Biomedical subjects

S Elgavish

Publications and source records attributed to S Elgavish.

5 recordsLinked to original sources

PubMed2001

Chemical characteristics of dimer interfaces in the legume lectin family.

The Erythrina corallodendron lectin (EcorL) crystallizes in monoclinic and hexagonal crystal forms. Comparison of the newly determined hexagonal form (PDB code 1fyu) with the monoclinic form shows that the dimeric structure of EcorL reflects the inherent biological structure of the protein and is not an artifact of the crystal packing. To further understand the factors determining the dimerization modes of legume lectins, EcorL, concanavalin A (ConA), and Griffonia simplicifolia (GS4) were taken as representatives of the three unique dimers found in the family. Six virtual homodimers were generated. The hydropathy, amino acid composition, and solvation energy were calculated for all nine homodimers. Each of the three native dimers has a distinct chemical composition. EcorL has a dominant hydrophobic component, and ConA has a strong polar component, but in GS4 the three components contribute equally to the interface. This distribution pattern at the interface is unique to the native dimers and distinct from the partition observed in the virtual dimers. Amino acid composition of other members of the family that dimerize like EcorL or ConA maintain the same pattern of amino acids distribution observed in EcorL and ConA. However, lectins that dimerize like GS4 do not show a particularly distinct distribution. In all cases, the calculated solvation energy of the native dimer was lower than that of the virtual dimers, suggesting that the observed mode of dimerization is the most stable organization for the given sequence and tertiary structure. The dimerization type cannot be predicted by sequence analysis.

Amino Acids↗

PubMed1998

Structures of the Erythrina corallodendron lectin and of its complexes with mono- and disaccharides.

The structures of the Erythrina corallodendron lectin (EcorL) and of its complexes with galactose, N-acetylgalactosamine, lactose and N-acetyllactosamine were determined at a resolution of 1.9 to 1.95 A. The final R-values of the five models are in the range 0.169 to 0.181. The unusual, non-canonical, dimer interface of EcorL is made of beta-strands from the two monomers, which face one another in a "hand-shake" mode. The galactose molecule in the primary binding site is bound in an identical way in all four complexes. Features of the electrostatic potential of the galactose molecule match those of the potential in the combining site, thus probably pointing to the contribution of the electrostatic energy to determining the orientation of the ligand. No conformational change occurs in the protein upon binding the ligand. Subtle variations in the binding mode of the second monosaccharide (glucose in the complex with lactose and N-acetylglucosamine in the complex with N-acetyllactosamine) were observed. The mobility of Gln219 is lower in the complexes with the disaccharides than in the complexes with the monosaccharides, indicating further recruitment of this residue to ligand binding through more extensive hydrogen bonding in the former complexes. Water molecules that have been located in the combining sites of the five structures undergo rearrangement in response to binding of the different ligands. The new structural information is in qualitative agreement with thermodynamic data on the binding to EcorL.

Acetylgalactosamine↗

PubMed1997

Rescue of a mutant G-protein by substrate-assisted catalysis.

Signaling by guanine-nucleotide-binding proteins (G-proteins) occurs when they are charged with GTP, while hydrolysis of the bound nucleotide turns the signaling off. Despite a wealth of biochemical and structural information, the mechanism of GTP hydrolysis by G-proteins remains controversial. We have employed substrate-assisted catalysis as a novel approach to study catalysis by G-proteins. In these studies, we have used diaminobenzophenone-phosphonoamidate-GTP, a unique GTP analog bearing the functional groups that are missing in the GTPase-deficient [Leu227]G(s alpha) mutant. This mutant, found in various human tumors, fails to hydrolyze GTP for an extended period. In contrast, the GTP analog is hydrolyzed by this mutant and by the wild-type enzyme at the same rate. On the other hand, modification of G(s alpha) by cholera toxin, which catalyses ADP-ribosylation of Arg201 of G(s alpha), decreased the rates of hydrolysis of both GTP and its analog by 95%. These results attest to the specificity of the GTP analog as a unique substrate for the [Leu227]G(s alpha) mutant and to the essential role of Gln227 in GTP hydrolysis. Furthermore, the finding that the GTP analog was hydrolyzed at the same rate as GTP by the wild-type enzyme, favors a model in which formation of a pentavalent transition state intermediate, presumably stabilized by the catalytic glutamine, is not the rate-limiting step of the GTPase reaction.

Animals↗

PubMed1997

In vitro phosphorylation of acetylcholinesterase at non-consensus protein kinase A sites enhances the rate of acetylcholine hydrolysis.

Here, we report that the catalytic subunit of cAMP-dependent protein kinase (PKA) but not casein kinase II or protein kinase C phosphorylates recombinant human acetylcholinesterase (AChE) in vitro. This enhances acetylthiocholine hydrolysis up to 10-fold as compared to untreated AChE, while leaving unaffected the enzyme's affinity for this substrate and for various active and peripheral site inhibitors. Alkaline phosphatase treatment enhanced the electrophoretic migration, under denaturing conditions, of part of the AChE proteins isolated from various mammalian sources and raised the isoelectric point of some of the treated AChE molecules, indicating that part of the AChE molecules are also phosphorylated in vivo. Enhancement of acetylthiocholine hydrolysis also occurred with Torpedo AChE, which has no consensus motif for PKA phosphorylation. Further, mutating the single PKA site in human AChE (threonine-249) did not prevent this enhancement, suggesting that in both cases it was due to phosphorylation at non-consensus sites. In vivo suppression of the acetylcholine hydrolyzing activity of AChE and consequent impairment in cholinergic neurotransmission occur under exposure to both natural and pharmacological compounds, including organophosphate and carbamate insecticides and chemical warfare agents. Phosphorylation of AChE may possibly offer a rapid feedback mechanism that can compensate for impairments in cholinergic neurotransmission, modulating the hydrolytic activity of this enzyme and enabling acetylcholine hydrolysis to proceed under such challenges.

Acetylcholine↗

PubMed1997

Lectin-carbohydrate interactions: different folds, common recognition principles.

Lectins can be found in many organisms and are involved in a multitude of cellular processes that depend on specific recognition of complex carbohydrates. The stereochemical principles underlying the recognition process have been the subject of extensive biochemical and structural studies. When examined from the viewpoint of the bound sugar, the structural information accumulated so far on lectins and other proteins that are specific to galactose and glucose (or mannose), provides suggestive evidence for distinct ligand-dependent distribution of hydrogen-bond partners in the combining site.

Binding Sites↗