Salt bridge relay triggers defective LDL receptor binding by a mutant apolipoprotein

BACKGROUND

Apolipoprotein-E (apo-E), a 34kDa blood plasma protein, plays a key role in directing cholesterol transport via its interaction with the low density lipoprotein (LDL) receptor. The amino-terminal domain of apo-E forms an unusually elongated four-helix bundle arranged such that key basic residues involved in LDL receptor binding form a cluster at the end of one of the helices. A common apo-E variant, apo-E2, corresponding to the single-site substitution Arg158-->Cys, displays minimal LDL receptor binding and is associated with significant changes in plasma cholesterol levels and increased risk of coronary heart disease. Surprisingly, the site of mutation in this variant is physically well removed (> 12A) from the cluster of LDL receptor binding residues.

RESULTS

We now report the refined crystal structure of the amino-terminal domain of apo-E2, at a nominal resolution of 3.0A. This structure reveals significant conformational changes relative to the wild-type protein that may account for reduced LDL receptor binding. Removal of the Arg158 side chain directly disrupts a pair of salt bridges, causing a compensatory reorganization of salt bridge partners that dramatically alters the charge surface presented by apo-E to its receptor.

CONCLUSIONS

It is proposed that the observed reorganization of surface salt bridges is responsible for the decreased receptor binding by apo-E2. This reorganization, essentially functioning as a mutationally induced electrostatic switch to turn off receptor binding, represents a novel mechanism for the propagation of conformational changes over significant distances.

Structure of alpha-lytic protease complexed with its pro region

While the majority of proteins fold rapidly and spontaneously to their native states, the extracellular bacterial protease alpha-lytic protease (alphaLP) has a t(1/2) for folding of approximately 2,000 years, corresponding to a folding barrier of 30 kcal mol(-1). AlphaLP is synthesized as a pro-enzyme where its pro region (Pro) acts as a foldase to stabilize the transition state for the folding reaction. Pro also functions as a potent folding catalyst when supplied as a separate polypeptide chain, accelerating the rate of alphaLP folding by a factor of 3 x 10(9). In the absence of Pro, alphaLP folds only partially to a stable molten globule-like intermediate state. Addition of Pro to this intermediate leads to rapid formation of native alphaLP. Here we report the crystal structures of Pro and of the non-covalent inhibitory complex between Pro and native alphaLP. The C-shaped Pro surrounds the C-terminal beta-barrel domain of the folded protease, forming a large complementary interface. Regions of extensive hydration in the interface explain how Pro binds tightly to the native state, yet even more tightly to the folding transition state. Based on structural and functional data we propose that a specific structural element in alphaLP is largely responsible for the folding barrier and suggest how Pro can overcome this barrier.

Effect of preoperative supplementation with alpha-tocopherol and ascorbic acid on myocardial injury in patients undergoing cardiac operations

Augmentation of antioxidant defenses may help protect tissues against ischemia-reperfusion injury associated with operations involving cardiopulmonary bypass. In this study we examined the effect of pretreating patients with alpha-tocopherol (vitamin E) and ascorbic acid (vitamin C) or placebo on injury to the myocardium. Seventy-six subjects undergoing elective coronary artery bypass grafting participated in a prospective, double-blind, placebo-controlled randomized trial, receiving either placebo or both 750 IU dl-alpha-tocopherol per day for 7 to 10 days and 1 gm ascorbic acid 12 hours before the operation. Plasma alpha-tocopherol concentrations, raised fourfold by supplementation, fell by 70% after the operation in the supplemented group and to negligible levels in the placebo group. There were no significant differences between the groups with respect to release of creatine kinase MB isoenzyme over 72 hours, nor in the reduction of the myocardial perfusion defect determined by thallium 201 uptake. Electrocardiography provided no evidence of a benefit from antioxidant supplementation. Thus the supplementation regimen prevented the depletion of the primary lipid soluble antioxidant in plasma, but provided no measurable reduction in myocardial injury after the operation.

Effects of DNA binding and metal substitution on the dynamics of the GAL4 DNA-binding domain as studied by amide proton exchange

Backbone amide proton exchange rates in the DNA-binding domain of GAL4 have been determined using 1H-15N heteronuclear correlation NMR spectroscopy. Three forms of the protein were studied-the native Zn-containing protein, the Cd-substituted protein, and a Zn-GAL4/DNA complex. Exchange rates in the Zn-containing protein are significantly slower than in the Cd-substituted protein. This shows that Cd-substituted GAL4 is destabilized relative to the native Zn-containing protein. Upon DNA binding, global retardation of amide proton exchange with solvent was observed, indicating that internal fluctuations of the DNA-recognition module are significantly reduced by the presence of DNA. In all forms of the protein, the internal dyad symmetry of the DNA-recognition module of GAL4 is reflected by the backbone amide proton exchange rates.

Partitioning roles of side chains in affinity, orientation, and catalysis with structures for mutant complexes: asparagine-229 in thymidylate synthase

Thymidylate synthase (TS) methylates only dUMP, not dCMP. The crystal structure of TS.dCMP shows sCMP 4-NH2 excluded from the space between Asn-229 and His-199 by the hydrogen bonding and steric properties and Asn-229. Consequently, 6-C of dCMP is over 4 A from the active site sulfhydryl. The Asn-229 side chain is prevented from flipping 180 degrees to and orientation the could hydrogen bond to dCMP by a hydrogen bond network between conserved residues. Thus, the specific binding of dUMP by TS results from occlusion of competing substrates by steric and electronic effects of residues in the active site cavity. When Asn-229 is replaced by a cysteine, the Cys-229 S gamma rotates out of the active site, and the mutant enzyme binds both dCMP and dUMP tightly but does not methylate dCMP. Thus simply admitting dCMP into the dUMP binding site of TS is not sufficient for methylation of dCMP. Structures of nucleotide complexes of TS N229D provide a reasonable explanation for the preferential methylation of dCMP instead of dUMP by this mutant. In TS N229D.dCMP, Asp-229 forms hydrogen bonds to 3-N and 40NH2 of dCMP. Neither the Asp-229 carboxyl moiety nor ordered water appears to hydrogen bond to 4-O of dUMP. Hydrogen bonds to 4-O (or 4-NH2) have been proposed to stabilize reaction intermediates. If their absence in TS N229D.dUMP persists in the ternary complex, it could explain the 10(4)-fold decrease in kcat/Km for dUMP.

Recognition of DNA by GAL4 in solution: use of a monomeric protein-DNA complex for study by NMR

The complex of a monomer of GAL4 with DNA has been investigated by two-dimensional 1H nuclear magnetic resonance (NMR) spectroscopy. Previous X-ray analysis has revealed a structure in which a dimer of the N-terminal 65-residue fragment of GAL4 forms a complex, 27 kDa in molecular mass, with a 19 base pair full-binding-site DNA [Marmorstein, R., Carey, M., Ptashne, M., & Harrison, S. C. (1992) Nature 356, 408-414]. We have developed a smaller system, half in molecular mass, which is amenable for detailed analysis using NMR. Titration of a 10 base pair half-binding-site DNA with GAL4-(65) shows 1:1 binding, illustrating that one monomer of the protein binds in a specific manner to half-site DNA. The components of the protein-DNA complex are mainly in fast exchange on the NMR chemical shift time scale, with an equilibrium dissociation constant of 161 +/- 12 microM. With a basis of chemical shift data for free GAL4 protein and for the free half-site DNA, the fast exchange facilitates 1H resonance assignments in the complex since cross-peak positions can be examined at different protein:DNA ratios. Chemical shift changes in the DNA reveal the base pairs that are important for recognition by GAL4. Intermolecular NOE cross-peaks are also observed in spectra of the protein-DNA complex. Their identification places the C-terminal end of the first alpha-helix (residues 12-17) in a position such that the amino acids are able to read the DNA sequence in a manner entirely consistent with the X-ray structure of the related complex.(ABSTRACT TRUNCATED AT 250 WORDS)

Development of a sterilizing in-place application for a production machine using Vaporized Hydrogen Peroxide

The use of steam in sterilization processes is limited by the implementation of heat-sensitive components inside the machines to be sterilized. Alternative low-temperature sterilization methods need to be found and their suitability evaluated. Vaporized Hydrogen Peroxide (VHP) technology was adapted for a production machine consisting of highly sensitive pressure sensors and thermo-labile air tube systems. This new kind of "cold" surface sterilization, known from the Barrier Isolator Technology, is based on the controlled release of hydrogen peroxide vapour into sealed enclosures. A mobile VHP generator was used to generate the hydrogen peroxide vapour. The unit was combined with the air conduction system of the production machine. Terminal vacuum pumps were installed to distribute the gas within the production machine and for its elimination. In order to control the sterilization process, different physical process monitors were incorporated. The validation of the process was based on biological indicators (Geobacillus stearothermophilus). The Limited Spearman Karber Method (LSKM) was used to statistically evaluate the sterilization process. The results show that it is possible to sterilize surfaces in a complex tube system with the use of gaseous hydrogen peroxide. A total microbial reduction of 6 log units was reached.