Biomedical subjects

C Eigenbrot

Publications and source records attributed to C Eigenbrot.

23 records · Page 2Linked to original sources

PubMed1991

X-ray structure of human relaxin at 1.5 A. Comparison to insulin and implications for receptor binding determinants.

The X-ray crystal structure of relaxin at 1.5 A resolution is reported for the physiologically active form of the human hormone. Relaxin is a small, two-chain polypeptide that is a member of the protein hormone family that also includes insulin and the insulin-like growth factors IGF-I and IGF-II. These hormones have biologically diverse activities but are structurally similar, sharing a distinctive pattern of cysteine and glycine residues. The predicted structural homology of relaxin to insulin is confirmed by this structural analysis; however, there are significant differences in the terminal regions of the b-chain. Although relaxin, like insulin, crystallizes as a dimer, the orientation of the molecules in the respective dimers is completely different. The region of the relaxin molecule proposed to be involved in receptor binding is part of the dimer interface, suggesting that some of the other residues contained in the dimer contact surface might be receptor binding determinants as well. The proposed receptor binding determinants for insulin likewise include residues at its dimer interface. However, because the dimer contacts of relaxin and insulin are quite different, it appears that these two structurally related hormones have evolved somewhat dissimilar mechanisms for receptor binding.

Amino Acid Sequence↗

PubMed1991

Neutron structure of subtilisin BPN': effects of chemical environment on hydrogen-bonding geometries and the pattern of hydrogen-deuterium exchange in secondary structure elements.

The neutron structure of subtilisin BPN' has been refined and analyzed at 2.0-A resolution. The structure studied was a mutant variant of subtilisin, Met222----Gln, and was used because large, uninhibited crystals could be grown, which was not the case for the native molecule. Comparison of the structure with that of the native molecule indicated that the two structures are essentially the same. Using the capability of the neutron method to locate hydrogen and deuterium atoms, the protonation states of the six histidine residues were assigned. The active site histidine, His64, was found to be neutral at the pH of the analysis (pH 6.1). This group has an unexpectedly low pKa compared to assignments made by other techniques. The altered pKa of the group could result from electrostatic effects of other molecules in the crystal lattice. The dihedral conformations of a majority of the hydroxyl rotors were assigned. The preferred orientation was trans (180 degrees) with the other two low-energy conformers (60 degrees, 300 degrees) about equally populated. For the serines, about 21% of the hydroxyls act exclusively as H-bond acceptors and 37% as H-bond donors, and in 42% the group functions as both. The experimentally observed dihedral conformations were compared to predicted conformations based on calculated energy criteria and showed a strong correspondence. Deviation from low-energy states could usually be explained by local electrostatic effects. The hydrogen exchange pattern of subtilisin identified the beta-sheet and alpha-helix secondary structure elements to be the most resistant to exchange. Fifty-five percent of the peptide amide hydrogens were fully exchanged, 15% unexchanged, and 30% partially exchanged. The largest concentration of unexchanged sites was in the seven-stranded parallel beta-sheet, in which there were 11 fully protected groups. Little correlation was found between H-bond length and angle and a peptide group's susceptibility toward exchange. Of the five alpha-helices the most protected from exchange is the one defined by residues 224-236. The pattern of exchange identifies regions in this helix where the H-bonding regularity is disrupted.

Binding Sites↗

PubMed1990

X-ray crystal structure of the protease inhibitor domain of Alzheimer's amyloid beta-protein precursor.

Alzheimer's amyloid beta-protein precursor contains a Kunitz protease inhibitor domain (APPI) potentially involved in proteolytic events leading to cerebral amyloid deposition. To facilitate the identification of the physiological target of the inhibitor, the crystal structure of APPI has been determined and refined to 1.5-A resolution. Sequences in the inhibitor-protease interface of the correct protease target will reflect the molecular details of the APPI structure. While the overall tertiary fold of APPI is very similar to that of the Kunitz inhibitor BPTI, a significant rearrangement occurs in the backbone conformation of one of the two protease binding loops. A number of Kunitz inhibitors have similar loop sequences, indicating the structural alteration is conserved and potentially an important determinant of inhibitor specificity. In a separate region of the protease binding loops, APPI side chains Met-17 and Phe-34 create an exposed hydrophobic surface in place of Arg-17 and Val-34 in BPTI. The restriction this change places on protease target sequences is seen when the structure of APPI is superimposed on BPTI complexed to serine proteases, where the hydrophobic surface of APPI faces a complementary group of nonpolar side chains on kallikrein A versus polar side chains on trypsin.

Alzheimer Disease↗

PubMed1990

Structural effects induced by removal of a disulfide-bridge: the X-ray structure of the C30A/C51A mutant of basic pancreatic trypsin inhibitor at 1.6 A.

The X-ray structure of a variant of basic pancreatic trypsin inhibitor (BPTI) has been analyzed to determine the structural accommodation resulting from removal of a disulfide cross-link in a protein. The disulfide removed, Cys30-Cys51, has been implicated in both the folding pathway of the protein and its overall thermal stability. In the variant studied, C30A/C51A, the disulfide cysteines were replaced by less bulky alanines. The atomic displacements observed for C30A/C51A indicate a set of concerted shifts of two segments of chains, which together significantly diminish a packing defect at the site of the removed cysteine sulfur atoms. The observed structural changes are distributed asymmetrically around the sites of mutation, indicating that the adjacent beta-sheet is more resistant to the perturbation than the alpha-helix on the opposite side of the disulfide bond. The thermal parameters of groups involved in the structural accommodation are not significantly altered. A comparison of the X-ray structures reported for native BPTI determined in three different crystal forms indicates that the magnitude of its conformational variability exceeds that of the structural changes caused by the disulfide removal. This emphasizes the necessity of using isomorphous crystal systems to determine the relatively small effects due to mutation.

Aprotinin↗

PubMed1987

Heteroatom-activated beta-lactam antibiotics: considerations of differences in the biological activity of [[3(S)-(acylamino)-2-oxo-1-azetidinyl]oxy]acetic acids (oxamazins) and the corresponding sulfur analogues (thiamazins).

The considerable antibacterial activity of [[3(S)-(acylamino)-2-oxo-1-azetidinyl]oxy]acetic acids (oxamazins) in contrast to the lack of activity of the corresponding sulfur analogues (thiamazins) is examined in terms of physicochemical parameters, including electronegativity, IR carbonyl stretching frequencies, base hydrolysis rates, and three-dimensional molecular geometries. An X-ray structure determination of a protected thiamazin together with molecular graphics and molecular orbital calculations on model structures reveals that thiamazins would not fit as well as oxamazins in the active site of target bacterial transpeptidases. As a result of thiamazins' long N-S and S-C bond lengths, the pharmacophoric beta-lactam ring and carboxylate functionality cannot adopt the spatial relationship they have in penicillins and cephalosporins. The beta-lactam nitrogen of the monocyclic, crystalline thiamazin is 0.18 A out of the plane of its three substituents, and this distance (h) is predicted by computational chemistry methods to be higher in oxamazins. The rates of beta-lactam ring opening of an oxamazin, thiamazin, and aztreonam are comparable, even though the pyramidal character and IR data both indicate the electronegative oxygen analogue has reduced amide resonance. MNDO, AM1, and MINDO/3 correctly give a twofold potential for rotation about the N-S bond in model sulfenamides, with barrier heights ranging up to 12 kcal/mol.

Acetates↗