Parallel 3-sheets

As they occur in known protein structures, almost all P sheets—parallel, antiparallel, and mixed—have twisted strands. This twist always has the same handedness as that shown in Figure 2.7, which is defined as a right-handed twist. 

Figure 5.30 Schematic diagrams of the structure of the enzyme pectate lyase C, which has a three-sheet parallel P-helix topology.

Parallel /3-sheets tend to be more regular than antiparallel /3-sheets. The range of (f) and i/t angles for the peptide bonds in parallel sheets is much smaller than that for antiparallel sheets. Parallel sheets are typically large structures those composed of less than five strands are rare. Antiparallel sheets, however, may consist of as few as two strands. Parallel sheets characteristically distribute... 

The second major class of protein structures contains structures based around parallel or mixed j8-sheets. Parallel /3-sheet arrays, as previously discussed, distribute hydrophobic side chains on both sides of the sheet. This means that neither side of parallel /3-sheets can be exposed to solvent. Parallel /3-sheets are thus typically found as core structures in proteins, with little access to solvent. 

Hartung et al. [120] determined the crystal structure of the mesogenic 6-hexylamino-l,2,4,5-tetrazin-3-yl 4-pentyloxybenzoate. The plane of the bridging carboxylic group is inclined to those of the phenyl and the tetrazine rings by 12.1 and 76.3°, respectively. The molecules are arranged in sheets parallel to (021). 

Molybdenum trioxide has a layered structure with orthorhombic symetry [16] (a=3.963, b=ll855, c=3.696 A), this structure consists of double layer sheets parallel to the (010) cleavage plane. The building unit is a distorted M0O6 octahedron, with Mo-0 distances 1.67, 1.73, 1.95 (twice), 2.25 and 2.33 A (Fig.l)... 

Two models have been proposed for how this dimeric structure may relate to the structure of cystatin C in the fibril. The first (Janowski et at, 2001) proposes that run-away domain swapping (like that shown in Fig. 11C) can account for the assembly and stability of the fibril. In this model, one monomer would swap /(I-a 1-/12 into a second monomer, the second would swap its /(I-a 1-/12 into a third, and so on. The second model (Staniforth et al., 2001) proposes a direct stacking of domain-swapped dimers, where /i5 of each subunit of the dimer would interact with /(I of a subunit of the adjacent dimer. In this way, the dimers would stack to form continuous /1-sheets. Both models arrange the /(-sheets parallel to the fibril axis with component /(-strands perpendicular to the axis, as in a cross-/ structure, although no diffraction pattern has been reported for cystatin fibrils. 

In the previous section it was seen that the Helmholtz-Perrin model fixes the solution charges onto a sheet parallel to the metal. I Iowcver, this model was too rigid... 

The stable phase in the Zn3(P04)2-Zn3(V04)2 system has the composition Zn3P1.5V0.sOs. The structure consists of sheets parallel to (100) formed of chains containing rings of comer-shared (Zn-1)04 and (V,P)04 tetrahedra the (Zn-2)04 groups form chains parallel to the b axis through corner sharing (Zn—O = 1.95, 1.96 A). 

Property Perpendicular to c ( plane of sheet) Parallel to c ( L plane of sheet)... 

For polypeptide, the B program provides options for building various protein conformations including 3-10 helix, alpha helix, alpha helix (L-H), beta sheet (anti-prl), beta sheet (parallel), various beta turns, extended, gamma turns, omega helix, pi helix, polyglycine, and polyproline. Choose the desired conformation and isomer (l or d) and then add amino acids from N-terminus to construct polypeptide chain. 

The P pleated sheets (parallel and antiparallel), as exemplified by the p form of stretched keratin and silk protein... 

In effect, a hypothetical superstructure of TEA cations at low temperature, implying long-range ordering, has not been ruled out [22,23]. Diffuse scattering experiments could help clarify this point (in fact, at least two families of diffuse sheets, parallel to c and parallel to a, and with a modulated intensity, have been observed but never explained) [23]. 

Figure 28.11 The parallel and antiparallel forms of the p-pleated sheet Parallel p-pleated sheet...

Amyloid fibrils show a characteristic diffraction pattern, the so called P-cross pattern (35), which is indicative of P-sheets parallel to the fibril axis with the protein strands perpendicular to the fibril s long axis (36, 37). The pattern of amyloid is characterized by reflections at 4.75 A (along the fibril axis) and 10 A (perpendicular to the fibril axis) which occur from regular repeats and stacking of monomers. 

Experiment has shown that every molecule of water in ice is located in the center of a tetrahedron and has four hydrogen bonds with its neighbors, which are located at the vertexes of the tetrahedron. This gives rise to a structure which can be represented as multiple interconnected sheets parallel to the ay-plane (see Eigure 1 for an illustration of a single sheet). Each sheet consists of a grid of nonplanar hexagons with... 

The U polyhedra share edges with two SO4 tetrahedra and eomers only with two other SO4 tetrahedra to form heteropolyhedral sheets parallel to (010) (Fig. lOe, d). The adjacent sheets are linked through a complex system of hydrogen bonds. All sulfate tetrahedra are bidentate. Again, the (NH4)U (S04)2(H20)4 phase is isostructural with trivalent rare earth sulfates with the composition AM XS04)2(H20)4 (A = NH4, Rb, Cs, Tl M = rare earth metal) [70, 77, 79-82]. 

Structures of some sulfates of tetravalent actinides are isotypic with their tetravalent rare earth analogues (presumably, Ce(IV)). For example, phases with the composition M" (S04)2(H20)4 (M = U, Np, Pu) [58, 63, 65] are isostructural with Ce(S04)2(H20)4 [83]. In their structures, the M04(H20)4 square antiprisms share comers with SO4 tetrahedra to produce sheets parallel to (100) (Fig. 11). 

This results in the formation of the i[U20io] chains with SBU consisting of UO7 and an UOe polyhedra connected through one oxygen (Fig. 12). These chains run along the a and b directions and are connected through VO4 tetrahedra. As in Li2(U02)3(V04)20, the UOe octahedra and VO4 tetrahedra form autunite anion-topology sheets parallel to (001) (Fig. 13a), but with an inverted occupation of the squares by the UOe octahedra and the V(2)04 tetrahedra. The V(l)04 tetrahedra are attached above and below the empty squares to form the [(U02)2(V04)3] double sheets (named D) Fig. 13b. 

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