Proteins with pleated sheets

Spider web is composed mostly of fibroin, a protein with pleated-sheet secondary structure. The pleated-sheet arrangement allows for multiple hydrogen bonds between molecules, conferring great strength. 

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. ... 

The pleated sheet structure applies to proteins of the silk fibroin-/3-keratin group. For silk fibroin itself it may be assumed that the structure has been proved correct by Pauling s calculations (antiparallel chains). A pleated sheet structure with parallel chains is proposed for stretched hair (/3-keratin). Investigations on some other natural representatives have not yet been concluded. Possibly, amorphous or differently arranged sections occur in combination with pleated-sheet areas. 

Most sequence-specific regulatory proteins bind to their DNA targets by presenting an a helix or a pair of antiparallel p strands to the major groove of DNA. Recognition of the TATA box by TBP is therefore exceptional it utilizes a concave pleated sheet protein surface that interacts with the minor groove of DNA. Since the minor groove has very few sequence-specific... 

Figure 26.6 (a) The /3-pleated sheet secondary structure of proteins is stabilized by hydrogen bonds between parallel or antiparallel chains, (b) The structure of concanavalin A, a protein with extensive regions of antiparallel / sheets, shown as flat ribbons. 

Ribbon views of proteins with varying amounts of helices and pleated sheets. Immunoglobulin, an antibody, is made up almost entirely of pleated sheets (magenta). Myoglobin, which stores oxygen in muscle tissue, is composed of about 70% helix (blue). G-Actin, a component of muscle protein fibers, is a complex mixture of helices and pleated sheets. Regions with no specific secondaiy stmcture are shown in orange. 

The essential distinction between the approaches used to formulate and evaluate proteins, compared with conventional low molecular weight drugs, lies in the need to maintain several levels of protein structure and the unique chemical and physical properties that these higher-order structures convey. Proteins are condensation polymers of amino acids, joined by peptide bonds. The levels of protein architecture are typically described in terms of the four orders of structure [23,24] depicted in Fig. 2. The primary structure refers to the sequence of amino acids and the location of any disulfide bonds. Secondary structure is derived from the steric relations of amino acid residues that are close to one another. The alpha-helix and beta-pleated sheet are examples of periodic secondary structure. Tertiary... 

Klunk, W. E., Pettegrew, J. W., and Abraham, D.J. (1989). Quantitative evaluation of Congo red binding to amyloid-like proteins with a beta-pleated sheet conformation. J. Histochem. Cytochem. 37, 1273-1281. 

In the last one and one-half decades many studies have been made on the ORD and CD of ribosomal proteins. Early studies (McPhie and Grat-zer, 1966 Sarkar et al., 1967 Cotter and Gratzer, 1969) were made on a mixture of proteins, and the general conclusion was that both in the ribosome and in the isolated state (usually after acetic acid and urea extraction) the protein moiety contained approximately 25% a helix together with some -pleated sheet and random-coil conformation. 

Silk is produced from the spun threads from silkworms (the larvae of the moth Bombyx mori and related species). The main protein in silk, fibroin, consists of antiparallel pleated sheet structures arranged one on top of the other in numerous layers (1). Since the amino acid side chains in pleated sheets point either straight up or straight down (see p. 68), only compact side chains fit between the layers. In fact, more than 80% of fibroin consists of glycine, alanine, and serine, the three amino acids with the shortest side chains. A typical repetitive amino acid sequence is (Gly-Ala-Gly-Ala-Gly-Ser). The individual pleated sheet layers in fibroin are found to lie alternately 0.35 nm and 0.57 nm apart. In the first case, only glycine residues (R = H) are opposed to one another. The slightly greater distance of 0.57 nm results from repulsion forces between the side chains of alanine and serine residues (2). 

The fact that a denatured protein can spontaneously return to its native conformation was demonstrated for the first time with ribonuclease, a digestive enzyme (see p. 266) consisting of 124 amino acids. In the native form (top right), there are extensive pleated sheet structures and three a helices. The eight cysteine residues of the protein are forming four disulfide bonds. Residues His-12, Lys-41 and His-119 (pink) are particularly important for catalysis. Together with additional amino acids, they form the enzyme s active center. 

Figure 4.2 This three-dimensional image of a protein shows the many twists and folds in its structure. The coils, called alpha helices, and the ribbons, called beta pleated sheets, are generally determined by the amino acid sequence of the protein and how the amino acids in different parts form weak bonds with each other. The shape of a protein is often critical for its function.

Dihydrofolate reductase (E.C. 1.5.1.3) is a relatively small monomeric protein (MW = 18,000-36,000) containing no disulfide bonds. It is present in all cells in different isozyme forms that depend on the organism. The best studied form contains 159 amino acids, with 30% of them in an eight-stranded pleated sheet. The binding site is a 1.5 nm... 

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