Binding of the substrate analog methanol in the oxygen-evolving complex of photosystem II in the D1-N87A genetic variant of cyanobacteria.

The solar water-splitting protein complex, photosystem II (PSII), catalyzes one of the most energetically demanding reactions in nature by using light energy to drive a catalyst capable of oxidizing water. The water oxidation reaction is catalyzed at the Mn4Ca-oxo cluster in the oxygen-evolving complex (OEC), which cycles through five light-driven S-state intermediates (S0-S4). A detailed mechanism of the reaction remains elusive as it requires knowledge of the delivery and binding of substrate water in the higher S-state intermediates. In this study, we use two-dimensional (2D) hyperfine sublevel correlation spectroscopy, in conjunction with quantum mechanics/molecular mechanics (QM/MM) and density functional theory (DFT), to probe the binding of the substrate analog, methanol, in the S2 state of the D1-N87A variant of PSII from Synechocystis sp. PCC 6803. The results indicate that the size and specificity of the "narrow" channel is altered in D1-N87A PSII, allowing for the binding of deprotonated 13C-labeled methanol at the Mn4(IV) ion of the catalytic cluster in the S2 state. This has important implications on the mechanistic models for water oxidation in PSII.

Incretins in fibrocalculous pancreatic diabetes: A unique subtype of pancreatogenic diabetes.

BACKGROUND: Studies evaluating endocrine and exocrine functions in fibrocalculous pancreatic diabetes (FCPD) are scarce. METHODS: Insulin, C-peptide, glucagon, incretin hormones (glucagon-like peptide 1 [GLP-1] and gastric inhibitory peptide [GIP]), and dipeptidyl peptidase IV (DPP-IV) were estimated in patients with FCPD (n = 20), type 2 diabetes mellitus (T2DM) (n = 20), and controls (n = 20) in fasting and 60 minutes after 75 g glucose. RESULTS: Fasting and post-glucose C-peptide and insulin in FCPD were lower than that of T2DM and controls. Plasma glucagon decreased after glucose load in controls (3.72, 2.29), but increased in T2DM (4.01, 5.73), and remained unchanged in FCPD (3.44, 3.44). Active GLP-1 (pmol/L) after glucose load increased in FCPD (6.14 to 9.72, P = <.001), in T2DM (2.87 to 4.62, P < .001), and in controls (3.91 to 6.13, P < .001). Median active GLP-1 in FCPD, both in fasting and post-glucose state (6.14, 9.72), was twice that of T2DM (2.87, 4.62) and 1.5 times that of controls (3.91, 6.13) (P < .001 for all). Post-glucose GIP (pmol/L) increased in all: FCPD (15.83 to 94.14), T2DM (21.85 to 88.29), and control (13.00 to 74.65) (P < .001 for all). GIP was not different between groups. DPP-IV concentration (ng/mL) increased in controls (1578.54, 3012.00) and FCPD (1609.95, 1995.42), but not in T2DM (1204.50, 1939.50) (P = .131). DPP-IV between the three groups was not different. Fecal elastase was low in FCPD compared with T2DM controls. CONCLUSIONS: In FCPD, basal C-peptide and glucagon are low, and glucagon does not increase after glucose load. GLP-1, but not GIP, in FCPD increases 1.5 to 2 times as compared with T2DM and controls (fasting and post glucose) without differences in DPP-IV.

D1-S169A substitution of photosystem II reveals a novel S2-state structure.

In photosystem II (PSII), photosynthetic water oxidation occurs at the O2-evolving complex (OEC), a tetramanganese-calcium cluster that cycles through light-induced redox intermediates (S0-S4) to produce oxygen from two substrate water molecules. The OEC is surrounded by a hydrogen-bonded network of amino-acid residues that plays a crucial role in proton transfer and substrate water delivery. Previously, we found that D1-S169 was crucial for water oxidation and its mutation to alanine perturbed the hydrogen-bonding network. In this study, we demonstrate that the activation energy for the S2 to S1 transition of D1-S169A PSII is higher than wild-type PSII with a ~1.7-2.7× slower rate of charge recombination with QA- relative to wild-type PSII. Arrhenius analysis of the decay kinetics shows an Ea of 5.87 ± 1.15 kcal mol-1 for decay back to the S1 state, compared to 0.80 ± 0.13 kcal mol-1 for the wild-type S2 state. In addition, we find that ammonia does not affect the S2-state EPR signal, indicating that ammonia does not bind to the Mn cluster in D1-S169A PSII. Finally, a QM/MM analysis indicates that an additional water molecule binds to the Mn4 ion in place of an oxo ligand O5 in the S2 state of D1-S169A PSII. The altered S2 state of D1-S169A PSII provides insight into the S2➔S3 state transition.

Identification of a Na+-Binding Site near the Oxygen-Evolving Complex of Spinach Photosystem II.

The oxygen-evolving complex (OEC) of photosystem II (PSII) is an oxomanganese cluster composed of four redox-active Mn ions and one redox-inactive Ca2+ ion, with two nearby bound Cl- ions. Sodium is a common counterion of both chloride and hydroxide anions, and a sodium-specific binding site has not been identified near the OEC. Here, we find that the oxygen-evolution activity of spinach PSII increases with Na+ concentration, particularly at high pH. A Na+-specific binding site next to the OEC, becomes available after deprotonation of the D1-H337 amino acid residue, is suggested by the analysis of two recently published PSII cryo-electron microscopy maps in combination with quantum mechanical calculations and multiconformation continuum electrostatics simulations.

Prevalence of End-Organ Damage, Beta Cell Reserve, and Exocrine Pancreas Defect in Fibrocalculous Pancreatic Diabetes: An Eastern India Perspective.

BACKGROUND: Data on prevalence and burden of end-organ damage in fibrocalculous pancreatic diabetes (FCPD) from eastern India is scant. This study investigated the burden of end-organ damage and exocrine pancreatic defect in FCPD patients in Eastern India. MATERIALS AND METHODS: Consecutive FCPD patients underwent evaluation of glycemic control, C-peptide, fecal elastase, body fat percent, tests for cardiac autonomic neuropathy (CAN), neuropathy, nephropathy, and retinopathy which were compared with data from type-1 diabetes (T1DM) and type-2 diabetes (T2DM). RESULTS: Data from 101 FCPD, 41 T1DM, 40 T2DM, and 40 controls were analyzed. Body fat percent was lowest in FCPD and T1DM. Similarly, fasting and stimulated C-peptide was significantly lowest in T1DM, followed by FCPD. Significant elevations in stimulated C-peptide were observed in FCPD. Fecal elastase was lowest in FCPD. Exocrine pancreas defect in FCPD, T1DM, and T2DM was 100%, 53.66%, 27.5%, respectively. HbA1c was worst in FCPD. About 40% of FCPD patients had CAN while 13.33% had borderline CAN. Isolated parasympathetic dysfunction was the commonest (66.67%) among them. FCPD patients with CAN had lower fecal elastase, higher HbA1c, microalbuminuria, steatorrhea, neuropathy, retinopathy, and nephropathy, compared to those without CAN. On binary logistic regression, diabetes duration was a significant predictor of end-organ damage in FCPD. Fecal elastase and body fat percent were independent predictors for insulin therapy in FCPD. CONCLUSION: CAN is common in FCPD while exocrine pancreas defect is most severe in FCPD followed by T1DM and T2DM. Fecal elastase has an important prognostic role for insulinization in FCPD. Role of pancreatic enzyme replacement on glycemic control in diabetes with exocrine pancreas defect needs investigation.

Insights into Proton-Transfer Pathways during Water Oxidation in Photosystem II.

Water oxidation by photosystem II (PSII) involves the release of O2, electrons, and protons at the oxygen-evolving complex (OEC). These processes are facilitated by a hydrogen-bonded network of amino acid residues and waters surrounding the OEC. It is crucial to probe the proton-transfer pathways from the OEC as proton release helps to maintain the charge balance required for efficient water oxidation. In this study, we generate point mutations in the cyanobacterium Synechocystis sp. PCC 6803 at secondary-shell amino acid residues surrounding the OEC: D2-K317, D1-S169, CP43-R357, D1-D61, and D1-N181. We employ direct experimental methods to study the O2 evolution rate under varying pH ranging from 3-8. The pH dependence follows a bell-shaped curve in both wild-type and mutated PSII from which we can derive the effective acidic pKa. The effective acidic pKa provides insights into the protonation states of the amino acid residues participating in the proton-transfer process during the rate-determining step of water oxidation. The presence of an additional effective pKa in D1-S169A PSII and D2-K317A PSII indicates the possibility of multiple proton-transfer pathways during the rate-determining step of water oxidation. We also studied the O2 evolution rate in H2O and D2O with varying pL (L = H or D) to identify the amino acid residues participating in the proton-transfer process. We find that replacing the positively charged lysine with a neutral alanine in D2-K317A PSII and aspartate with alanine in D1-D61A PSII significantly enhances the kinetic solvent isotope effect (KSIE), indicating that proton transfer becomes rate-limiting at the optimal pH in these mutated PSII. However, the KSIE remains unchanged for D1-N181A, D1-S169A, and CP43-R357K PSII. Thus, perturbing the channel defined by the D1-D61 and D2-K317 residues strongly hampers the proton-transfer mechanism, and in turn, the water oxidation reaction of PSII. Hence, our study provides a direct experimental probe to identify that the D1-D61 and D2-K317 residues participate in the proton-transfer process. These results, thereby, provide us a deeper understanding of the proton-transfer processes in the water oxidation mechanism.

Bicarbonate rescues damaged proton-transfer pathway in photosystem II.

The membrane-protein complex photosystem II (PSII) catalyzes photosynthetic water oxidation. Proton transfer plays an integral role in the catalytic cycle of water oxidation by maintaining charge balance to regulate and ensure the efficiency of the process. The hydrogen-bonded amino-acid residues that surround the oxygen-evolving complex (OEC) provide an efficient pathway for proton removal. Hence, it is crucial to identify these pathways to provide deeper insights into the proton-transfer mechanisms. In this study, we have used bicarbonate as a mobile exogenous proton-transfer reagent to recover the activity lost by site-directed mutations in order to identify amino-acid residues participating in the proton-transfer pathway. We find that bicarbonate restores efficient S-state cycling in D2-K317A PSII core complexes, but not in D1-D61A and CP43-R357K PSII core complexes, indicating that bicarbonate chemical rescue can be used to differentiate single-point mutations affecting the pathways of proton transfer from mutations that affect other aspects of the water-oxidation mechanism.

D1-S169A Substitution of Photosystem II Perturbs Water Oxidation.

In photosystem II (PSII), photosynthetic water oxidation occurs at the tetramanganese-calcium cluster that cycles through light-induced intermediates (S0-S4) to produce oxygen from two substrate waters. The surrounding hydrogen-bonded amino acid residues and waters form channels that facilitate proton transfer and substrate water delivery, thereby ensuring efficient water oxidation. The residue D1-S169 lies in the "narrow" channel and forms hydrogen bonds with the Mn4CaO5 cluster via waters W1 and Wx. To probe the role of the narrow channel in substrate-water binding, we studied the D1-S169A mutation. PSII core complexes isolated from mutant cells exhibit inefficient S-state cycling and delayed oxygen evolution. The S2-state multiline EPR spectrum of D1-S169A PSII core complexes differed significantly from that of wild-type, and FTIR difference spectra showed that the mutation strongly perturbs the extensive network of hydrogen bonds that extends at least from D1-Y161 (YZ) to D1-D61. These results imply a possible role of D1-S169 in proton egress or substrate water delivery.

Evaluation of prognostic utility of Ki-67, P53, and O-6-methylguanine-DNA methyltransferase expression in pituitary tumors.

BACKGROUND OR CONTEXT: Pituitary adenoma (PA) is the most common pathology of the pituitary gland. Pituitary tumors were historically considered benign, however, from recent advances in pathological and molecular analyses, numerous prognostic markers have been identified, allowing a better characterization of tumor behavior and prediction of response to treatment and recurrences. AIMS AND OBJECTIVES: Evaluation of the epidemiological occurrence of pituitary tumors in our center and prediction of the benign, aggressive, or malignant nature of the tumor with the help of immunohistochemical markers (IHC) Ki-67, P53, and O-6-methylguanine-DNA methyltransferase (MGMT) along with radiology. MATERIALS AND METHODS: A prospective study was done in 33 cases. Patients with clinically suspected pituitary tumors and related symptoms and signs are referred from the endocrine outpatient department and subsequently operated at the neurosurgery department were selected. We have studied the clinical features, radiology, histopathology, and IHC with the help of Ki-67, P53, and MGMT of PA over 2 years. RESULTS: We have 94% (31/33) cases of PA among them 94% (29/31) cases are macroadenoma. The IHC was conducted on 30 cases (excluding 1 case of pituitary apoplexy) where Ki-67, p53, and MGMT have been used for IHC in order to analyze the prognosis of the PA, irrespective of the immunological subtype of the PA. In our study, only 13% (4 patients) had MGMT score 0 and 2 patients, among these 4 patients having above cutoff level of Ki-67 and p53 value, considered as aggressive (in case of Ki-67 >3% and >50% in case of p53). When comparing MGMT expression with recurrence, a high degree of significance was found (Mann-Whitney U-test, P = 0.0038). Most of the recurrent tumors (6/9) had MGMT score 1 or below and most of the nonrecurring tumor had MGMT score 2 or above. When comparing MGMT expression with aggressiveness, a high degree of significance was found (Mann-Whitney U-test, P < 0.0001). Finally, combining the radiological Ki-67, p53, and MGMT values, two cases of aggressive adenoma have been seen in our study, the remaining being benign adenomas (WHO classification 2004). We did not encounter any case of pituitary carcinoma. MGMT did not show any significant correlation with radiological grading and histology. CONCLUSION: The benign, aggressive, or malignant nature of PA can be effectively predicted with the help of IHC, such as Ki-67, p53, and MGMT. This helps in better patient management and predicts recurrences and prognosis.

Thyroid Status in Patients with Type 2 Diabetes Attending a Tertiary Care Hospital in Eastern India.

OBJECTIVE: Type 2 diabetes mellitus and thyroid dysfunction (TD) are two major public health endocrine problem, but the prevalence of TD and iodine status in patients with T2 DM in India is less studied. The study objective was to explore the prevalence of TD and to evaluate iodine health in type 2 diabetes patients attending a tertiary care center in Eastern India. METHODS: Consecutive 100 patients with diabetes attending outpatient department were evaluated clinically and biochemically (thyrotropin [TSH], free thyroxine, anti-TPO antibody, and urinary iodine). We excluded pregnant women or patients taking drugs that can alter thyroid function. Subclinical hypothyroid and overt hypothyroidism were diagnosed as per standard definitions. RESULTS: Out of 100 patients were analyzed, 51 (51%) were male. Mean (±standard deviation) age was 45.4 ± 11.2 years, body mass index 24.1 ± 4.28 kg/m2, and duration of diabetes 7.76 ± 5.77 years. The prevalence of subclinical hypothyroidism and overt hypothyroidism was 23/100 (23%) and 3/100 (3%), respectively. Thyroid autoantibody was positive in 13 (13.1%) patients. All patients were iodine sufficient. A trend toward increased neuropathy (r = 0.45) and nephropathy (r = -0.29) was associated with rising TSH. CONCLUSION: Almost one in four people living with diabetes are suffering from TD. Thus, routine screening should be implemented. Salt iodination program is a huge success in this part of the country.

Substitution of the D1-Asn87 site in photosystem II of cyanobacteria mimics the chloride-binding characteristics of spinach photosystem II.

Photoinduced water oxidation at the O2-evolving complex (OEC) of photosystem II (PSII) is a complex process involving a tetramanganese-calcium cluster that is surrounded by a hydrogen-bonded network of water molecules, chloride ions, and amino acid residues. Although the structure of the OEC has remained conserved over eons of evolution, significant differences in the chloride-binding characteristics exist between cyanobacteria and higher plants. An analysis of amino acid residues in and around the OEC has identified residue 87 in the D1 subunit as the only significant difference between PSII in cyanobacteria and higher plants. We substituted the D1-Asn87 residue in the cyanobacterium Synechocystis sp. PCC 6803 (wildtype) with alanine, present in higher plants, or with aspartic acid. We studied PSII core complexes purified from D1-N87A and D1-N87D variant strains to probe the function of the D1-Asn87 residue in the water-oxidation mechanism. EPR spectra of the S2 state and flash-induced FTIR spectra of both D1-N87A and D1-N87D PSII core complexes exhibited characteristics similar to those of wildtype Synechocystis PSII core complexes. However, flash-induced O2-evolution studies revealed a decreased cycling efficiency of the D1-N87D variant, whereas the cycling efficiency of the D1-N87A PSII variant was similar to that of wildtype PSII. Steady-state O2-evolution activity assays revealed that substitution of the D1 residue at position 87 with alanine perturbs the chloride-binding site in the proton-exit channel. These findings provide new insight into the role of the D1-Asn87 site in the water-oxidation mechanism and explain the difference in the chloride-binding properties of cyanobacterial and higher-plant PSII.