Proteins pleated-sheet model

Michael reactions and, 895 Beta-keto ester, 851 alkylation of, 859-860 cyclic, 892-893 decarboxylation of, 857, 860 Michael reactions and. 895 pKd of, 852 synthesis of, 892-893 Beta-lactam antibiotics, 824-825 Beta oxidation pathway, 1133-1137 mechanism of, 1133-1136 Beta-pleated sheet (protein), 1038 molecular model of, 1039 secondary protein structure and, 1038-1039 Betaine, 720 Bextra. structure of, 544 BHA, synthesis of, 629 BHT, synthesis of. 629 Bicycloalkane. 129 Bijvoet. J. M., 299 Bimolecular, 363... 

In an early inaccurate model for the structure of biological membranes, a phospholipid bilayer was coated on both sides by protein in an unfolded or jS-pleated sheet conformation. This model reflected the prevailing view of membrane structure from about 1940 until the early 1970s. 

In Fig. 30, a three-dimensional model is presented in which only the organic phases are shown. Hexagonal plates of MM alternate with pleated sheets of CP. The hydrophobic sides of MM are facing each other and encase the mineral phase. The relationship between hydrophobic bonding and accessible surface area in proteins, and the effect of polar and non-polar side groups on free energy values has recently been discussed246. For informations on hydrophobicity in protein systems see Refs.247-252. ... 

In pleated sheets, we know that successive carbonyl oxygens point in opposite directions. One or two carbonyls whose orientations are clearly revealed by the map can allow sensible guesses as to the positions of others within the same sheet. As mentioned previously with respect to map fitting, we use knowledge of protein structure to infer more than the map shows us. If our inferences are correct, subsequent maps, computed with phases calculated from the model, will show enhanced evidence for the inferred features and will show additional features as well, leading to further improvement of the model. Poor inferences degrade the map, so where electron density conflicts with intuition, we follow the density as closely as possible. 

The result was one of the most extraordinary sets of papers in 20th-century science. Seven appeared together, dominating the May 1951 issue of the Proceedings of the National Academy of Sciences. There was a detailed description of the pleated sheet for silk. There was a new model for the protein in feathers, and new ideas about the structure of artificial proteins, globular proteins, and muscle. 

Oarter has reviewed the comparative crystallography of oxidized and reduced C. vinosum HiPIP (1), and the dimensional changes of the iron-sulfur cube following oxidation or reduction have also been extensively tabulated and discussed for both model complexes and protein-bound clusters (118). In spite of the low sequence homology in HiPIPs, there is a remarkable similarity in tertiary structure, especially around the cluster (114). No significant secondary structure is observed in the HiPIPs, with only two short a-helical segments, three strands of antiparallel /3-pleated sheet, and one small helix near the N terminus (Fig. 1). The 4Fe-4S cluster is buried in the protein interior and is inaccessible to solvent (Fig. 2). This feature has been pro-... 

Figure 7.37. Possible structural model of scrapie prion protein from electron crystallography studies, showing full sixfold symmetric top view with triangular arrangement of parallel P-pleated sheets at the center surrounded by additional partially a-helical sequences. (Reproduced with permission from Wille et al. Copyright 2002 National Academy of Sciences, U.S.A.)...

Poly(a-amino acids) serve as model substances for proteins. In the solid state, they occur in two forms. The a-form is a helix stabilized by intramolecular hydrogen bonding (see also Section 4.2.1). The )8-form has the pleated sheet structure (see also Figure 5.10). Because of intermolecular hydrogen bonding, this form is infusible and insoluble. The a form yields wool-like, the j3-form silklike fibers. 

On the basis of models and X-ray diffraction data, Pauling deduced that there are two common structures for protein molecules, called the a helix and the pleated sheet. The a-heUcal structure of a polypeptide chain is shown in Figure 25.11. The helix... 

The most important part of the cuticle from the point of view of polymer deposition is, of course, its outermost surface, i.e., the epicuticle. The epicuticle has been defined by Lindberg (11) as the membrane that contains sacs or bubbles raised upon immersion of wool fibers in chlorine water, i.e., the so-called Allwoerden reaction. It has been shown, however, that the removal of strongly bound surface lipids does not affect this phenomenon (12). It is assumed therefore, that the epicuticle, which is about 25 A thick, is a residue of the cuticle cell membrane and is at least partly proteinaceous in nature and, together with surface-bound lipids (9), forms the F-layer. A recent model of the epicuticle membrane proposed by Negri and co-workers (13) incorporates new evidence and defines the epicuticle in terms of a membrane consisting of 25% lipids and 75% protein, the latter having an ordered, possibly P-pleated sheet structure with 12% cystine, as shown in Figure 4. 

From WAXS and SAED data of both ProNectin F lyophilized powder and sprayed fibrils, the current model indicates that ProNectin F crystallizes into a chain folded pleated sheet of beta strands (Anderson et a/. 1994). The strands are oriented antiparallel. The beta strands are not fully extended, but have a more compressed crankshaft conformation. This conformation agrees with the predicted conformation of unoriented silk fibroin protein, the Silk I structure (Lotz and Keith 1971). The crystal dimension in the c direction (along the peptide backbone) is consistent with a theoretical length of 11.6 nm for nine SEP blocks (54 amino acids) in this conformation. This predicts that the width of the ProNectin F tile is controlled at least in part by the number of amino acids in the silklike block domains. 

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