Isha Raj

Virtual Screening, Identification and In Vitro Testing of Novel Inhibitors of O-Acetyl-L-Serine Sulfhydrylase of Entamoeba histolytica

The explosive epidemicity of amoebiasis caused by the facultative gastrointestinal protozoan parasite Entamoeba histolytica is a major public health problem in developing countries. Multidrug resistance and side effects of various available antiamoebic drugs necessitate the design of novel antiamobeic agents. The cysteine biosynthetic pathway is the critical target for drug design due to its significance in the growth, survival and other cellular activities of E. histolytica. Here, we have screened 0.15 million natural compounds from the ZINC database against the active site of the EhOASS enzyme (PDB ID. 3BM5, 2PQM), whose structure we previously determined to 2.4 Å and 1.86 Å resolution. For this purpose, the incremental construction algorithm of GLIDE and the genetic algorithm of GOLD were used. We analyzed docking results for top ranking compounds using a consensus scoring function of X-Score to calculate the binding affinity and using ligplot to measure protein-ligand interactions. Fifteen compounds that possess good inhibitory activity against EhOASS active site were identified that may act as potential high affinity inhibitors. In vitro screening of a few commercially available compounds established their biological activity. The first ranked compound ZINC08931589 had a binding affinity of ∼8.05 µM and inhibited about 73% activity at 0.1 mM concentration, indicating good correlation between in silico prediction and in vitro inhibition studies. This compound is thus a good starting point for further development of strong inhibitors.

Structural and biochemical studies of serine acetyltransferase reveal why the parasite Entamoeba histolytica cannot form a cysteine synthase complex.

Cysteine (Cys) plays a major role in growth and survival of the human parasite Entamoeba histolytica. We report here the crystal structure of serine acetyltransferase (SAT) isoform 1, a cysteine biosynthetic pathway enzyme from E. histolytica (EhSAT1) at 1.77 Å, in complex with its substrate serine (Ser) at 1.59 Å and inhibitor Cys at 1.78 Å resolution. EhSAT1 exists as a trimer both in solution as well as in crystal structure, unlike hexamers formed by other known SATs. The difference in oligomeric state is due to the N-terminal region of the EhSAT1, which has very low sequence similarity to known structures, also differs in orientation and charge distribution. The Ser and Cys bind to the same site, confirming that Cys is a competitive inhibitor of Ser. The disordered C-terminal region and the loop near the active site are responsible for solvent-accessible acetyl-CoA binding site and, thus, lose inhibition to acetyl-CoA by the feedback inhibitor Cys. Docking and fluorescence studies show that EhSAT1 C-terminal-mimicking peptides can bind to O-acetyl serine sulfhydrylase (EhOASS), whereas native C-terminal peptide does not show any binding. To test further, C-terminal end of EhSAT1 was mutated and found that it inhibits EhOASS, confirming modified EhSAT1 can bind to EhOASS. The apparent inability of EhSAT1 to form a hexamer and differences in the C-terminal region are likely to be the major reasons for the lack of formation of the large cysteine synthase complex and loss of a complex regulatory mechanism in E. histolytica.

The narrow active-site cleft of O-acetylserine sulfhydrylase from Leishmania donovani allows complex formation with serine acetyltransferases with a range of C-terminal sequences

Cysteine is a crucial substrate for the synthesis of glutathione and trypanothione, which in turn maintain intracellular redox homeostasis and defend against oxidative stress in the pathogen Leishmania donovani. Here, the identification, sequencing, characterization and crystal structure at 1.79 Å resolution of O-acetylserine sulfhydrylase (OASS), a cysteine-biosynthetic pathway enzyme from L. donovani (LdOASS), are reported. It shows binding to the serine acetyltransferase (SAT) C-terminal peptide, indicating that OASS and SAT interact with each other to form a cysteine synthase complex, further confirmed by the structure of LdOASS in complex with SAT C-terminal octapeptide at 1.68 Å resolution. Docking and fluorescence binding studies show that almost all SAT C-terminus mimicking tetrapeptides can bind to LdOASS. Some peptides had a higher binding affinity than the native peptide, indicating that SAT-OASS interactions are not sequence-specific. The structure of LdOASS with a designed peptide (DWSI) revealed that LdOASS makes more interactions with the designed peptide than with the native peptide. In almost all known SAT-OASS interactions the SAT C-terminal sequence was shown to contain amino acids with large side chains. Structural comparison with other OASSs revealed that LdOASS has a relatively less open active-site cleft, which may be responsible for its interaction with the smaller-amino-acid-containing C-terminal LdSAT peptide. Biochemical studies confirmed that LdOASS interacts with SATs from Entamoeba histolytica and Brucella abortus, further displaying its sequence-independent and versatile mode of interaction with SATs. This implicates a critical role of the size of the active-site cleft opening in OASS for SAT-OASS interaction and thus cysteine synthase complex formation.

Crystal Structure and Interaction of Phycocyanin with β-Secretase: A Putative Therapy for Alzheimer's Disease

Alzheimers disease (AD) represents a neurological disorder, which is caused by enzymatic degradation of an amyloid precursor protein into short peptide fragments that undergo association to form insoluble plaques. Preliminary studies suggest that cyanobacterial extracts, especially the light-harvesting protein phycocyanin, may provide a means to control the progression of the disease. However, the molecular mechanism of disease control remains elusive. In the present study, intact hexameric phycocyanin was isolated and crystallized from the cyanobacterium Leptolyngbya sp. N62DM, and the structure was solved to a resolution of 2.6 A. Molecular docking studies show that the phycocyanin αβ-dimer interacts with the enzyme β-secretase, which catalyzes the proteolysis of the amyloid precursor protein to form plaques. The molecular docking studies suggest that the interaction between phycocyanin and β-secretase is energetically more favorable than previously reported inhibitor-β-secretase interactions. Transgenic Caenorhabditis elegans worms, with a genotype to serve as an AD-model, were significantly protected by phycocyanin. Therefore, the present study provides a novel structure-based molecular mechanism of phycocyanin-mediated therapy against AD.

Interaction analysis identifies semenogelin I fragments as new binding partners of PIP in human seminal plasma.

Identification of protein-protein interactions is vital for complete understanding of a biological process and for functional characterization of a protein in related biochemical pathways. In this study, we performed analysis of prolactin inducible protein (PIP) interactions in human seminal plasma. PIP and its interacting partners were co-immunoprecipitated, analyzed by SDS-PAGE and identified by MALDI-TOF mass spectrometry. Three major interacting partners were identified, viz. human serum albumin, zinc-α-2 glycoprotein and semenogelin I fragments. This is the first report of interaction between PIP and semenogelin I fragments in human seminal plasma or elsewhere with a suggestive role in reproductive physiology which might be helpful for spermatozoa to acquire their motility.

Molecular basis of ligand recognition by OASS from E. histolytica: insights from structural and molecular dynamics simulation studies

BACKGROUND O-acetyl serine sulfhydrylase (OASS) is a pyridoxal phosphate (PLP) dependent enzyme catalyzing the last step of the cysteine biosynthetic pathway. Here we analyze and investigate the factors responsible for recognition and different conformational changes accompanying the binding of various ligands to OASS. METHODS X ray crystallography was used to determine the structures of OASS from Entamoeba histolytica in complex with methionine (substrate analog), isoleucine (inhibitor) and an inhibitory tetra-peptide to 2.00Å, 2.03Å and 1.87Å resolutions, respectively. Molecular dynamics simulations were used to investigate the reasons responsible for the extent of domain movement and cleft closure of the enzyme in presence of different ligands. RESULTS Here we report for the first time an OASS-methionine structure with an unmutated catalytic lysine at the active site. This is also the first OASS structure with a closed active site lacking external aldimine formation. The OASS-isoleucine structure shows the active site cleft in open state. Molecular dynamics studies indicate that cofactor PLP, N88 and G192 form a triad of energy contributors to close the active site upon ligand binding and orientation of the Schiff base forming nitrogen of the ligand is critical for this interaction. CONCLUSIONS Methionine proves to be a better binder to OASS than isoleucine. The β branching of isoleucine does not allow it to reorient itself in suitable conformation near PLP to cause active site closure. GENERAL SIGNIFICANCE Our findings have important implications in designing better inhibitors against OASS across all pathogenic microbial species.

Isolation and identification of Concanavalin A binding glycoproteins from human seminal plasma: A step towards identification of male infertility marker proteins

Human seminal plasma contains a large array of proteins of clinical importance which are essentially needed to maintain the reproductive physiology of spermatozoa and for successful fertilization. Thus, isolation and identification of seminal plasma proteins is of paramount significance for their biophysical characterization and functional analysis in reproductive physiological processes. In this study, we have isolated Concanavalin-A binding glycoproteins from human seminal plasma and subsequently identified them by MALDI-TOF/MS analysis. The major proteins, as identified in this study, are Aminopeptidase N, lactoferrin, prostatic acid phosphatase, zinc-alpha-2-glycoprotein, prostate specific antigen, progestagen-associated endometrial protein, Izumo sperm-egg fusion protein and prolactin inducible protein. This paper also reports preliminary studies to identify altered expression of these proteins in oligospermia and azoospermia in comparison to normospermia. In oligospermia, five proteins were found to be downregulated while in azoospermia, four proteins were downregulated and two proteins were upregulated. Thus, this study is of immense biomedical interest towards identification of potential male infertility marker proteins in seminal plasma.

Crystal structures and kinetics of Type III 3-phosphoglycerate dehydrogenase reveal catalysis by lysine.

d‐Phosphoglycerate dehydrogenase (PGDH) catalyzes the first committed step of the phosphorylated serine biosynthesis pathway. Here, we report for the first time, the crystal structures of Type IIIK PGDH from Entamoeba histolytica in the apo form, as well as in complexes with substrate (3‐phosphoglyceric acid) and cofactor (NAD+) to 2.45, 1.8 and 2.2 Å resolution, respectively. Comparison of the apo structure with the substrate‐bound structure shows that the substrate‐binding domain is rotated by ~ 20° to close the active‐site cleft. The cofactor‐bound structure also shows a closed‐cleft conformation, in which NAD+ is bound to the nucleotide‐binding domain and a formate ion occupies the substrate‐binding site. Superposition of the substrate‐ and cofactor‐bound structures represents a snapshot of the enzyme in the active form, where C2 of the substrate and C4N of the cofactor are 2.2 Å apart, and the amino group of Lys263 is close enough to the substrate to remove the proton from the hydroxyl group of PGA, indicating the role of Lys in the catalysis. Mutation of Lys263 to Ala yields just 0.8% of the specific activity of the wild‐type enzyme, revealing that Lys263 indeed plays an integral role in the catalytic activity. The detectable activity of the mutant, however, indicates that after 20° rotation of the substrate‐binding domain, the resulting positions of the substrate and cofactor are sufficiently close to make a productive reaction.

Structure-based mutational studies of O-Acetylserine Sulfhydrylase reveal the reason for the loss of cysteine synthase complex formation in Brucella abortus

Cysteine biosynthesis takes place via a two-step pathway in bacteria, fungi, plants and protozoan parasites, but not in humans, and hence, the machinery of cysteine biosynthesis is an opportune target for therapeutics. The decameric cysteine synthase complex (CSC) is formed when the C-terminal tail of serine acetyltransferase (SAT) binds in the active site of O-acetylserine sulfydrylase (OASS), playing a role in the regulation of this pathway. Here, we show that OASS from Brucella abortus (BaOASS) does not interact with its cognate SAT C-terminal tail. Crystal structures of native BaOASS showed that residues Gln96 and Tyr125 occupy the active-site pocket and interfere with the entry of the SAT C-terminal tail. The BaOASS (Q96A-Y125A) mutant showed relatively strong binding (Kd = 32.4 μM) to BaSAT C-terminal peptides in comparison with native BaOASS. The mutant structure looks similar except that the active-site pocket has enough space to bind the SAT C-terminal end. Surface plasmon resonance results showed a relatively strong (7.3 μM Kd) interaction between BaSAT and the BaOASS (Q96A-Y125A), but no interaction with native BaOASS. Taken together, our observations suggest that the CSC does not form in B. abortus.

Structural Biology of Cysteine Biosynthetic Pathway Enzymes

The cysteine biosynthetic pathway is of central importance for the growth, survival, and pathogenicity of the anaerobic protozoan parasite Entamoeba histolytica. This pathway is present across all species but is absent in mammals. Cysteine, the product of this pathway, is the only antioxidative thiol responsible for fighting oxidative stress in E. histolytica. Serine acetyl transferase (SAT) and O-acetyl serine sulfhydrylase (OASS) are the two enzymes catalyzing the de novo cysteine biosynthetic pathway. In all organisms in which so far this pathway is known to exist, both these enzymes associate to form a regulatory complex, but in E. histolytica this complex is not formed. The cysteine biosynthetic pathway has been optimized in this organism to adapt to and fulfill its cysteine requirements. Here we describe recent studies of the structure, function, and complex formation of cysteine biosynthetic enzymes in E. histolytica. The findings reveal subtle modifications that lend both cysteine biosynthetic enzymes their unique characteristics to escape inhibitory regulation; allowing E. histolytica to maintain high levels of cysteine at all times.