A NOVEL APPROACH TO SOLID PHASE CHEMICAL SYNTHESIS OF OLIGONUCLEOTIDE mRNA CAP ANALOGS

A novel approach for the synthesis of 5′-capped 2′-O-methyloligoribonucleotides on a disulfide-tethered solid support is described. The key step of the synthesis is ZnCl₂ promoted coupling of m⁷GDP imidazolide to a fully deprotected oligonucleotide 5′-phosphate on-support. By this methodology m⁷G⁵′pppm²′Apm²′Upm^2′Ap has been prepared.     

Lack of an Efficient Endoplasmic Reticulum-localized Recycling System Protects Peroxiredoxin IV from Hyperoxidation

Typical 2-Cys peroxiredoxins are required to remove hydrogen peroxide from several different cellular compartments. Their activity can be regulated by hyperoxidation and consequent inactivation of the active-site peroxidatic cysteine. Here we developed a simple assay to quantify the hyperoxidation of peroxiredoxins. Hyperoxidation of peroxiredoxins can only occur efficiently in the presence of a recycling system, usually involving thioredoxin and thioredoxin reductase. We demonstrate that there is a marked difference in the sensitivity of the endoplasmic reticulum-localized peroxiredoxin to hyperoxidation compared with either the cytosolic or mitochondrial enzymes. Each enzyme is equally sensitive to hyperoxidation in the presence of a robust recycling system. Our results demonstrate that peroxiredoxin IV recycling in the endoplasmic reticulum is much less efficient than in the cytosol or mitochondria, leading to the protection of peroxiredoxin IV from hyperoxidation.     

Identification of thiol groups and a disulfide crosslink site in bovine myelin proteolipid protein

The existence of disulfide crosslinks limits the number of possible folded structures a protein can assume. Thus localization of disulfide and thiol groups is a key to understanding the conformation and orientation of myelin proteolipid protein (PLP) in the myelin membrane. [¹⁴C]Carboxamidomethylated PLP was fragmented with chymotrypsin, and the resulting mixture was partially separated by reversed-phase HPLC. Purified ¹⁴C-labeled peptides and a disulfide containing peptide were characterized by amino acid analysis. These experiments showed that Cys-32 and Cys-34 are free thiols, and are presumably on the interior of the cell or within the membrane bilayer, and that Cys-200 and Cys-219 are joined by a disulfide bond, and are probably located on the extracellular face of the membrane. Sequence analysis experiments indicate that Cys-5, Cys-6 and Cys-9 are linked by disulfides, probably to other parts of the protein on the extracellular face of the membrane.     

Helix packing of leucine-rich peptides: a parallel leucine ladder in the structure of Boc-Aib-Leu-Aib-Aib-Leu-Leu-Leu-Aib-Leu-Aib-OMe.

The packing of peptide helices in crystals of the leucine-rich decapeptide Boc-Aib-Leu-Aib-Aib-Leu-Leu-Leu-Aib-Leu-Aib-OMe provides an example of ladder-like leucylleucyl interactions between neighboring molecules. The peptide molecule forms a helix with five 5----1 hydrogen bonds and two 4----1 hydrogen bonds near the C terminus. Three head-to-tail NH ... O = C hydrogen bonds between helices form continuous columns of helices in the crystal. The helicial columns associate in an antiparallel fashion, except for the association of Leu ... Leu side chains, which occurs along the diagonal of the cell where the peptide helices are parallel. The peptide, with formula C56H102N10O13, crystallizes in space group P2(1)2(1)2(1) with Z = 4 and cell parameters a = 16.774(3) A, b = 20.032(3) A and c = 20.117(3) A; overall agreement factor R = 10.7% for 2014 data with magnitude of F(obs) greater than 3 sigma (F); resolution 1.0 A.     

Crystal structure of the Y52F/Y73F double mutant of phospholipase A2: increased hydrophobic interactions of the phenyl groups compensate for the disrupted hydrogen bonds of the tyrosines.

The enzyme phospholipase A2 (PLA2) catalyzes the hydrolysis of the sn-2 ester bond of membrane phospholipids. The highly conserved Tyr residues 52 and 73 in the enzyme form hydrogen bonds to the carboxylate group of the catalytic Asp-99. These hydrogen bonds were initially regarded as essential for the interfacial recognition and the stability of the overall catalytic network. The elimination of the hydrogen bonds involving the phenolic hydroxyl groups of the Tyr-52 and -73 by changing them to Phe lowered the stability but did not significantly affect the catalytic activity of the enzyme. The X-ray crystal structure of the double mutant Y52F/Y73F has been determined at 1.93 A resolution to study the effect of the mutation on the structure. The crystals are trigonal, space group P3(1)21, with cell parameters a = b = 46.3 A and c = 102.95 A. Intensity data were collected on a Siemens area detector, 8,024 reflections were unique with an R(sym) of 4.5% out of a total of 27,203. The structure was refined using all the unique reflections by XPLOR to a final R-factor of 18.6% for 955 protein atoms, 91 water molecules, and 1 calcium ion. The root mean square deviation for the alpha-carbon atoms between the double mutant and wild type was 0.56 A. The crystal structure revealed that four hydrogen bonds were lost in the catalytic network; three involving the tyrosines and one involving Pro-68. However, the hydrogen bonds of the catalytic triad, His-48, Asp-99, and the catalytic water, are retained. There is no additional solvent molecule at the active site to replace the missing hydroxyl groups; instead, the replacement of the phenolic OH groups by H atoms draws the Phe residues closer to the neighboring residues compared to wild type; Phe-52 moves toward His-48 and Asp-99 of the catalytic diad, and Phe-73 moves toward Met-8, both by about 0.5 A. The closing of the voids left by the OH groups increases the hydrophobic interactions compensating for the lost hydrogen bonds. The conservation of the triad hydrogen bonds and the stabilization of the active site by the increased hydrophobic interactions could explain why the double mutant has activity similar to wild type. The results indicate that the aspartyl carboxylate group of the catalytic triad can function alone without additional support from the hydrogen bonds of the two Tyr residues.     

Rat heart gap junctions as disulfide-bonded connexon multimers: Their depolymerization and solubilization in deoxycholate

Summary Unproteolyzed gap junctions isolated from rat heart and liver were analyzed for the presence of inter-subunit disulfide bonds by sodium dodecylsulfate polyacrylamide gel electrophoresis. Rat cardiac junctions contained multiple disulfide bonds connecting theM _r 47,000 subunits of the same connexon and of different connexons. Inter-subunit disulfide bonds were absent in liver junctions. Unproteolyzed rat heart gap junctions were resistant to deoxycholate in their oxidized state, but dissolved readily in the detergent when the disulfide bonds were cleaved with -mercaptoethanol. Disulfide bonding in proteolyzed cardiac junctions was limited to pairs ofM _r 29,500 subunits. These junctions were not soluble in deoxycholate even in the presence of -mercaptoethanol. These results show that heart and liver junctions differ in their quarternary organization.     

Location in Muscarinic Acetylcholine Receptors of Sites for [3H]Propylbenzilcholine Mustard Binding and for Phosphorylation with Protein Kinase C

Muscarinic acetylcholine receptors purified from porcine cerebra or atria were covalently labeled with [3H]propylbenzilylcholine mustard ([3H]PrBCM), and then the labeled receptors were subjected to limited hydrolysis with trypsin, V8 protease, and lysyl endopeptidase, followed by analysis involving sodium dodecyl sulfate-polyacrylamide gel electrophoresis, fluorography, autoradiography, or immunostaining. The labeled peptides were located on the basis of their reactivity with antibodies raised against three synthetic peptides with partial sequences of the m1 or m2 receptor, and of their sensitivity to endoglycosidase F, which was taken as evidence that they contain glycosylation sites near the N terminus. The [3H]PrBCM-binding site in both cerebral and atrial receptors was found to be located between the N terminus and the second intracellular loop, because the size of the smallest deglycosylated peptide that contained both the [3H]PrBCM-binding and glycosylation sites was approximately 16 kDa. Cerebral receptors were 32P-phosphorylated with protein kinase C, and the major phosphorylation sites in cerebral muscarinic receptors were found to be located in a C-terminal segment including a part of the third intracellular loop, because a 32P-labeled peptide of 12-14 kDa reacted with anti-(m1 C-terminal peptide) antiserum. The presence of an intramolecular disulfide bond, probably between Cys 98 and Cys 178 in the first and second extracellular loops, respectively, was suggested by the finding that a peptide of approximately 17 kDa containing the [3H]PrBCM-binding site, but not the glycosylation sites, was partly converted to a peptide of approximately 12 kDa on treatment with beta-mercaptoethanol.     

Haptoglobin in Carnivora: a unique molecular structure in bear, cat and dog haptoglobins

Haptoglobin (Hp), a hemoglobin-binding protein in plasma, consists of α and β subunits and has a tetra-chain arrangement (β-α-α-β) connected by disulfide bridges in most mammals so far examined. Dog Hp has been reported to be unique compared with other Hps in respect that (1) the two αβ units are joined by a non-covalent interaction rather than a disulfide bridge and (2) the α chain has an oligosaccharide-binding sequence (Asn-X-Ser/Thr) and is glycosylated. To determine whether the unique structures of dog Hp are common in the Carnivora, we purified Hps from sera of bear and cat, and analyzed their subunit structure and partial amino acid sequences. The analyses by gel filtration and sodium dodecyl sulfate-polyacrylamide gel electrophoresis, under both reducing and non-reducing conditions, revealed that bear and cat Hps have similar subunit arrangements to dog Hp, suggesting the absence of a disulfide bridge between two α chains. This was confirmed by amino acid sequence analysis of the α chains: that is, Cys₁₅ participating in the inter-α chain disulfide bridge was replaced by Val in bear or Leu in cat and dog. Thus, the unique subunit arrangement of Hp reported in dog may be common in the Carnivora. In contrast to dog Hp, however, α chains of bear and cat Hps were found not to have the typical oligosaccharide binding sequence on their α chains and were not glycosylated.     

Native disulfide bonds greatly accelerate secondary structure formation in the folding of lysozyme.

To assess the respective roles of local and long-range interactions during protein folding, the influence of the native disulfide bonds on the early formation of secondary structure was investigated using continuous-flow circular dichroism. Within the first 4 ms of folding, lysozyme with intact disulfide bonds already had a far-UV CD spectrum reflecting large amounts of secondary structure. Conversely, reduced lysozyme remained essentially unfolded at this early folding time. Thus, native disulfide bonds not only stabilize the cfinal conformation of lysozyme but also provide, in early folding intermediates, the necessary stabilization that favors the formation of secondary structure.     

Effect of disulfide bridge formation on the NMR spectrum of a protein: Studies on oxidized and reduced Escherichia coli thioredoxin

Summary As a prelude to complete structure calculations of both the oxidized and reduced forms of Escherchia coli thioredoxin (M_r 11 700), we have analyzed the NMR data obtained for the two proteins under identical conditions. The complete aliphatic ¹³C assignments for both oxidized and reduced thioredoxin are reported. Correlations previously noted between ¹³C chemical shifts and secondary structure are confirmed in this work, and significant differences are observed in the C and C shifts between cis- and trans-proline, consistent with previous work that identifies this as a simple and unambiguous method of identifying cis-proline residues in proteins. Reduction of the disulfide bond in the active-site Cys³²-Gly-Pro-Cys³⁵ sequence causes changes in the ¹H, ¹⁵N and ¹³C chemical shifts of residues close to the active site, some of them quite far distant in the amino acid sequence. Coupling constants, both backbone and side chain, show some differences between the two proteins, and the NOE connectivities and chemical shifts are consistent with small changes in the positions of several side chains, including the two tryptophan rings (Trp²⁸ and Trp³¹). These results show that, consistent with the biochemical behavior of thioredoxin, there are minimal differences in backbone configuration between the oxidized and reduced forms of the protein.