Morphology, molecular and systematic analyses of Bychowskyella (Monogenea: Dactylogyridae) in siluriform fish from India.

Three species of genus Bychowskyella Akhmerov (1952), i.e. B. fossilisi Majumdar & Agarwal, 1989, B. tchangi Gusev, 1976 and B. wallagonia (Jain, 1959a) Gusev, 1961, were found to parasitize the gill filaments of siluriform fish in India. This redescription based on light microscopic examination of B. fossilisi, B. tchangi and B. wallagonia provides detailed taxonomic data for these species. We also amplified the 18S ribosomal RNA gene to evaluate the phylogenetic relationships of these three species. The morphological and genetic profiles confirmed the validation and taxonomical relationships of the above-mentioned species belonging to the genus Bychowskyella.

Morphology and molecular analysis of Mizelleus indicus Jain () and M. longicirrus (Tripathi, ) Venkatanarasaiah & Kulkarni 1981 (Monogenea, Dactylogyridae) from the freshwater shark Wallago attu in the Ganga River, India.

Species of the genus Mizelleus Jain (1957) have always been controversial regarding identification and validity. Members of this group of species differ from each other in the morphology of their hard parts, which can be misleading and subject to differing interpretation among scientists. Therefore, the main objective of present study was to identify Mizelleus worms by morphological methods and molecular analysis on the basis of 18S ribosomal DNA to clarify their phylogenetic status. In this study, specimens were isolated from the gill filaments of Wallago attu (Siluriformes) and studied morphologically. In accordance with morphological characters, the specimens were found to be Mizelleus indicus and Mizelleus longicirrus. Partial sequences of nuclear 18S rDNA of these two species were amplified. The results confirm the phylogenetic relationships and taxonomic validation of M. indicus and M. longicirrus in India.

Aspergillus fumigatus harbouring the sole Y121F mutation shows decreased susceptibility to voriconazole but maintained susceptibility to itraconazole and posaconazole.

OBJECTIVES: Voriconazole, itraconazole and posaconazole are members of the azole family and widely used for the treatment of aspergillosis. They act by inhibiting the activity of the fungal Cyp51A enzyme. The emergence of environmental azole-resistant Aspergillus fumigatus strains raises major concerns for human health. METHODS: Recently, a new cyp51A-mediated resistance mechanism (namely TR46/Y121F/T289A) was described in clinical samples and patient-frequented environmental sites. In an azole-naive patient, we isolated an A. fumigatus strain that was not susceptible to voriconazole but was susceptible to itraconazole and posaconazole. RESULTS: A molecular analysis indicated a single Y121F substitution without the TR46 or T289A alterations, which to our knowledge has never been reported. Structure modelling and molecular dynamics offered an explanation for the resistance profile consistent with the structural differences between the three azoles. CONCLUSIONS: Taken together, these observations suggest an original mechanism conferring resistance to azoles mediated by cyp51A of environmental origin. This uncommon susceptibility pattern might represent a 'missing link' between the wild-type A. fumigatus and the fully azole-resistant strain harbouring the TR46/Y121F/T289A mutations.

Determinants of rural women's participation in India's National Leprosy Eradication Programme.

A multistage representative random sample of women and men from each of the 3 states of Bihar, Uttar Pradesh and West Bengal, from the rural blocks where the Leprosy Mission Hospitals were located were selected during 2010 to identify relevant factors that are preventing active participation of women and suggest corrective steps. Adult men and women were interviewed in depth, using a detailed checklist by the first author. A total of 1239 respondents 634 women and 605 men, were interviewed, only 44 women (7%) claimed that they had earlier participated in leprosy work, about 92% of the women felt that they had the potential to take part in leprosy work, and 70% showed willingness to participate. Factors that would encourage and facilitate more women to participate in leprosy work, included financial support (32.8%), convincing the family to grant permission (88%), and delegating them to work in proximity to their residences (15%). Some women respondents (11.0%) felt that they would provide their services voluntarily for social good. Women suggested that work should be delegated as per their capabilities and skills, and they should be given proper orientation, training and guidance. Hardly 5% of ASHA's in the clusters examined participated in leprosy related work, which needs stringent steps to re-orient and encourage them to undertake leprosy related work. It is concluded that rural Indian women are keen to play an important role in the national leprosy eradication program, with minimal support from the government and nongovernmental agencies in a truly community-based approach. This will benefit vast numbers of leprosy affected women as well as others.

The structural parameters for antimicrobial activity, human epithelial cell cytotoxicity and killing mechanism of synthetic monomer and dimer analogues derived from hBD3 C-terminal region.

Understanding the molecular mechanisms of antimicrobial peptide-membrane interactions is crucial in predicting the design of useful synthetic antimicrobial peptide analogues. Defensins are small (3-5 kDa) cysteine-rich cationic proteins which constitute the front line of host innate immunity. In this study, a series of eight 10 AA C-terminal analogues of hBD3 [sequence: RGRKXXRRKK, X = W, F, Y, V, L, I, H, C(Acm); net charge = +7, coded as W2, F2, Y2, V2, L2, I2, H2, and C2] and covalent V2-dimer [(RGRKVVRR)(2)KK] (18 AA, net charge = +11) were synthesized using solid phase peptide synthesis (SPPS) in Fmoc chemistry. Wild-type hBD3 was used as a control in all analyses. W2, V2, and especially Y2 showed high activity selectively against Gram-negative bacteria Pseudomonas aeruginosa in the concentration range of 4.3-9.7 microM. The covalent dimeric form of V2-monomer, V2-dimer, showed increased antibacterial killing compared to the monomeric form, V2-monomer. Cytotoxicity assays on a human conjunctival epithelial cell line (IOBA-NHC cells) showed that no change in viable cell number 24 h after constant exposure to all the eight peptide analogues even at concentrations up to 200 microg/ml. Fluorescence correlation spectroscopy (FCS) was used to study the interaction of these peptides against POPC vesicles (neutral; mammalian cell membrane mimic) and POPG vesicles (negatively charged; bacterial cell membrane mimic). Using FCS, significant aggregation and some leakage of Rhodamine dye were observed with POPG with Y2, W2 and V2 at the concentration of 5-10 mmicroM and no significant aggregation or disruption of vesicles was observed for all peptide analogues tested against POPC. V2-dimer induced more leakage and aggregation than the monomeric form. Overall, V2-dimer is the most effective antimicrobial peptide, with aggregation of POPG vesicles observed at concentrations as low as 1 microM. The concentration of 5-10 microM for Y2 from FCS correlated with the concentration of 5 microM (6.25 microg/ml), at which Y2 showed a cooperative increase in the activity. This suggests a structural transition of Y2 in the 2.5-5 microM concentration range resulting in the correlated increased antimicrobial activity. These results and the FCS together with previous NMR and molecular dynamics (MD) suggested that the charge density-based binding affinity, stable covalent dimerization, the ability to dimerize or even oligomerize and adopt a well-defined structure are important physicochemical properties distinguishing more effective cationic antimicrobial peptides.

Public awareness on integration of leprosy services at primary health centres in Uttar Pradesh, India.

Leprosy services were integrated into the general health a decade ago but it seems that a majority of public are still ignorant of this development. Hence, a study was done in Uttar Pradesh, India to determine the awareness about integration and its relationships to various socio-demographic factors. A multistage representative random sample of 3000 persons was chosen in Faizabad district, selecting a sample of 3 villages each situated within 1 km, 1-3 km and beyond 3 km of a PHC. A systematic random sample of 10% of households was chosen from selected villages and an adult male and an adult female from each household interviewed by a qualified investigator. Data were computerized and cross- tabulated against distance from the PHC, sex, age, education and occupational status. Only 45.7% in Uttar Pradesh are aware of the availability of leprosy treatment facilities at PHC but most knew that MDT was free. A smaller proportion was also aware of other facilities such as ulcer dressing and treatment of complications. Family members and health workers and PHC were the main source of information. It is concluded that massive efforts are urgently needed to educate the rural public on integration.

To determine the correlation between echocardiographic diastolic parameters and invasively measured left ventricular end diastolic pressure in patients with heart failure with preserved ejection fraction- an observational, descriptive study. (CEAL-HFpEF study).

OBJECTIVES: Though invasive monitoring is the most accurate to estimate diastolic dysfunction but it has its own risk. The purpose of this study was to find out any standardized correlation between invasive and non -invasive parameters. METHODS: It is an observational, descriptive study comprising of a total of 500 patients. The primary objective of the study was to determine the correlation between echocardiographic diastolic parameters and invasively measured left ventricular end diastolic pressure (LVEDP). RESULTS: On studying correlation of different invasive and non-invasive data it was reported that there was a weak correlation between peak E velocity (r = 0.14, p = 0.631), Peak A velocity (r = 0.67, p = 0.59), IVRT (r = -0.35, p = 0.178), Mitral deceleration time (DT) (r = -0.06, p = 0.842), pulmonary venous peak systolic (r = -0.02, p = 0.966) and diastolic flows (r = 0.47, p = 0.201) to LVEDP. There was a good positive correlation between elevated LVEDP and difference in duration of pulmonary venous and mitral flow at atrial contraction (A-Ard) and E/Ea at all four longitudinal segments of the left ventricle. The sensitivity and specificity for detecting an elevated LVEDP of more than 12 mm Hg, using a cut off value of E/Ea< 8, were 89% and 90%.Lateral E/Ea ≥ 12, LAVI ≥34 mL/m2, and Ard-Ad > 30 msec have the greatest diagnostic value for diagnosing diastolic dysfunction in HFpEF patients. CONCLUSION: Lateral E/Ea ≥ 12, LAVI ≥34 mL/m2, and Ard-Ad > 30 msec have the greatest diagnostic value for diagnosing diastolic dysfunction in HFpEF patients and have good correlation with invasively measured LVEDP.

An echocardiographic evaluation to determine the immediate and short-term changes in biventricular systolic and diastolic functions after PDA device closure-an observational analytical prospective study (echo- PDA study).

OBJECTIVES: -This prospective study with a sizable cohort was undertaken to assess changes in left and right ventricle systolic and diastolic functions after percutaneous patent ductus arteriosus device closure with appropriate follow up evaluation. METHODS: - It is an observational analytical prospective study. Ninety-eight patients were recruited out of which sixty-eight patients underwent percutaneous PDA device closure and were taken for final analysis. The primary objective was to study the left and right ventricular systolic and diastolic functions pre- and post-procedure at 48 h with follow up analysis at six months. RESULTS: - The mean age of the patients was 7.88 ± 5.05 years with the female to male ratio was 3.85:1. Thirty-three (48.52%) of the patients had immediate post PDA device closure LV systolic dysfunction. It was more common in those having pre-procedure mean low LVEF and those having a significant reduction in mitral A velocity. It became normal at six months follow up. The study reported immediate decrease in mea/n LVEF from 63.55 ± 8.11% to 48.19 ± 7.9%. The changes in LVEDD, LVEF, LVFS and LVEDV were statistically significant (p < 0.0001). In diastolic functions, there were significant reductions in peak early and late diastolic velocities. There was no statistically significant difference in right chamber functional assessment. CONCLUSION: Asymptomatic LV systolic and diastolic dysfunction in immediate post PDA closure period is a common complication and reported in around 48.5% cases. It was more common in those having pre-procedure mean low LVEF and those having a significant reduction in mitral A velocity.

Hyperpigmentation as a cutaneous manifestation of fungal sepsis in neonates: Case series report.

With improved and prolonged survival of very and extremely low birth weight infants, invasive fungal infection has emerged as an important concern in the neonatal intensive care units. Candidiasis is the third leading cause of late onset sepsis in these neonates and is associated with 20-30% mortality. Extreme prematurity, central venous catheters, prolonged antibiotic exposure, parenteral nutrition are important risk factors. Various forms of cutaneous manifestations of candidiasis have been described ranging from local diaper dermatitis and oral thrush to widespread erosive and ulcerative lesions with extensive crusting in invasive fungal dermatitis. We report a series of four cases with cutaneous hyperpigmentation as manifestation of systemic candidiasis.

A comparative assessment of Dorsal radial artery access versus classical radial artery access for percutaneous coronary angiography-a randomized control trial (DORA trial).

OBJECTIVES: This is an open-label randomized control trial with a parallel assignment with single masking comparing patients undergoing coronary angiography via dorsal radial and classical radial access. METHODS: Study done at three tertiary cardiac care centers for two years. A total of 970 patients were finally recruited for the study. Patients were randomly selected for dorsal radial artery access Group A (485 patients) and classical radial artery access Group B (485 patients) without any bias for age & sex. RESULTS: On comparative assessment both techniques are found to be equal in terms of procedural success rate. While dorsal access was superior in terms of fewer incidences of forearm radial artery occlusion, radial artery spasm, less post-procedure persistence of pain, and hand clumsiness. In comparison to this, the number of puncture attempts and time to achieve post-procedure hemostasis is less in classical radial access. CONCLUSION: So both techniques have pros and coins and it is the discretion of interventionists to adopt which technique.

A novel approach for dimension reduction of microarray.

This paper proposes a new hybrid search technique for feature (gene) selection (FS) using Independent component analysis (ICA) and Artificial Bee Colony (ABC) called ICA+ABC, to select informative genes based on a Naïve Bayes (NB) algorithm. An important trait of this technique is the optimization of ICA feature vector using ABC. ICA+ABC is a hybrid search algorithm that combines the benefits of extraction approach, to reduce the size of data and wrapper approach, to optimize the reduced feature vectors. This hybrid search technique is facilitated by evaluating the performance of ICA+ABC on six standard gene expression datasets of classification. Extensive experiments were conducted to compare the performance of ICA+ABC with the results obtained from recently published Minimum Redundancy Maximum Relevance (mRMR) +ABC algorithm for NB classifier. Also to check the performance that how ICA+ABC works as feature selection with NB classifier, compared the combination of ICA with popular filter techniques and with other similar bio inspired algorithm such as Genetic Algorithm (GA) and Particle Swarm Optimization (PSO). The result shows that ICA+ABC has a significant ability to generate small subsets of genes from the ICA feature vector, that significantly improve the classification accuracy of NB classifier compared to other previously suggested methods.

A fuzzy based feature selection from independent component subspace for machine learning classification of microarray data.

Feature (gene) selection and classification of microarray data are the two most interesting machine learning challenges. In the present work two existing feature selection/extraction algorithms, namely independent component analysis (ICA) and fuzzy backward feature elimination (FBFE) are used which is a new combination of selection/extraction. The main objective of this paper is to select the independent components of the DNA microarray data using FBFE to improve the performance of support vector machine (SVM) and Naïve Bayes (NB) classifier, while making the computational expenses affordable. To show the validity of the proposed method, it is applied to reduce the number of genes for five DNA microarray datasets namely; colon cancer, acute leukemia, prostate cancer, lung cancer II, and high-grade glioma. Now these datasets are then classified using SVM and NB classifiers. Experimental results on these five microarray datasets demonstrate that gene selected by proposed approach, effectively improve the performance of SVM and NB classifiers in terms of classification accuracy. We compare our proposed method with principal component analysis (PCA) as a standard extraction algorithm and find that the proposed method can obtain better classification accuracy, using SVM and NB classifiers with a smaller number of selected genes than the PCA. The curve between the average error rate and number of genes with each dataset represents the selection of required number of genes for the highest accuracy with our proposed method for both the classifiers. ROC shows best subset of genes for both the classifier of different datasets with propose method.

The Parasol Protocol for computational mutagenesis.

To aid in the discovery and development of peptides and proteins as therapeutic agents, a virtual screen can be used to predict trends and direct workflow. We have developed the Parasol Protocol, a dynamic method implemented using the AMBER MD package, for computational site-directed mutagenesis. This tool can mutate between any pair of amino acids in a computationally expedient, automated manner. To demonstrate the potential of this methodology, we have employed the protocol to investigate a test case involving stapled peptides, and have demonstrated good agreement with experiment.

Domain motions in dihydrofolate reductase: a molecular dynamics study.

Molecular dynamics simulations have been carried out on the enzyme dihydrofolate reductase from Lactobacillus casei complexed with methotrexate, NADPH and 264 crystallographic water molecules. Analysis of correlations in atomic fluctuations reveal the presence of highly correlated motion (correlation coefficient > 0.6) in the region between residues 30 to 35 and 85 to 90 leading to the identification of two domains, an "adenosine-binding domain" and a "large domain", which rotate by 3 to 4 degrees with respect to each other. The strongest correlation (> 0.6) within the large domain involves a coupling between the motions of the "teen-loop", and the spatially contiguous loops linking beta 6-beta 7 and beta 7-beta 8. Moreover, there is a significant correlation (approximately 0.5) between the adenosine fragment of NADPH and the pteridine and p-aminobenzoyl fragments of methotrexate, which are separated by approximately 17 A, and is lost on removal of "rigid-body" motion from the original trajectory. This provides support for the idea that the relative motion of the two domains is a means by which the occupation of the binding site for the adenosine end of the coenzyme can affect methotrexate binding and vice versa. Quasiharmonic vibrational analysis of the trajectory reveals that the overall dynamics of the system are governed by domain motions whose contributions are dominant at low frequencies. In addition, different low-frequency modes are responsible for separately coupling the adenosine-binding site and parts of methotrexate.

Structure of a slow processing precursor penicillin acylase from Escherichia coli reveals the linker peptide blocking the active-site cleft.

Penicillin G acylase is a periplasmic protein, cytoplasmically expressed as a precursor polypeptide comprising a signal sequence, the A and B chains of the mature enzyme (209 and 557 residues respectively) joined by a spacer peptide of 54 amino acid residues. The wild-type AB heterodimer is produced by proteolytic removal of this spacer in the periplasm. The first step in processing is believed to be autocatalytic hydrolysis of the peptide bond between the C-terminal residue of the spacer and the active-site serine residue at the N terminus of the B chain. We have determined the crystal structure of a slowly processing precursor mutant (Thr263Gly) of penicillin G acylase from Escherichia coli, which reveals that the spacer peptide blocks the entrance to the active-site cleft consistent with an autocatalytic mechanism of maturation. In this mutant precursor there is, however, an unexpected cleavage at a site four residues from the active-site serine residue. Analyses of the stereochemistry of the 260-261 bond seen to be cleaved in this precursor structure and of the 263-264 peptide bond have suggested factors that may govern the autocatalytic mechanism.

The crystal structure of the L1 metallo-beta-lactamase from Stenotrophomonas maltophilia at 1.7 A resolution.

The structure of the L1 metallo-beta-lactamase from the opportunistic pathogen Stenotrophomonas maltophilia has been determined at 1.7 A resolution by the multiwavelength anomalous dispersion (MAD) approach exploiting both the intrinsic binuclear zinc centre and incorporated selenomethionine residues. L1 is unique amongst all known beta-lactamases in that it exists as a tetramer. The protein exhibits the alphabeta/betaalpha fold found only in the metallo-beta-lactamases and displays several unique features not previously observed in these enzymes. These include a disulphide bridge and two substantially elongated loops connected to the active site of the enzyme. Two closely spaced zinc ions are bound at the active site with tetrahedral (Zn1) and trigonal bipyramidal (Zn2) co-ordination, respectively; these are bridged by a water molecule which we propose acts as the nucleophile in the hydrolytic reaction. Ligation of the second zinc ion involves both residues and geometry which have not been previously observed in the metallo-beta-lactamases. Simulated binding of the substrates ampicillin, ceftazidime and imipenem suggests that the substrate is able to bind to the enzyme in a variety of different conformations whose common features are direct interactions of the beta-lactam carbonyl oxygen and nitrogen with the zinc ions and of the beta-lactam carboxylate with Ser187. We describe a catalytic mechanism whose principal features are a nucleophilic attack of the bridging water on the beta-lactam carbonyl carbon, electrostatic stabilisation of a negatively charged tetrahedral transition state and protonation of the beta-lactam nitrogen by a second water molecule co-ordinated by Zn2. Further, we propose that direct metal:substrate interactions provide a substantial contribution to substrate binding and that this may explain the lack of specificity which is a feature of this class of enzyme.

Crystal structure of GerE, the ultimate transcriptional regulator of spore formation in Bacillus subtilis.

The small, DNA-binding protein GerE regulates gene transcription in the terminally differentiated mother-cell compartment during late stages of sporulation in Bacillus subtilis. This versatile transcription factor shares sequence homology with the LuxR/FixJ/UhpA family of activators and modulates the expression of a number of genes, in particular those encoding the components of the coat that surrounds the mature spore. GerE orchestrates the final stages of coat deposition and maturation that lead to a spore with remarkable resistance properties but that must be responsive to low levels of germination signals. As this germination process is largely passive and can occur in the absence of de novo protein synthesis, the correct assembly of germination machinery, including germinant receptors and energy storage compounds, is crucial to the survival of the cell. The crystal structure of GerE has been solved at 2.05 A resolution using multi-wavelength anomalous dispersion techniques and reveals the nature of the GerE dimer. Each monomer comprises four alpha-helices, of which the central pair forms a helix-turn-helix DNA-binding motif. Implications for DNA-binding and the structural organisation of the LuxR/FixJ/UhpA family of transcription activator domains are discussed.

Understanding the mechanism of ice binding by type III antifreeze proteins.

Type III antifreeze proteins (AFPs) are present in the body fluids of some polar fishes where they inhibit ice growth at subzero temperatures. Previous studies of the structure of type III AFP by NMR and X-ray identified a remarkably flat surface on the protein containing amino acids that were demonstrated to be important for interaction with ice by mutational studies. It was proposed that this protein surface binds onto the (1 0 [\bar 1] 0) plane of ice with the key amino acids interacting directly with the water molecules in the ice crystal. Here, we show that the mechanism of type III AFP interaction with ice crystals is more complex than that proposed previously. We report a high-resolution X-ray structure of type III AFP refined at 1.15 A resolution with individual anisotropic temperature factors. We report the results of ice-etching experiments that show a broad surface coverage, suggesting that type III AFP binds to a set of planes that are parallel with or inclined at a small angle to the crystallographic c-axis of the ice crystal. Our modelling studies, performed with the refined structure, confirm that type III AFP can make energetically favourable interactions with several ice surfaces.

The crystal structure of the globular domain of sheep prion protein.

The prion protein PrP is a naturally occurring polypeptide that becomes transformed from a normal conformation to that of an aggregated form, characteristic of pathological states in fatal transmissible spongiform conditions such as Creutzfeld-Jacob Disease and Bovine Spongiform Encephalopathy. We report the crystal structure, at 2 A resolution, of residues 123-230 of the C-terminal globular domain of the ARQ allele of sheep prion protein (PrP). The asymmetric unit contains a single molecule whose secondary structure and overall organisation correspond to those structures of PrPs from various mammalian species determined by NMR. The globular domain shows a close association of helix-1, the C-terminal portion of helix-2 and the N-terminal portion of helix-3, bounded by the intramolecular disulphide bond, 179-214. The loop 164-177, between beta2 and helix-2 is relatively well structured compared to the human PrP NMR structure. Analysis of the sheep PrP structure identifies two possible loci for the initiation of beta-sheet mediated polymerisation. One of these comprises the beta-strand, residues 129-131 that forms an intra-molecular beta-sheet with residues 161-163. This strand is involved in lattice contacts about a crystal dyad to generate a four-stranded intermolecular beta-sheet between neighbouring molecules. The second locus involves the region 188-204, which modelling suggests is able to undergo a partial alpha-->beta switch within the monomer. These loci provide sites within the PrPc monomer that could readily give rise to early intermediate species on the pathway to the formation of aggregated PrPSc containing additional intermolecular beta-structure.

Characterization of the electrostatic perturbation of a catalytic site (Cys)-S-/(His)-Im+H ion-pair in one type of serine proteinase architecture by kinetic and computational studies on chemically mutated subtilisin variants.

We have used two structurally well-characterized serine proteinase variants, subtilisins Carlsberg and BPN', to produce (Cys)-S-/(His)-Im+H ion-pairs by chemical mutation in well defined, different, electrostatic microenvironments. These ion-pairs have been characterized by pH-dependent rapid reaction kinetics using, as reactivity probes, thiol-specific time dependent inhibitors, 2,2'-dipyridyl disulfide and 4,4'-dipyrimidyl disulfide, that differ in the protonation states of their leaving groups in acidic media, computer modelling and electrostatic potential calculations. Both ion-pairs possess nucleophilic character, identified by the striking rate maxima in their reactions with 2,2'-dipyridyl disulfide in acid media. In the Carlsberg enzyme, the (Cys220)-S-/(His63)-Im+H ion-pair is produced by protonic dissociation with pKa 4.1 and its reactivity is not perturbed by any detectable electrostatic influence other than the deprotonation of His63 (pKa 10.2). In the BPN' enzyme, the analogous, (Cys221)-S-/(His64)-Im+H ion-pair is produced by protonic dissociation with pKa 5.1 and its reactivity is affected by an ionization with pKa 3.5 in addition to the deprotonation of His64 (pKa > or = 10.35). It is a striking result that calculations using finite difference solutions of the Poisson-Boltzmann equation provide a value of the pKa difference between the two enzyme catalytic sites (0.97) in close agreement with the value (1.0) determined by reactivity probe kinetics when a protein dielectric constant of 2 is assumed and water molecules within 5 A of the catalytic site His residue are included. The pKa difference is calculated to be 0.84 when the water molecules are not included and a protein dielectric constant of 20 is assumed. The calculations also identify Glu156 in the BPN' enzyme (which is Ser in the Carlsberg enzyme) as the main individual source of the pKa shift. The additional kinetically influential pKa of 3.5 is assigned to Glu156 by examining the non-covalent interactions between the 2-pyridyl disulfide reactivity probe and the enzyme active centre region.