Albumin binding of photobilirubin II.

Photobilirubin II, a stereoisomer of bilirubin, binds to human serum albumin at a single binding site (K = 2.2 x 10(6)M-1), presumably the high-affinity bilirubin-binding site. Binding in the secondary (class II) binding sites is of minor importance. The results are discussed with respect to photometabolism of bilirubin and as a possible source of error in the determination of bilirubin unbound to albumin.     

Concerning the structure of apoE

Apolipoprotein E (apoE), first described in 1973, is a truly fascinating protein. While studies initially focused on its role in cholesterol and lipid metabolism, one apoE isoform (apoE4) is a major risk factor for development of late onset Alzheimer's disease. Yet the difference between apoE3, the common form, and apoE4 is a single amino acid of the 299 in this 34 kDa protein. Structure determination of the two domain full length apoE3 protein was only accomplished in 2011 and supports the notion that mutations in the N‐terminal domain can be propagated through the structure to the C‐terminal domain. Understanding the structural differences between apoE3 and apoE4 is critical for finding ways to modulate the deleterious effect of apoE4.     

Fine structure of the secretory and non-secretory ameloblasts in the frog. II. Fine structure of the non-secretory ameloblast.

The non-secretory ameloblasts present at the enamel-free surfaces of maxillary teeth in the frog Rana pipiens were examined by electron microscopy at different stages of tooth development. Their main fine structural features seem to reflect a transport function. During early tooth development, the non-secretory ameloblasts adjacent to odontoblasts and predentin exhibit extensive lateral surface specializations and numerous cytoplasmic vesicles. During late tooth development, the non-secretory ameloblasts adjacent to mineralizing dentin show numerous cellular junctions, well-developed intercellular channels with numerous interdigitating processes and labyrinthine configurations at their distal surfaces. An intact basal lamina is present between the non-secretory ameloblasts and the dentin surface until the dentin becomes fully mineralized. At this stage the adjacent cells no longer exhibit surface specializations. It is suggested that the non-secretory ameloblasts may participate in the mineralization of adjacent dentin at the enamel-free surfaces. This surface dentin becomes fully mineralized at a later stage of development than the underlying dentin.     

The structure of capsular polysaccharide of the Pneumococcus type II.

The pneumococcus type II capsular polysaccharide (SII) is composed of singly-branched hexasaccharide repeating units, for which three alternative structures have been proposed. The correct structure has now been determined by consecutive eliminations of the sugar residues in the side chain. The terminal D-glucuronic acid group was eliminated by treating the fully methylated and esterified SIIpolysaccharide first with base, and then with weak acid. The hydroxyl group at C-6 in the penultimate D-glucose residue released by this elimination was transformed into the 6-deoxy-6-tosyl derivative, and the residue thereafter eliminated by treatment with base. As the side-chains were eliminated by these reactions, it is considered that they contain only two sugar residues, which thus excludes two of the three alternative structures. Structure 1 was further confirmed by subjecting SII to a Smith degradation, which yielded the tetrasaccharide L-Rhap-(1 yields 3)-L-Rhap-(1 yields 3)-L-Rhap-(1 yields 2-erythritol, characterised by methylation analysis.     

Vibrational analysis of the structure of gramicidin A. II. Vibrational spectra.

Raman and infrared spectra have been obtained of gramicidin A (GA) in the crystalline state both in the native form and complexed with CsSCN and KSCN, in solution in dioxane, and incorporated into lipid vesicles. Based on predictions from normal mode calculations of a number of relevant single- and double-stranded beta-helix conformations (Naik and Krimm, 1986), it has been possible to assign the structures of GA that are present under the above conditions. In the crystalline state, native GA has a double-stranded increases decreases beta 5.6 structure, whereas complexes with CsSCN or KSCN adopt a increases decreases beta 7.2 structure. In dioxane solution, the increases decreases beta 5.6 structure predominates. In lipid vesicles, the single-stranded beta 6.3-helix is found, which converts to a double-stranded helix on drying the sample. These results support our previous studies in showing that normal mode analysis can be a powerful technique in obtaining three-dimensional structural information from vibrational spectra.     

Predicting the structure of the light-harvesting complex II of Rhodospirillum molischianum.

We attempted to predict through computer modeling the structure of the light-harvesting complex II (LH-II) of Rhodospirillum molischianum, before the impending publication of the structure of a homologous protein solved by means of X-ray diffraction. The protein studied is an integral membrane protein of 16 independent polypeptides, 8 alpha-apoproteins and 8 beta-apoproteins, which aggregate and bind to 24 bacteriochlorophyll-a's and 12 lycopenes. Available diffraction data of a crystal of the protein, which could not be phased due to a lack of heavy metal derivatives, served to test the predicted structure, guiding the search. In order to determine the secondary structure, hydropathy analysis was performed to identify the putative transmembrane segments and multiple sequence alignment propensity analyses were used to pinpoint the exact sites of the 20-residue-long transmembrane segment and the 4-residue-long terminal sequence at both ends, which were independently verified and improved by homology modeling. A consensus assignment for the secondary structure was derived from a combination of all the prediction methods used. Three-dimensional structures for the alpha- and the beta-apoprotein were built by comparative modeling. The resulting tertiary structures are combined, using X-PLOR, into an alpha beta dimer pair with bacteriochlorophyll-a's attached under constraints provided by site-directed mutagenesis and spectral data. The alpha beta dimer pairs were then aggregated into a quaternary structure through further molecular dynamics simulations and energy minimization. The structure of LH-II so determined is an octamer of alpha beta heterodimers forming a ring with a diameter of 70 A.     

On the computation of the tertiary structure of globular proteins II.

The semi-empirical approach proposed earlier (Y?as et al., 1978) to compute the tertiary structures of globular proteins is here amplified and developed further. Using, as input, information on sequence and certain averages of interatomic distances which can be semi-empirically estimated, structures have been computed for pancreatic trypsin inhibitor, lysozyme and staphylococcal nuclease which resemble those determined by X-ray diffraction methods. The approach used is compared and contrasted with others proposed recently.     

Metaphase chromosome structure. Involvement of topoisomerase II

SCI is a prominent, 170,000 Mr, non-histone protein of HeLa metaphase chromosomes. This protein binds DNA and was previously identified as one of the major structural components of the residual scaffold structure obtained by differential protein extraction from isolated chromosomes. The metaphase scaffold maintains chromosomal DNA in an organized, looped conformation. We have prepared a polyclonal antibody against the SC1 protein. Immunolocalization studies by both fluorescence and electron microscopy allowed identification of the scaffold structure in gently expanded chromosomes. The micrographs show an immunopositive reaction going through the kinetochore along a central, axial region that extends the length of each chromatid. Some micrographs of histone-depleted chromosomes provide evidence of the substructural organization of the scaffold; the scaffold appears to consist of an assembly of foci, which in places form a zig-zag or coiled arrangement. We present several lines of evidence that establish the identity of SC1 as topoisomerase II. Considering the enzymic nature of this protein, it is remarkable that it represents 1% to 2% of the total mitotic chromosomal protein. About 60% to 80% of topoisomerase II partitions into the scaffold structure as prepared from isolated chromosomes, and we find approximately three copies per average 70,000-base loop. This supports the proposed structural role of the scaffold in the organization of the mitotic chromosome. The dual enzymic and apparent structural function of topoisomerase II (SC1) and its location at or near the base of chromatin loops allows speculation as to its involvement in the long-range control of chromatin structure.