Biomedical subjects

Ibrahim Moustafa

Publications and source records attributed to Ibrahim Moustafa.

4 recordsLinked to original sources

PubMed2006

Atomic resolution crystallography reveals how changes in pH shape the protein microenvironment.

Hydrogen atoms are a vital component of enzyme structure and function. In recent years, atomic resolution crystallography (>or=1.2 A) has been successfully used to investigate the role of the hydrogen atom in enzymatic catalysis. Here, atomic resolution crystallography was used to study the effect of pH on cholesterol oxidase from Streptomyces sp., a flavoenzyme oxidoreductase. Crystallographic observations of the anionic oxidized flavin cofactor at basic pH are consistent with the UV-visible absorption profile of the enzyme and readily explain the reversible pH-dependent loss of oxidation activity. Furthermore, a hydrogen atom, positioned at an unusually short distance from the main chain carbonyl oxygen of Met122 at high pH, was observed, suggesting a previously unknown mechanism of cofactor stabilization. This study shows how a redox active site responds to changes in the enzyme's environment and how these changes are able to influence the mechanism of enzymatic catalysis.

Catalysis↗

PubMed2005

Evolution of living donor liver transplantation in Egypt.

OBJECTIVE: To date, cadaveric organ donation is illegal in Egypt. Therefore, Egypt recently introduced living donor liver transplantation (LDLT), aiming to save those who are suffering from end stage liver disease. Herein, we study the evolution of LDLT in Egypt. METHODS: In Egypt, between August 2001 and February 2004, we approached all centers performing LDLT through personal communication and sent a questionnaire to each center asking for limited information regarding their LDLT experience. RESULTS: We identified and approached 7 LDLT centers, which collectively performed a total of 130 LDLT procedures, however, 3 major centers performed most of the cases (91%). Overseas surgical teams, mainly from Japan, France, Korea, and Germany, either performed or supervised almost all procedures. Out of those 7 LDLT centers, 5 centers agreed to provide complete data on their patients including a total of 73 LDLT procedures. Out of those 73 recipients, 50 (68.5%) survived after a median follow-up period of 305 days (range 15-826 days). They reported single donor mortality. Hepatitis C virus cirrhosis, whether alone or mixed with schistosomiasis, was the main indication for LDLT. CONCLUSION: Egypt recently introduced LDLT with reasonable outcomes; yet, it carries considerable risks to healthy donors, it lacks cadaveric back up, and is not feasible for all patients. We hope that the initial success in LDLT will not deter the efforts to legalize cadaveric organ donation in Egypt.

Cultural Characteristics↗

PubMed2004

Sialic acid recognition by Vibrio cholerae neuraminidase.

Vibrio cholerae neuraminidase (VCNA) plays a significant role in the pathogenesis of cholera by removing sialic acid from higher order gangliosides to unmask GM1, the receptor for cholera toxin. We previously showed that the structure of VCNA is composed of a central beta-propeller catalytic domain flanked by two lectin-like domains; however the nature of the carbohydrates recognized by these lectin domains has remained unknown. We present here structures of the enzyme in complex with two substrates, alpha-2,3-sialyllactose and alpha-2,6-sialyllactose. Both substrate complexes reveal the alpha-anomer of N-acetylneuraminic acid (Neu5Ac) bound to the N-terminal lectin domain, thereby revealing the role of this domain. The large number of interactions suggest a relatively high binding affinity for sialic acid, which was confirmed by calorimetry, which gave a Kd approximately 30 microm. Saturation transfer difference NMR using a non-hydrolyzable substrate, Neu5,9Ac2-2-S-(alpha-2,6)-GlcNAcbeta1Me, was also used to map the ligand interactions at the VCNA lectin binding site. It is well known that VCNA can hydrolyze both alpha-2,3- and alpha-2,6-linked sialic acid substrates. In this study using alpha-2,3-sialyllactose co-crystallized with VCNA it was revealed that the inhibitor 2-deoxy-2,3-didehydro-N-acetylneuraminic acid (Neu5Ac2en) was bound at the catalytic site. This observation supports the notion that VCNA can produce its own inhibitor and has been further confirmed by 1H NMR analysis. The discovery of the sialic acid binding site in the N-lectin-like domain suggests that this might help target VCNA to sialic acid-rich environments, thereby enhancing the catalytic efficiency of the enzyme.

Binding Sites↗

PubMed2002

Probing the sialic acid binding site of the hemagglutinin-neuraminidase of Newcastle disease virus: identification of key amino acids involved in cell binding, catalysis, and fusion.

We recently reported the first crystal structure of a paramyxovirus hemagglutinin-neuraminidase (HN) from Newcastle disease virus. This multifunctional protein is responsible for binding to cellular sialyl-glycoconjugate receptors, promotion of fusion through interaction with the second viral surface fusion (F) glycoprotein, and processing progeny virions by removal of sialic acid from newly synthesized viral coat proteins. Our structural studies suggest that HN possesses a single sialic acid recognition site that can be switched between being a binding site and a catalytic site. Here we examine the effect of mutation of several conserved amino acids around the binding site on the hemagglutination, neuraminidase, and fusion functions of HN. Most mutations around the binding site result in loss of neuraminidase activity, whereas the effect on receptor binding is more variable. Residues E401, R416, and Y526 appear to be key for receptor binding. The increase in fusion promotion seen in some mutants that lack receptor binding activity presents a conundrum. We propose that in these cases HN may be switched into a fusion-promoting state through a series of conformational changes that propagate from the sialic acid binding site through to the HN dimer interface. These results further support the single-site model and suggest certain residues to be important for the triggering of fusion.

Amino Acids↗