespiratory arrest, edema, and anticoagulation. Although extensively characterized, the reaction mechanisms of enzymatic svPLA2 are still to be thoroughly understood. This review presents and analyses the most plausible reaction mechanisms for svPLA2, such as the "single-water mechanism" or the "assisted-water mechanism" initially proposed for the homologous human PLA2. All of the mechanistic possibilities are characterized by a highly conserved Asp/His/water triad and a Ca2+ cofactor. The extraordinary increase in activity induced by binding to a lipid-water interface, known as "interfacial activation," critical for the PLA2s activity, is also discussed. Finally, a potential catalytic mechanism for the postulated noncatalytic PLA2-like proteins is anticipated.

Impact of Immune-Related Adverse Events on Immune Checkpoint Inhibitors Treated Cancer Patients' Survival: Single Center Experience and Literature Review.

Immune-related adverse events have emerged as a new challenge and its correlation with survival remains unclear. The goal of our study was to investigate the effect of irAE on survival outcomes in solid tumor patients receiving ICI treatment. This was a retrospective, single-center study at a university hospital involving patients with malignancy who received immune checkpoint inhibitors. Chart review was performed on each patient, noting any irAE, including new events or worsening of previous autoimmune condition after starting treatment with ICI. A total of 155 patients were included, 118 (76.1%) were male, with median age of 64 years. Median follow up time was 36 months. Seventy patients (45.2%) had at least one irAE. Of all irAE, nine (8.1%) were classified as grade 3 or higher according to the CTCAE version 5.0. There was one death secondary to pneumonitis. Median ICI cycles until first irAE onset was 4 (range: 2-99). The objective response rate was higher for patients who developed irAE (18.7% vs. 9.0%; p = 0.001), as was median overall survival (18 months (95% CI, 8.67-27.32) vs. 10 (95% CI, 3.48-16.52) months; p < 0.016) and progression free survival (10 months (95% CI, 5.44-14.56) vs. 3 months (95% CI, 1.94-4.05); p = 0.000). The risk of death in patients with irAE was 33% lower when compared to patients without such events (hazard ratio (HR): 0.67; 95% CI, 0.46-0.99; p = 0.043). Development of irAE predicted better outcomes, including OS in patients with advanced solid tumors treated with ICI. Further prospective studies are needed to explore and validate this prognostic value.

Immunotherapy in Elderly Patients-Single-Center Experience.

Cancer management faces a substantial challenge posed by the aging demographic. Aging is marked by accumulated DNA damage, and this phenomenon is implicated in the process of tumorigenesis. The concept of immunosenescence, postulated to manifest in elderly individuals, is defined by an age-related decline in T cells and a simultaneous elevation in proinflammatory status, leading to a diminished efficacy in response to immunotherapy. Notably, despite the rising prevalence of cancer in the elderly population, their underrepresentation in clinical trials persists. This underscores the unmet need to evaluate the safety and efficacy of cancer treatment in the elderly. This retrospective, single-center cohort study aimed to assess and evaluate the effectiveness and safety of immunotherapy in patients compared to younger individuals with metastatic solid tumors receiving ICI. A total of 220 patients were included, mostly males, with a median age of 64. The proportion of patients ≥ 65 years old was 56.5%. The use of ICI showed no significant differences concerning overall survival (OS) and progression-free survival (PFS) among age groups across different cancer types (melanoma, non-small-cell lung cancer (NSCLC), renal, and bladder cancer; p = 0.388). Concerning the response to treatment in renal cancer patients, a significant difference was observed (p = 0.041), suggesting a potential negative impact of age on the treatment response. In patients that presented immune-related adverse events (irAEs), oral corticosteroid therapy was marginally associated (p = 0.059) with the elderly population. When evaluating the NSCLC population alone (n = 131, 59.5%), our study revealed a strong association between the development of irAEs, patients' PFS and OS, and the duration of ICI treatment, but not directly correlated with age. The NSCLC elderly population presented a marginally greater number of irAEs, although without statistical significance (p = 0.86). ICI maintained efficacy and safety in elderly patients, challenging the notion that age alone should determine treatment decisions. The findings emphasize the necessity of a comprehensive geriatric assessment rather than relying solely on chronological age for personalized cancer treatment in the elderly population. Further prospective studies are needed to better understand immune responses in older adults and derive predictive biomarkers for cancer treatment.

Relapsed Ovarian Cancer Patients with Ascites and/or Pleural Effusion Still Benefit from Treatment: A Real-Life Study.

(1) Background: Relapsed HGSOC with ascites and/or pleural effusion is a poor-prognostic population and poorly represented in clinical studies. We questioned if these patients are worth treating. In other words, if these patients received the most effective treatment, would it change the course of this disease? To our knowledge this is the first real-life study to evaluate this question in this low-survival population. (2) Methods: To tackle this question we performed a retrospective, multi-centric, real-life study, that reviewed relapsed HGSOC patients with ascites and/or pleural effusion. Our rationale was to compare the OS of two groups of patients: responders, i.e., patients who had an imagological response to treatment (complete/partial response/stable disease, RECIST criteria) versus non-responders (no response/progression upon treatment). We evaluated the predictive value of clinical variables that are available in a real-life setting (e.g., staging, chemotherapy, surgery, platinum-sensitivity). Multivariate logistic regression and survival analysis was conducted. A two-step cluster analysis SPSS tool was used for subgroup analysis. Platinum sensitivity/resistance was also analyzed, as well as multivariate and cluster analysis. (3) Results: We included 57 patients, 41.4% first line responders and 59.6% non-responders. The median OS of responders was 23 months versus 8 months in non-responders (p < 0.001). This difference was verified in platinum-sensitive (mOS 28 months vs. 8 months, p < 0.001) and platinum-resistant populations (mOS 16 months vs. 7 months, p < 0.001). Thirty-one patients reached the second line, of which only 10.3% responded to treatment. Three patients out of thirty-one who did not respond in the first line of relapse, responded in the second line. In the second line, the mOS for the responders' group vs. non-responders was 31 months versus 13 months (p = 0.02). The two step cluster analysis tool found two different subgroups with different prognoses based on overall response rate, according to consolidation chemotherapy, neoadjuvant chemotherapy, FIGO staging and surgical treatment. Cluster analysis showed that even patients with standard clinical and treatment variables associated with poor prognosis might achieve treatment response (the opposite being also true). (4) Conclusions: Our data clearly show that relapsed HGSOC patients benefit from treatment. If given an effective treatment upfront, this can lead to a ~3 times increase in mOS for these patients. Moreover, this was irrespective of patient disease and treatment characteristics. Our results highlight the urgent need for a sensitivity test to tailor treatments and improve efficacy rates in a personalized manner.

A combined experimental and computational study to discover novel tyrosinase inhibitors.

Depigmenting properties of tyrosinase inhibitors (TAi) boosted the search for new compounds applicable in cosmetics. Kojic acid, a 3-hydroxy-4-pyrone, is the most studied tyrosinase inhibitor but undesirable side effects, like dermatitis, and unspecified mechanism led to its exclusion in several countries. To discover safer and more efficient TA, we evaluated tyrosinase inhibitory effect of twelve 3-hydroxy-4-pyridinones (3,4-HPO) in vitro and considering the two reaction steps of inhibition in mushroom tyrosinase enzyme. In parallel we performed molecular docking studies in human and mushroom enzymes. Ligands I6 and I11 were the most effective compounds considering their inhibitory activity in both reaction steps. Our studies revealed that I6 has a non-competitive and mixed type of inhibition for monophenolase and diphenolase activity, while ligand I11 showed a mixed and competitive inhibition type for each reaction step. Molecular Docking results indicated that ligands tend to bind the enzyme by coordinating directly with the binuclear cooper centre and highlighted the relevance of voluminous and non-polar substituents at R2 to avoid the binding of the ligands to the enzyme. The work clarifies the type of inhibition established for kojic acid and points out the differences found for the set of 3,4-HPO chelators studied as prospective tyrosinase inhibitors.

Transmembrane Protease Serine 2 Proteolytic Cleavage of the SARS-CoV-2 Spike Protein: A Mechanistic Quantum Mechanics/Molecular Mechanics Study to Inspire the Design of New Drugs To Fight the COVID-19 Pandemic.

Despite the development of vaccines against the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus, there is an urgent need for efficient drugs to treat infected patients. An attractive drug target is the human transmembrane protease serine 2 (TMPRSS2) because of its vital role in the viral infection mechanism of SARS-CoV-2 by activation of the virus spike protein (S protein). Having in mind that the information derived from quantum mechanics/molecular mechanics (QM/MM) studies could be an important tool in the design of transition-state (TS) analogue inhibitors, we resorted to adiabatic QM/MM calculations to determine the mechanism of the first step (acylation) of proteolytic cleavage of the S protein with atomistic details. Acylation occurred in two stages: (i) proton transfer from Ser441 to His296 concerted with the nucleophilic attack of Ser441 to the substrate's P1-Arg and (ii) proton transfer from His296 to the P1'-Ser residue concerted with the cleavage of the ArgP1-SerP1' peptide bond, with a Gibbs activation energy of 17.1 and 15.8 kcal mol-1, relative to the reactant. An oxyanion hole composed of two hydrogen bonds stabilized the rate-limiting TS by 8 kcal mol-1. An analysis of the TMPRSS2 interactions with the high-energy, short-lived tetrahedral intermediate highlighted the limitations of current clinical inhibitors and pointed out specific ways to develop higher-affinity TS analogue inhibitors. The results support the development of more efficient drugs against SARS-CoV-2 using a human target, free from resistance development.

Discovery of a multi-target compound for estrogen receptor-positive (ER+) breast cancer: Involvement of aromatase and ERs.

Despite intense research, breast cancer remains the leading cause of cancer-related death in women worldwide, being estrogen receptor-positive (ER+) the most common subtype. Nowadays, aromatase inhibitors (AIs), the selective estrogen receptor modulator (SERM) tamoxifen and the selective estrogen receptor down-regulator (SERD) fulvestrant are used as therapeutic options for ER+ breast cancer, since they interfere directly with the production of estrogens and with the activation of estrogen-dependent signaling pathways. Despite the success of these treatments, the occurrence of resistance limits their clinical efficacy, demanding the development of novel therapies. Recently, multi-target compounds emerged as promising therapeutic strategies for ER+ breast cancer, as they can potentially modulate several important targets simultaneously. In line with this, in this work, the anti-cancer properties and multi-target action of 1,1-Bis(4-hydroxyphenyl)-2-phenylbut-1-ene, tamoxifen bisphenol (1,1-BHPE), were evaluated in an ER+ breast cancer cell model (MCF-7aro cells). Molecular docking analysis predicted that 1,1-BHPE was able to bind to aromatase, ERα and ERß. In vitro studies showed that, although it did not present anti-aromatase activity, 1,1-BHPE reduced aromatase protein levels and interfered with ERα and ERß signaling pathways, acting as an ERα antagonist and inducing ERß up-regulation. Through these mechanisms, 1,1-BHPE was able to impair breast cancer growth and induce apoptosis. This represents an important therapeutic advantage because the main players responsible for estrogen production and signaling are modulated by a single compound. To the best of our knowledge, this is the first study describing the anti-cancer properties of 1,1-BHPE as a multi-target compound specific for ER+ breast cancer.

Cannabidiol (CBD) but not tetrahydrocannabinol (THC) dysregulate in vitro decidualization of human endometrial stromal cells by disruption of estrogen signaling.

Decidualization, which comprises proliferation and differentiation of endometrial stromal cells (ESCs), is essential for the establishment of a receptive endometrium and pregnancy to occur. A deregulation of decidualization has been associated with miscarriage, infertility and other pregnancy-related disorders. The role of estradiol (E2) on decidualization has already been shown, since it regulates proliferation of ESCs and expression of progesterone receptor. In this study, we investigated the effects of phytocannabinoids, tetrahydrocannabinol (THC) and cannabidiol (CBD), in proliferation and differentiation of ESCs, as well as, in E2 metabolism/signaling. We found that CBD, but not THC, inhibits ESCs differentiation. We also show that CBD prevents the increase on transcript levels of CYP19A1 gene and the elevation of E2 levels that are observed in differentiating ESCs. Moreover, we found that CBD presents anti-aromatase activity. In overall, we highlight a novel effect of CBD on human endometrial differentiation, which may lead to infertility problems.

Conformational diversity induces nanosecond-timescale chemical disorder in the HIV-1 protease reaction pathway.

The role of conformational diversity in enzyme catalysis has been a matter of analysis in recent studies. Pre-organization of the active site has been pointed out as the major source for enzymes' catalytic power. Following this line of thought, it is becoming clear that specific, instantaneous, non-rare enzyme conformations that make the active site perfectly pre-organized for the reaction lead to the lowest activation barriers that mostly contribute to the macroscopically observed reaction rate. The present work is focused on exploring the relationship between structure and catalysis in HIV-1 protease (PR) with an adiabatic mapping method, starting from different initial structures, collected from a classical MD simulation. The first, rate-limiting step of the HIV-1 PR catalytic mechanism was studied with the ONIOM QM/MM methodology (B3LYP/6-31G(d):ff99SB), with activation and reaction energies calculated at the M06-2X/6-311++G(2d,2p):ff99SB level of theory, in 19 different enzyme:substrate conformations. The results showed that the instantaneous enzyme conformations have two independent consequences on the enzyme's chemistry: they influence the barrier height, something also observed in the past in other enzymes, and they also influence the specific reaction pathway, which is something unusual and unexpected, challenging the "one enzyme-one substrate-one reaction mechanism" paradigm. Two different reaction mechanisms, with similar reactant probabilities and barrier heights, lead to the same gem-diol intermediate. Subtle nanosecond-timescale rearrangements in the active site hydrogen bonding network were shown to determine which reaction the enzyme follows. We named this phenomenon chemical disorder. The results make us realize the unexpected mechanistic consequences of conformational diversity in enzymatic reactivity.

The Catalytic Mechanism of Human Transketolase.

We have computationally determined the catalytic mechanism of human transketolase (hTK) using a cluster model approach and density functional theory calculations. We were able to determine all the relevant structures, bringing solid evidences to the proposed experimental mechanism, and to add important detail to the structure of the transition states and the energy profile associated with catalysis. Furthermore, we have established the existence of a crucial intermediate of the catalytic cycle, in agreement with experiments. The calculated data brought new insights to hTK's catalytic mechanism, providing free-energy values for the chemical reaction, as well as adding atomistic detail to the experimental mechanism.

Anandamide targets aromatase: A breakthrough on human decidualization.

In each menstrual cycle endometrial stromal cells (hESC) proliferate and differentiate into specialized decidual cells, a process termed decidualization, which regulates endometrial receptivity. Decidualization is mainly controlled by sex ovarian hormones, estradiol (E2) and progesterone. E2 plays an important role in the expression of the progesterone receptor and promotes the endometrial stromal cells differentiation. Our group previously reported that anandamide (AEA) impairs decidualization through cannabinoid receptor 1 (CB1). In this study, we hypothesized whether AEA inhibitory effect on cell decidualization could be mediated through interaction with aromatase and consequent interference in estradiol production/signaling. We used an immortalized human endometrial stromal cell line (St-T1b) and human decidual fibroblasts (HdF) derived from human term placenta. In cells exposed to a differentiation stimulus, AEA-treatment prevents the increase of the expression of CYP19A1 gene encoding aromatase, E2 levels and of estradiol receptor expression, that are observed in differentiating cells. Regarding CYP19A1 mRNA levels, the effect was partially reverted by a CB1 receptor antagonist and by a COX2 inhibitor. In addition, we report that AEA presents anti-aromatase activity in placental microsomes, the nature of the inhibition being the uncommon mixed type as revealed by the kinetic studies. Structural analysis of the AEA-Aromatase complexes determined that AEA may bind to the active site pocket of the enzyme. In overall we report that AEA inhibits aromatase activity and may affect E2 signaling crucial for the decidualization process, indicating that a deregulation of the endocannabinoid system may be implicated in endometrial dysfunction and in fertility/infertility disorders.

Benchmark of Density Functionals for the Calculation of the Redox Potential of Fe3+/Fe2+ Within Protein Coordination Shells.

Iron is a very important transition metal often found in proteins. In enzymes specifically, it is often found at the core of reaction mechanisms, participating in the reaction cycle, more often than not in oxidation/reduction reactions, where it cycles between its most common Fe(III)/Fe(II) oxidation states. QM and QM/MM computational methods that study these catalytic reaction mechanisms mostly use density functional theory (DFT) to describe the chemical transformations. Unfortunately, density functional is known to be plagued by system-specific and property-specific inaccuracies that cast a shadow of uncertainty over the results. Here we have modeled 12 iron coordination complexes, using ligands that represent amino acid sidechains, and calculated the accuracy with which the most common density functionals reproduce the redox properties of the iron complexes (specifically the electronic component of the redox potential at 0 K, Δ E elec F e 3 + / F e 2 + ), using the same property calculated with CCSD(T)/CBS as reference for the evaluation. A number of hybrid and hybrid-meta density functionals, generally with a large % of HF exchange (such as BB1K, mPWB1K, and mPW1B95) provided systematically accurate values for Δ E elec F e 3 + / F e 2 + , with MUEs of ~2 kcal/mol. The very popular B3LYP density functional was found to be quite precise as well, with a MUE of 2.51 kcal/mol. Overall, the study provides guidelines to estimate the inaccuracies coming from the density functionals in the study of enzyme reaction mechanisms that involve an iron cofactor, and to choose appropriate density functionals for the study of the same reactions.

Structural and mechanistic aspects of S-S bonds in the thioredoxin-like family of proteins.

Disulfide bonds play a critical role in a variety of structural and mechanistic processes associated with proteins inside the cells and in the extracellular environment. The thioredoxin family of proteins like thioredoxin (Trx), glutaredoxin (Grx) and protein disulfide isomerase, are involved in the formation, transfer or isomerization of disulfide bonds through a characteristic thiol-disulfide exchange reaction. Here, we review the structural and mechanistic determinants behind the thiol-disulfide exchange reactions for the different enzyme types within this family, rationalizing the known experimental data in light of the results from computational studies. The analysis sheds new atomic-level insight into the structural and mechanistic variations that characterize the different enzymes in the family, helping to explain the associated functional diversity. Furthermore, we review here a pattern of stabilization/destabilization of the conserved active-site cysteine residues presented beforehand, which is fully consistent with the observed roles played by the thioredoxin family of enzymes.

Visualizing the Microscopic World.

Visualization can be a motivating way of teaching students about the microscopic world. This can become even more exciting if the information is based on accurate computational results rather than on crude approximations that eventually might create unreal alternative perceptions. Here, we report on a VMD plug-in, named vmdMagazine, which can turn computational simulations into stunning high-impact video presentations, suitable for classes/lectures and even conferences. The software will help students/audience to understand atoms and molecules better and learn to like them. The present paper is meant to give a general idea of the software's potential, showing how it works and how it can be used for educational purposes. The software is freely available at: http://www.fc.up.pt/PortoBioComp/database/doku.php?id=vmdmagazine .

Improving the Catalytic Power of the DszD Enzyme for the Biodesulfurization of Crude Oil and Derivatives.

The enhancement of the catalytic power of enzymes is a subject of enormous interest both for science and for industry. The latter, in particular, due to the vast applications enzymes can have in industrial processes, for instance in the desulfurization of crude oil, which is mandatory by law in many developed countries and is currently performed using costly chemical processes. In this work we sought to enhance the turnover rate of DszD from Rhodococcus erythropolis, a NADH-FMN oxidoreductase responsible for supplying FMNH2 to DszA and DszC in the biodesulfurization process of crude oil, the 4S pathway. For this purpose, we replaced the wild type spectator residue of the rate limiting step of the reduction of FMN to FMNH2 , a process catalysed by DszD and known to play an important role in the reaction energy profile. As replacements, we used all the naturally occurring amino acids, one at a time, using computational methodologies, and repeated the above-mentioned reaction with each mutant. To calculate the different free energy profiles, one for each mutated model, we applied quantum mechanics/molecular mechanics (QM/MM) methods within an ONIOM scheme. The free energy barriers obtained varied between 15.1 and 29.9 kcal mol-1 . Multiple factors contributed to the different ΔG values. The most relevant were electrostatic interactions and the induction of a favourable alignment between substrate and cofactor. These results confirm the great potential that chirurgic mutations have for increasing the catalytic power of DszD in relation to the wild type (wt) enzyme.

Molecular motion regulates the activity of the Mitochondrial Serine Protease HtrA2.

HtrA2 (high-temperature requirement 2) is a human mitochondrial protease that has a role in apoptosis and Parkinson's disease. The structure of HtrA2 with an intact catalytic triad was determined, revealing a conformational change in the active site loops, involving mainly the regulatory LD loop, which resulted in burial of the catalytic serine relative to the previously reported structure of the proteolytically inactive mutant. Mutations in the loops surrounding the active site that significantly restricted their mobility, reduced proteolytic activity both in vitro and in cells, suggesting that regulation of HtrA2 activity cannot be explained by a simple transition to an activated conformational state with enhanced active site accessibility. Manipulation of solvent viscosity highlighted an unusual bi-phasic behavior of the enzymatic activity, which together with MD calculations supports the importance of motion in the regulation of the activity of HtrA2. HtrA2 is an unusually thermostable enzyme (TM=97.3 °C), a trait often associated with structural rigidity, not dynamic motion. We suggest that this thermostability functions to provide a stable scaffold for the observed loop motions, allowing them a relatively free conformational search within a rather restricted volume.