Epstein-Barr Virus BGLF2 commandeers RISC to interfere with cellular miRNA function.

The Epstein-Barr virus (EBV) BGLF2 protein is a tegument protein with multiple effects on the cellular environment, including induction of SUMOylation of cellular proteins. Using affinity-purification coupled to mass-spectrometry, we identified the miRNA-Induced Silencing Complex (RISC), essential for miRNA function, as a top interactor of BGLF2. We confirmed BGLF2 interaction with the Ago2 and TNRC6 components of RISC in multiple cell lines and their co-localization in cytoplasmic bodies that also contain the stress granule marker G3BP1. In addition, BGLF2 expression led to the loss of processing bodies in multiple cell types, suggesting disruption of RISC function in mRNA regulation. Consistent with this observation, BGLF2 disrupted Ago2 association with multiple miRNAs. Using let-7 miRNAs as a model, we tested the hypothesis that BGLF2 interfered with the function of RISC in miRNA-mediated mRNA silencing. Using multiple reporter constructs with 3'UTRs containing let-7a regulated sites, we showed that BGLF2 inhibited let-7a miRNA activity dependent on these 3'UTRs, including those from SUMO transcripts which are known to be regulated by let-7 miRNAs. In keeping with these results, we showed that BGLF2 increased the cellular level of unconjugated SUMO proteins without affecting the level of SUMO transcripts. Such an increase in free SUMO is known to drive SUMOylation and would account for the effect of BGLF2 in inducing SUMOylation. We further showed that BGLF2 expression inhibited the loading of let-7 miRNAs into Ago2 proteins, and conversely, that lytic infection with EBV lacking BGLF2 resulted in increased interaction of let-7a and SUMO transcripts with Ago2, relative to WT EBV infection. Therefore, we have identified a novel role for BGLF2 as a miRNA regulator and shown that one outcome of this activity is the dysregulation of SUMO transcripts that leads to increased levels of free SUMO proteins and SUMOylation.

Bilateral Dilated Superior Ophthalmic Veins in a Patient With an Arteriovenous Dialysis Fistula.

A 64-year-old man presented with 4 months of diplopia. He had end-stage renal disease requiring a cephalic transposition brachiocephalic fistula that was no longer in use following successful renal transplantation. On presentation, he had bilateral proptosis, extraocular movement restriction, chemosis, tortuous episcleral vessels, and caruncular injection. Non-contrast CT of the orbits demonstrated dilation of both superior ophthalmic veins, and CT angiography showed asymmetric enlargement of both cavernous sinuses and superior ophthalmic veins. A carotid-cavernous fistula was suspected, but cerebral angiography revealed shunting from the old fistula with intracranial drainage and cerebral venous hypertension. Aberrant retrograde drainage resulted from anatomical compression of the left brachiocephalic vein. The fistula was ligated, and at 1-week follow-up, the patient had marked improvement in extraocular movements and orbital congestion with near complete resolution of diplopia. Postoperative CT angiography obtained 2 months later demonstrated decreased size of both superior ophthalmic veins, consistent with improvement of venous hypertension.

Establishment and Characterization of Three Human Ocular Adnexal Sebaceous Carcinoma Cell Lines.

Ocular adnexal sebaceous carcinoma (SebCA) represents one of the most clinically problematic periocular tumors, often requiring aggressive surgical resection. The pathobiology of this tumor remains poorly understood, and few models exist that are suitable for preclinical testing. The aim of this study was to establish new cell lines to serve as models for pathobiological and drug testing. With patient consent, freshly resected tumor tissue was cultured using conditional reprogramming cell conditions. Standard techniques were used to characterize the cell lines in terms of overall growth, clonogenicity, apoptosis, and differentiation in vitro. Additional analyses including Western blotting, short tandem repeat (STR) profiling, and next-generation sequencing (NGS) were performed. Drug screening using mitomycin-C (MMC), 5-fluorouricil (5-FU), and 6-Diazo-5-oxo-L-norleucine (DON) were performed. JHH-SebCA01, JHH-SebCA02, and JHH-SebCA03 cell lines were established from two women and one man undergoing surgical resection of eyelid tumors. At passage 15, they each showed a doubling time of two to three days, and all could form colonies in anchorage-dependent conditions, but not in soft agar. The cells contained cytoplasmic vacuoles consistent with sebaceous differentiation, and adipophilin protein was present in all three lines. STR profiling confirmed that all lines were derived from their respective patients. NGS of the primary tumors and their matched cell lines identified numerous shared mutations, including alterations similar to those previously described in SebCA. Treatment with MMC or 5-FU resulted in dose-dependent growth inhibition and the induction of both apoptosis and differentiation. MYC protein was abundant in all three lines, and the glutamine metabolism inhibitor DON, previously shown to target high MYC tumors, slowed the growth of all our SebCA models. Ocular adnexal SebCA cell lines can be established using conditional reprogramming cell conditions, and our three new models are useful for testing therapies and interrogating the functional role of MYC and other possible molecular drivers. Current topical chemotherapies promote both apoptosis and differentiation in SebCA cells, and these tumors appear sensitive to inhibition or MYC-associated metabolic changes.

Trends in orbital cellulitis severity and management: 2016 through 2022.

PURPOSE: To characterize trends in orbital cellulitis (OC) severity and management from 2016 through 2022, including assessment of effects from the COVID-19 pandemic. METHODS: A retrospective chart review was conducted at a tertiary care center using an electronic medical record search of OC from January 1 2016 to January 1 2023. Radiography was used to categorize patients into three cohorts - OC without abscess, OC with subperiosteal abscess (SPA), and OC with orbital abscess (OA). Infection cohort and age group [pediatric (<9 years old), adolescent (9-18 years), adult (>18 years)] were compared across years. Subgroup analysis was performed in the SPA cohort. Comparisons were made between the time periods prior to and after the start of the COVID-19 pandemic. RESULTS: From 2016-2022, there were 247 (67.3%) cases of OC, 103 (28.1%) SPA, and 17 (4.6%) OA. The year 2022 incurred higher rates of SPA and OA (p = 0.035), pediatric infections (p = 0.008), and co-morbid sinusitis (p = 0.005) compared to all preceding years. Rates were similar from 2016 through 2021. Within the SPA cohort, rates of surgical intervention and outcomes were similar across all years. Rate of abscess formation was significantly higher during the COVID-19 pandemic compared to the pre-pandemic era (p = 0.033). CONCLUSION: The severity of OC leading up to the COVID-19 pandemic appeared steady. After pandemic onset, a delayed increase in abscess formation, predominantly affecting pediatric patients and correlating with a greater rate of sinusitis, was observed in 2022.

Ocular adnexal sebaceous carcinoma in a patient with Li-Fraumeni syndrome.

Li-Fraumeni syndrome (LFS) is caused by a pathogenic germline variant at the TP53 locus and is associated with an increased predisposition to a variety of cancers. The neoplasms most frequently associated with LFS are sarcomas, breast cancer, brain tumors, and adrenocortical carcinomas. In this case report, we present a 43-year-old male diagnosed with an ocular adnexal sebaceous carcinoma of the right upper eyelid who was confirmed to have LFS with subsequent genetic testing. The mutational profile of both the patient's genetic screen and tumor sequencing were congruent, demonstrating the same pathogenic loss-of-function TP53 variant. This case report highlights the importance of pursuing genetic testing in patients with a history of multiple tumor types, particularly those with uncommon diagnoses. In this case, confirmation of LFS had important implications for personalized patient care, including identification of contraindicated treatment interventions and the imaging modalities necessary for vigilant follow-up screening.

Off-Label Reduced Dose Apixaban in Older Adults With Atrial Fibrillation and Associated Outcomes.

BACKGROUND: Apixaban is commonly used to prevent stroke in older adults with nonvalvular atrial fibrillation (AF). Although its package insert has specific dose reduction criteria, providers may dose reduce outside of these parameters based on clinical scenarios. OBJECTIVE: The primary objective was to determine the incidence of apixaban off-label reduced dosing, while secondarily determining the safety and efficacy outcomes associated with such dosing. METHODS: A retrospective analysis of patients aged 65 and older with orders for apixaban for AF was institutional review board (IRB)-approved and conducted across 3 academic medical centers. Patients receiving off-label reduced-dose apixaban (ie, "underdosed") were matched to a cohort of patients dosed according to the package insert at the standard dosing (5 mg twice daily) using stratified random sampling. Secondary outcomes included 1-year incidence of major bleeding, clinically relevant non-major bleeding (CRNMB), stroke or transient ischemic attack (TIA), and mortality. The Fisher exact tests were used to compare between-group differences. RESULTS: Of the 1172 patients meeting initial inclusion criteria, 201 (17%) were dosed off-label, with 175 (15%) "underdosed." The 147 "underdosed" patients with documented follow-up were matched with 139 patients receiving standard Food and Drug Administration (FDA)-labeled dosing. There were no significant differences in incidence of stroke (2.7% vs 2.2%), major bleeding (0% vs 0.7%), and CRNMB (2.7% vs 1.4%) in the off-label reduced dosing versus standard dosing groups. All-cause mortality was higher in the off-label reduced-dose group (16 [10.9%] vs 2 [1.4%], P < 0.05). CONCLUSION AND RELEVANCE: Older adults with nonvalvular AF are commonly prescribed lower-than-recommended doses of apixaban. However, no significant association was found between empiric off-label reduced dosing and stroke or bleeding outcomes.

Teprotumumab and Hearing Loss: Case Series and Proposal for Audiologic Monitoring.

PURPOSE: To present a protocol for audiologic monitoring in the setting of teprotumumab treatment of thyroid eye disease, motivated by 4 cases of significant hearing loss, and review the relevant literature. METHODS: Cases of hearing loss in the setting of teprotumumab were retrospectively elicited as part of a multi-institutional focus group, including oculoplastic surgeons, a neurotologist and an endocrinologist. A literature review was performed. RESULTS: An aggregate of 4 cases of teprotumumab-associated hearing loss documented by formal audiologic testing were identified among 3 clinicians who had treated 28 patients. CONCLUSIONS: Teprotumumab may cause a spectrum of potentially irreversible hearing loss ranging from mild to severe, likely resulting from the inhibition of the insulin-like growth factor-1 and the insulin-like growth factor-1 receptor pathway. Due to the novelty of teprotumumab and the lack of a comprehensive understanding of its effect on hearing, the authors endorse prospective investigations of hearing loss in the setting of teprotumumab treatment. Until the results of such studies are available, the authors think it prudent to adopt a surveillance protocol to include an audiogram and tympanometry before, during and after infusion, and when prompted by new symptoms of hearing dysfunction.

Detection of Human Papillomavirus in Squamous Lesions of the Conjunctiva Using RNA and DNA In-Situ Hybridization.

In-situ hybridization provides a convenient and reliable method to detect human papillomavirus (HPV) infection in formalin-fixed paraffin-embedded tissue. Cases of conjunctival papillomas, conjunctival intraepithelial neoplasia (CIN), conjunctival carcinoma in situ (cCIS), and invasive squamous cell carcinoma (SCC), in which low-risk (LR) and/or high-risk (HR) HPV types were evaluated by RNA or DNA in-situ hybridization, were retrospectively identified. LR HPV types were frequently detected in conjunctival papillomas (25/30, 83%), including 17/18 (94%) with RNA probes, compared to 8/12 (75%) with DNA probes. None of the CIN/cCIS or SCC cases were positive for LR HPV by either method. HR HPV was detected by RNA in-situ hybridization in 1/16 (6%) of CIN/cCIS cases and 2/4 (50%) of SCC cases, while DNA in-situ hybridization failed to detect HPV infection in any of the CIN/cCIS lesions. Reactive atypia and dysplasia observed in papillomas was generally associated with the detection of LR HPV types. Collectively, our findings indicate RNA in-situ hybridization may provide a high-sensitivity approach for identifying HPV infection in squamous lesions of the conjunctiva and facilitate the distinction between reactive atypia and true dysplasia. There was no clear association between HPV infection and atopy in papillomas or dysplastic lesions.

Use of computer-assisted surgery in the orbit.

PURPOSE: To present the application of computer-assisted surgery (CAS) in pre-operative planning, intra-operative navigation, and post-operative assessment as an adjunct tool in orbital surgery. METHODS: An IRB-approved, retrospective review was performed to identify patients who had undergone orbital surgery by a single surgeon from July 2013 to December 2019 with attention to pre-operative virtual surgical planning, intra-operative navigation, and post-operative assessment. The reasons and methods of CAS use were classified. RESULTS: The use of computer-assisted technologies was identified in 91 cases out of 464 orbital surgeries (19.6%). This included 23 (25.3%) orbital decompression surgeries, 39 (42.9%) fracture repairs, and 25 (27.5%) orbital tumors. In all cases, pre-, intra-, and post-operative CAS allowed for increased operative efficiency and safety with good outcomes. CONCLUSIONS: Use of CAS in orbital surgery can allow for complex radiographic analysis and in select cases is a great tool to add to the orbital surgeon's armamentarium.

Trapping conformational states of a flavin-dependent N-monooxygenase in crystallo reveals protein and flavin dynamics.

The siderophore biosynthetic enzyme A (SidA) ornithine hydroxylase from Aspergillus fumigatus is a fungal disease drug target involved in the production of hydroxamate-containing siderophores, which are used by the pathogen to sequester iron. SidA is an N-monooxygenase that catalyzes the NADPH-dependent hydroxylation of l-ornithine through a multistep oxidative mechanism, utilizing a C4a-hydroperoxyflavin intermediate. Here we present four new crystal structures of SidA in various redox and ligation states, including the first structure of oxidized SidA without NADP(H) or l-ornithine bound (resting state). The resting state structure reveals a new out active site conformation characterized by large rotations of the FAD isoalloxazine around the C1-'C2' and N10-C1' bonds, coupled to a 10-Å movement of the Tyr-loop. Additional structures show that either flavin reduction or the binding of NADP(H) is sufficient to drive the FAD to the in conformation. The structures also reveal protein conformational changes associated with the binding of NADP(H) and l-ornithine. Some of these residues were probed using site-directed mutagenesis. Docking was used to explore the active site of the out conformation. These calculations identified two potential ligand-binding sites. Altogether, our results provide new information about conformational dynamics in flavin-dependent monooxygenases. Understanding the different active site conformations that appear during the catalytic cycle may allow fine-tuning of inhibitor discovery efforts.

Structure and function of a flavin-dependent S-monooxygenase from garlic (Allium sativum).

Allicin is a component of the characteristic smell and flavor of garlic (Allium sativum). A flavin-containing monooxygenase (FMO) produced by A. sativum (AsFMO) was previously proposed to oxidize S-allyl-l-cysteine (SAC) to alliin, an allicin precursor. Here, we present a kinetic and structural characterization of AsFMO that suggests a possible contradiction to this proposal. Results of steady-state kinetic analyses revealed that AsFMO exhibited negligible activity with SAC; however, the enzyme was highly active with l-cysteine, N-acetyl-l-cysteine, and allyl mercaptan. We found that allyl mercaptan with NADPH was the preferred substrate-cofactor combination. Rapid-reaction kinetic analyses showed that NADPH binds tightly (KD of ∼2 µm) to AsFMO and that the hydride transfer occurs with pro-R stereospecificity. We detected the formation of a long-wavelength band when AsFMO was reduced by NADPH, probably representing the formation of a charge-transfer complex. In the absence of substrate, the reduced enzyme, in complex with NADP+, reacted with oxygen and formed an intermediate with a spectrum characteristic of C4a-hydroperoxyflavin, which decays several orders of magnitude more slowly than the kcat The presence of substrate enhanced C4a-hydroperoxyflavin formation and, upon hydroxylation, oxidation occurred with a rate constant similar to the kcat The structure of AsFMO complexed with FAD at 2.08-Å resolution features two domains for binding of FAD and NADPH, representative of class B flavin monooxygenases. These biochemical and structural results are consistent with AsFMO being an S-monooxygenase involved in allicin biosynthesis through direct formation of sulfenic acid and not SAC oxidation.

Structural analysis of prolines and hydroxyprolines binding to the l-glutamate-γ-semialdehyde dehydrogenase active site of bifunctional proline utilization A.

Proline utilization A (PutA) proteins are bifunctional proline catabolic enzymes that catalyze the 4-electron oxidation of l-proline to l-glutamate using spatially-separated proline dehydrogenase and l-glutamate-γ-semialdehyde dehydrogenase (GSALDH, a.k.a. ALDH4A1) active sites. The observation that l-proline inhibits both the GSALDH activity of PutA and monofunctional GSALDHs motivated us to study the inhibition of PutA by proline stereoisomers and analogs. Here we report five high-resolution crystal structures of PutA with the following ligands bound in the GSALDH active site: d-proline, trans-4-hydroxy-d-proline, cis-4-hydroxy-d-proline, l-proline, and trans-4-hydroxy-l-proline. Three of the structures are of ternary complexes of the enzyme with an inhibitor and either NAD+ or NADH. To our knowledge, the NADH complex is the first for any GSALDH. The structures reveal a conserved mode of recognition of the inhibitor carboxylate, which results in the pyrrolidine rings of the d- and l-isomers having different orientations and different hydrogen bonding environments. Activity assays show that the compounds are weak inhibitors with millimolar inhibition constants. Curiously, although the inhibitors occupy the aldehyde binding site, kinetic measurements show the inhibition is uncompetitive. Uncompetitive inhibition may involve proline binding to a remote site or to the enzyme-NADH complex. Together, the structural and kinetic data expand our understanding of how proline-like molecules interact with GSALDH, reveal insight into the relationship between stereochemistry and inhibitor affinity, and demonstrate the pitfalls of inferring the mechanism of inhibition from crystal structures alone.

High-risk human papillomavirus and ZEB1 in ocular adnexal sebaceous carcinoma.

BACKGROUND: Ocular adnexal (OA) sebaceous carcinoma is an aggressive malignancy. Oncologic drivers of ocular sebaceous carcinoma are incompletely understood. METHODS: A retrospective search of our pathology archives for OA sebaceous carcinoma identified 18 primary resection specimens. Immunohistochemistry for p16 and ZEB1 and RNA in situ hybridization for high-risk human papillomavirus (HPV) subtypes were performed. RESULTS: High-risk HPV was demonstrated in 2/11 (18%) cases. p16 overexpression was observed in 10/11 (91%). No association between gender, age at presentation, tumor location, intraepithelial spread, tumor size, and T stage was observed between HPV-driven and nonviral cases. High expression of ZEB1 was observed in the intraepithelial component of 4/14 (28%) cases and in the subepithelial component of 1/13 (7%) cases. ZEB1 overexpression was not associated with HPV status, T stage, or tumor size. CONCLUSION: As previously described by others, our findings suggest that a subset of OA sebaceous carcinomas may arise via an HPV-dependent pathway. However, unlike high-risk HPV-driven carcinomas of the oropharynx, we did not identify an association between HPV-status and prognostic features. Furthermore, p16 expression was not a useful surrogate marker for HPV-driven disease. ZEB1 overexpression is not associated with HPV in our cohort of ocular sebaceous carcinoma.

Compound Coherent Plane-Wave Ultrasound Imaging of Vascular Malformations of the Orbit.

PURPOSE: Prior color-flow Doppler ultrasound studies of the eye have been performed with systems that exceed US Food and Drug Administration permissible ophthalmic ultrasonic energy limits. The authors report a study of orbital vascular malformations using a novel, Food and Drug Administration compliant, ultrafast compound coherent plane-wave ultrasound device to produce power Doppler images. METHODS: Using a Verasonics Vantage 128 ultrasound engine and a user-developed MATLAB program with a 5-MHz linear-array probe, compound coherent plane-wave ultrasound data were collected on patients with orbital vascular malformations. Real-time color-flow Doppler visualized orbital blood flow. Power Doppler images were produced by post-processing compound coherent plane-wave ultrasound data acquired continuously for 2 seconds. RESULTS: Compound coherent plane-wave ultrasound was performed on 3 orbital vascular malformations (1 venolymphatic malformation, 1 infantile hemangioma, and 1 arteriovenous malformation). Compound coherent plane-wave ultrasound produced a high-resolution depiction of orbital blood flow for orbital vascular malformations with high sensitivity to slow flow. CONCLUSIONS: Analysis of blood flow within orbital lesions informs treatment planning. Compound coherent plane-wave ultrasound is an emerging ultrasound modality that falls within the Food and Drug Administration guidelines for use in the orbit and provides information to characterize orbital vascular malformations.

Chocolate Cysts Associated With Porous Polyethylene Orbital Implants.

Late-onset orbital hemorrhage is a rare complication of alloplastic implant use in orbital wall reconstruction following trauma. The authors report 3 patients with chocolate cysts presenting 3 to 9 years after orbital fracture repair with porous polyethylene implants. All patients were managed by implant removal and evacuation of cyst contents. Complete excision of the cyst was performed in 1 patient, while partial excision of the capsule was performed in 2 patients. Improvement of symptoms associated with the mass effects of the cyst was noted after surgical intervention. The authors also report the first case of orbital volume expansion from a chocolate cyst associated with a nonbarriered porous polyethylene implant. Delayed hemorrhage with capsule formation, although extremely rare, is a possible complication following orbital fracture repair with porous polyethylene implants.

NGS Analysis Confirms Common TP53 and RB1 Mutations, and Suggests MYC Amplification in Ocular Adnexal Sebaceous Carcinomas.

Ocular adnexal (OA) sebaceous carcinomas generally demonstrate more aggressive clinical and histopathological phenotypes than extraocular cases, but the molecular drivers implicated in their oncogenesis remain poorly defined. A retrospective review of surgical and ocular pathology archives identified eleven primary resection specimens of OA sebaceous carcinomas with adequate tissue for molecular analysis; two extraocular cases were also examined. Next-generation sequencing was used to evaluate mutations and copy number changes in a large panel of cancer-associated genes. Fluorescence in situ hybridization (FISH) confirmed MYC copy number gain in select cases, and immunohistochemistry to evaluate MYC protein expression. The commonest mutations occurred in TP53 (10/13) and RB1 (7/13). Additional mutations in clinically actionable genes, or mutations with a frequency of at least 25%, included the NF1 (3/12), PMS2 (4/12), ROS1 (3/12), KMT2C (4/12), MNX1 (6/12), NOTCH1 (4/12), PCLO (3/12), and PTPRT (3/12) loci. Low level copy number gain suggestive of amplification of the MYC locus was seen in two cases, and confirmed using FISH. MYC protein expression, as assessed by immunohistochemistry, was present in almost all sebaceous carcinoma cases. Our findings support the concept that alterations in TP53 and RB1 are the commonest alterations in sebaceous carcinoma, and suggest that MYC may contribute to the oncogenesis of these tumors.

Acute angle plication of optic nerve glioma as a mechanism of rapidly progressive visual loss.

Purpose: The majority of pediatric patients with optic nerve gliomas (ONG) remain asymptomatic; however, a subset of patients suffer rapid, irreversible visual loss. The purpose of this study was to determine anatomic differences of ONG noted either by imaging or by intra-operative observation between patients with gradual visual dysfunction and those with rapid visual loss. Methods: A retrospective review was performed in patients with visual loss secondary to ONG. The clinical records, pathology, and radiographic images were reviewed for all patients. The degree of folding or plication of the optic nerve (ON) was determined by analyzing the most acute angle present in the course of the ON. Outcome measures: The primary outcome measure was the angle of plication or folding of the ON and the relationship of this to the course of visual dysfunction. Results: Six patients with ONG were included in the study. A structural difference in the ON was identified in four patients with rapid vision loss compared with two patients with more gradual visual dysfunction. In patients with rapid progressive visual loss, the ONG had a 90-degree or more acute plication of the ON. Those with more gradual visual loss had more obtuse bends in the ONG. Conclusions: We have identified that the intrinsic structure of the ONG may contribute to the rare but devastatingly rapid progression of visual dysfunction in some patients. Recognizing these changes may guide clinicians to intervene prior to the development of irreversible visual loss.

High-risk human papillomavirus positive primary squamous cell carcinoma of the lacrimal gland: a case report.

A 78-year-old Caucasian female presented with a painless mass in the right orbit that had progressively enlarged over several months. Computed tomography scan of the orbits showed a right lacrimal gland mass with no bony erosion. Histopathologic analysis of the biopsy specimen revealed invasive squamous cell carcinoma positive via in-situ hybridization for high-risk human papillomavirus. The patient underwent successful removal of the right lacrimal gland tumor en bloc, followed by adjuvant radiotherapy. This is an extremely rare case of primary squamous cell carcinoma of the lacrimal gland and the first report describing human papillomavirus positivity in this tumor location.

Structural Determinants of Flavin Dynamics in a Class B Monooxygenase.

The ornithine hydroxylase known as SidA is a class B flavin monooxygenase that catalyzes the first step in the biosynthesis of hydroxamate-containing siderophores in Aspergillus fumigatus. Crystallographic studies of SidA revealed that the FAD undergoes dramatic conformational changes between out and in states during the catalytic cycle. We sought insight into the origins and purpose of flavin motion in class B monooxygenases by probing the function of Met101, a residue that contacts the pyrimidine ring of the in FAD. Steady-state kinetic measurements showed that the mutant variant M101A has a 25-fold lower turnover number. Pre-steady-state kinetic measurements, pH profiles, and solvent kinetic isotope effect measurements were used to isolate the microscopic step that is responsible for the reduced steady-state activity. The data are consistent with a bottleneck in the final step of the mechanism, which involves flavin dehydration and the release of hydroxy-l-ornithine and NADP+. Crystal structures were determined for M101A in the resting state and complexed with NADP+. The resting enzyme structure is similar to that of wild-type SidA, consistent with M101A exhibiting normal kinetics for flavin reduction by NADPH and wild-type affinity for NADPH. In contrast, the structure of the M101A-NADP+ complex unexpectedly shows the FAD adopting the out conformation and may represent a stalled conformation that is responsible for the slow kinetics. Altogether, our data support a previous proposal that one purpose of the FAD conformational change from in to out in class B flavin monooxygenases is to eject spent NADP+ in preparation for a new catalytic cycle.

Inhibition, crystal structures, and in-solution oligomeric structure of aldehyde dehydrogenase 9A1.

Aldehyde dehydrogenase 9A1 (ALDH9A1) is a human enzyme that catalyzes the NAD+-dependent oxidation of the carnitine precursor 4-trimethylaminobutyraldehyde to 4-N-trimethylaminobutyrate. Here we show that the broad-spectrum ALDH inhibitor diethylaminobenzaldehyde (DEAB) reversibly inhibits ALDH9A1 in a time-dependent manner. Possible mechanisms of inhibition include covalent reversible inactivation involving the thiohemiacetal intermediate and slow, tight-binding inhibition. Two crystal structures of ALDH9A1 are reported, including the first of the enzyme complexed with NAD+. One of the structures reveals the active conformation of the enzyme, in which the Rossmann dinucleotide-binding domain is fully ordered and the inter-domain linker adopts the canonical ß-hairpin observed in other ALDH structures. The oligomeric structure of ALDH9A1 was investigated using analytical ultracentrifugation, small-angle X-ray scattering, and negative stain electron microscopy. These data show that ALDH9A1 forms the classic ALDH superfamily dimer-of-dimers tetramer in solution. Our results suggest that the presence of an aldehyde substrate and NAD+ promotes isomerization of the enzyme into the active conformation.