Central strand

The four central strands of the p sheet are parallel and have the strand order 8 5 3 4 (see Figure 4.19). The strand order is thus reversed once, and there is a switch point in the middle of this p sheet between p strands 5 and 3 where we would expect the active site to be located. 

Fig. 15. Cartoon representation of the cross-/ spine model of HET-s fibrils, based on Fig. 2 of Ritter et al. (2005). The stretch of residues 218-289 is shown as the four central //-strands, //1-//4, plus the connecting loops. N- and C-termini are indicated.

Figure 14. Motifs of double crossover molecules. The top row contains the three parallel isomers of double crossover (DX) molecules, DPE, DPOW and DPON P in their name indicates their parallel structure. Arrowheads indicate 3 ends of strands. Strands drawn with the same thickness are related by the vertical dyad axis indicated in the plane of the paper. DPE contains crossovers separated by an even number (two) of half-turns of DNA, DPOx by an odd number in DPOW, the extra half turn is a major groove spacing, in DPON, it is a minor groove spacing. The middle row illustrates two other DX isomers, DAE, and DAO. The symmetry axis of DAE is normal to the page (and broken by the nick in the central strand) the symmetry axis of DAO is horizontal within the page in DAO, strands of opposite thickness are related by symmetry. DAE+J, in the second row, is a DAE molecule, in which an extra junction replaces the nick shown in DAE.

Figure 26-13 Synaptonemal complexes. (A) Aligned pairs of homologous chromatids lying 0.4 pm apart in Allium cepa. Arrows indicate "recombination nodules" which may be involved in initiating formation of crossovers. Portions of meiotic chromosomes of lily are shown at successive stages (B) Pachytene. (C) Portion of diplotene nucleus. (D) A bivalent at diplo-tene. (E) Two bivalents at diakinesis. Pairs of sister chromatids are coiled with appropriate handedness. (F) Sister chromatid cores are far apart in preparation for separation. A chiasma is present between the two central strands. (B) through (F) courtesy of Stephen Stack.279,279d (G) Pair of sister chromatids coiled with opposite handedness at metaphase. These are immun-ostained with anti-topoisomerase II antibodies. From Boy de la Tour and Laemmli.280 Courtesy of U. K. Laemmli.

We have already dealt with some general aspects of biochemical self-assembly in Section 2.10 including the remarkable formation of viral capsids. There are some biochemical examples, however, that translate readily into supramolecular chemical concepts and have been pivotal in defining the field. One such system is the tobacco mosaic virus, a virus that is very harmful to a variety of crops including tobacco, tomato, pepper, cucumbers and species such as ornamental flowers. This system consists of a helical virus particle measuring some 300 X 18 nm (Figure 10.6). A central strand of RNA is sheathed by 2130 identical protein subunits, each of which contains 158 amino acids. What is remarkable about... 

Although successful attempts to develop templates for fj-sheet structures are rare (see also Chapter 2.4), Kemp and coworkers introduced an epindolidione-based template that mimics the central strand of a fj-sheet by appropriately orienting three hydrogen bonds to enforce an extended conformation on the attached peptide chains, as shown in Figure 1.2.2 with the model system III [6], Direct attachment of peptide chains to the template leads to the formation of a parallel fj-sheet mimic, whereas antiparallel fj-sheet models can be obtained by incorporation of two urea groups for attachment of the peptide chains [7]. 

A high-resolution structure has been determined for the BI IgG-binding domain of protein G. The structure comprises of four stranded /3-sheet made up of two antiparallel j8-hairpins connected by an a-helix. The two central strands of the sheet are parallel and comprise the N- and C-terminal residues. Comparison of the protein A and protein G IgG-binding domain architectures reveals no immediately obvious region that could take the place of the two interacting helices of protein A and protein G complex. 

Polysaccharides are classified on the basis of the units that make up the central strand. In the first group are the galactans, whose core structures consist of 1 —> 3 and 1 6 linked galactopyranose units. Included... 

The central strand of the triplex must be purine rich since a pyrimidine does not have two hydrogen bonding surfaces with more than one hydrogen bond. Thus triple-stranded DNA requires a homopurine homopyrimidine region of DNA. If the third strand is purine rich, it forms reverse Hoogsteen hydrogen bonds in an antiparallel orientation with the purine strand of the Watson-Crick helix. If the third strand is pyrimidine rich, it forms Hoogsteen bonds in a parallel orientation with the Watson-Crick paired purine strand. 

Central strand cells (in cross sections without central cavities) present state 1 occasionally present state 2 absent state 3. 

Indeed a photoreactive triple-stranded ladder, [Pb3(bpe)3(02C-CF3)4(02CCH3)2] (26) containing well-aligned three bpe spacer ligands has been investigated. In this compound, Pb(II) in the central strand is bridged by an OAc and a TEA. ligands, and the terminal Pb(II)... 

Surrounded by the inner root sheath, the fiber cells multiply and the constant stream of cell production pushes the fiber cells upward toward the skin surface. As the fiber cells move up the hair folhcle, they begin to differentiate into particular cell types cuticle and cortex. In addition to cuticle and cortex cells, the folhcle also produces a central strand of cells that are loosely organized, forming the medulla in the center of the hair fiber. Eventually, the tip of the fiber penetrates the superficial layers of the epidermis and the mature fiber emerges from the skin surface. In the mature fiber, cells are keratinized and hardened. 

A great similarity of part of structure of subtilisin and lactate dehydrogenase has been noted (Rao and Rossmaim, 1973). In subtilisin and in carboxypeptidase four parallel central strands of p pleated sheets are super-posable (Rossmann and Argos, 1977). Since these similar structural patterns form structural domains in the corresponding proteins, their evolutionary relationship is discussed in the following paragraphs. 

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