Twisted sheets

Figure 4.1 Alpha/beta domains are found in many proteins. They occur in different classes, two of which are shown here (a) a closed barrel exemplified by schematic and topological diagrams of the enzyme trlosephosphate isomerase and (b) an open twisted sheet with helices on both sides, as in the coenzymebinding domain of some dehydrogenases. Both classes are built up from p-a-p motifs that are linked such that the p strands are parallel. Rectangles represent a helices, and arrows represent p strands in the topological diagrams, [(a) Adapted from J. Richardson, (b) Adapted from B. Furugren.j...

Figure 4.2 A p-a-p motif is a right-handed structure. Two such motifs can be joined into a four-stranded parallel p sheet in two different ways. They can be aligned with the a helices either on the same side of the p sheet (a) or on opposite sides (b). In case (a) the last p strand of motif I (red) is adjacent to the first p strand of motif 2 (blue), giving the strand order 1 2 3 4. The motifs are aligned in this way in barrel structures (see Figure 4.1a) and in the horseshoe fold (see Figure 4.11). In case (b) the first p strands of both motifs are adjacent, giving the strand order 4 3 12. Open twisted sheets (see Figure 4.1b) contain at least one motif alignment of this kind. In both cases the motifs ate joined by an ct helix (green).

SNAPs is an acronym for soluble NSF attachment proteins. They were originally discovered as cofactors for NSF that mediate the membrane binding of NSF in in vitro transport assays. Several isoforms of SNAPs exist in mammalian cells. SNAPs are also highly conserved proteins. Crystallographic studies indicated that the proteins form a very stiff and twisted sheet that is formed by a series of antiparallel and tightly packed helices connected by short loops. 

A cross-// spine structure consists of two or more flat or twisted //-sheets, composed of parallel (Nelson et al., 2005) or antiparallel (Makin et al., 2005) //-strands, in a cross-/ arrangement. The cross-/ spine model of fibril formation proposes that a short segment of the native protein changes conformation to form one or more //-strands of a cross-/ spine. The seven-residue peptide GNNQQNY, derived from the prion-determining domain... 

Parallel /3 structure usually forms large, moderately twisted sheets such as in Fig. 23, although occasionally it rolls up into a cylinder with helices around the outside (e.g., triosephosphate isomerase). Large antiparallel sheets, on the other hand, usually roll up either partially (as in the first domain of thermolysin or in ribonuclease) or completely around to join edges into a cylinder or barrel. Occurrence, topology, and classification of /3 barrels will be discussed in Section III,D, but here we will consider the interaction between the /3 sheets on opposite sides of the barrel, especially in terms of the angle at which opposite strands cross. 

Fig. 2.4.13. Left typical amyloid fibrils are twisted / -sheets [27], Right differential ultracentrifugation experiment with A/ (1-42, 33 and 16 xm) in the absence (right) or presence (left) of dimeric aminopyrazole 9 (Ampox, 1 mM). S = solution,...

Reitzer et al., 1999) and a MeCbl-binding fragment of E. coli methionine synthase (Drennan et al., 1994), the cofactor is sandwiched between two domains (Figure 8). The conserved domain possesses an a/ 3 structure reminiscent of the Rossmann fold of nucleotide-binding proteins (Rossmann et al., 1974) and consists of a twisted )-sheet of five parallel strands encased by five a-helices. It binds the lower, a-face of the corrin macrocycle and the substituents projecting idowni from this face, notably the dimethylbenz-imidazole ribofuranosyl nucleotide loop. 

CasAleOw has twisted sheets of distorted AIO4 tetrahedra that are linked by their comers. The calcium atoms are six-coordinate and lie between the AleOw sheets. 

Twisted sheets. X-ray diffraction studies have shown that P pleated sheets are usually not flat but are twisted. In a twisted sheet the individual polypeptide chains make a shallow left-handed helix. However, when successive carbonyl groups are viewed along the direction of the chain, a rig t-handed twist is seen (Fig. 2-12). ° Such twisted P sheets are often found in the globular proteins. An example (Fig. 2-13) is the "nucleotide-binding" domain of a dehydrogenase enzyme. The twist of the sheet is seen clearly in this stereoscopic view. When such chains are associated into P sheets, whether parallel or antiparallel, and are viewed in a direction perpendicular to the chains and looking down the edge of the sheet, a left-handed "propeller" is seen. Such a propeller is visible in the... 

We can see one short segment of a-helix at residues 234-245 another (mostly hidden) lies at 164-170. There is a hint of a twisted sheet beginning with residues 91 -86 and 103-108, and extending to their ri t. 

The folding of a/jS-proteins falls into two major classes, illustrated in Figure 23 the closed ( a)g barrel (the subject of this article) and the open twisted sheet which has a helices on both sides of it. They both involve parallel p sheets that are connected in most cases by an a heUx (the )3a/J-motif described earlier). 

Figure 23 Some a/ -motifs (a) the barrel and (b) the open twisted sheet...

If X Xo the plane of polarization of incident light is rotated by an angle and converted to elliptically polarized light. De Vries [13] applied Maxwell s equations to twisted sheets and derived for 0, the optical rotatory power... 

Fig. 1.21. Diagrammatic presentation of a twisted sheet structure of parallel peptide chains (according to Schulz and Schirmer, 1979)...

The two chain termini are in strands 1 and 5 of a twisted sheet structure. The NH2-terminus is inferred to be at N (Fig.4) from the disposition of side chains in the helices and by analogy with the pepsin sequence (vide infra). The first 140 residues of the chain fold to give the lobe on the right-hand side of Figures 4 and 5. The chain is led back into the sheet structure (P in Fig.4) at strand 3 of the sheet and out at strand 2 (Q in Fig.4). The carboxyl-terminal half of the polypeptide is folded to give the lobe on the left-hand side of Figure 4 and finally rejoins the sheet at strand 4 folding to place the COOH-terminus in strand 5 at C (Fig.4). 

Contacts between the two lobes are very extensive. The strands in the g-sheet at the COOH-terminus contribute not only to the barrel, but also extend tangentially into the twisted sheet containing the NH2 terminus. The fold in the NH2-terminal 20 residues extends the sheet structure into the NH2-terminal lobe. The enzyme can therefore be described as an extended sheet folding into a sandwich structure in the NH2-terminal lobe and the barrel in the COOH-terminal lobe. There are further interactions between the bottom of the g-barrel and the bottom of the sandwich, and many large, almost certainly aromatic groups, are thrown into the regions between and within the lobes and presumably give a hydrophobic core. 

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