S-Pleated Sheet

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

Figure 25. Circular dichroism spectra of the classical polypeptide conformations extended into the vacuum ultraviolet region. Solid curve, a-helical pattern averaged from poly-L-alanine and poly(y-methyl-L-glutamate) data. Dashed curve, antiparallel /5-pleated sheet CD pattern due to films of Boc-(l -Ala)7-OMe [78]. Dotted curve, parallel /S-pleated sheet patterns were calibrated by solution spectra. Dash-dot curve, disordered collagen to provide a measure of a random structure. Reproduced, with permission, from [79].

Figure 4. Protein secondary structural elements (a) right-handed a-helix showing intrachain hydrogen bonds as dotted lines (crR 0 = —60°, 0 = —60°) (b) antiparallel /S-pleated sheet showing interchain hydrogen bonds as dashed lines WA 0 = —120°, 0 = 120°) (c) /3-turns of types I and II, differing in the orientation of the central peptide group. [Part (a) is adapted from A. L. Lehninger, Biochemistry (Worth Publishers, Inc., New York, 1975) (b) from Ref. 81 and (c) from Ref. 53.J...

L-amino acid (p. 964) amino acid analyzer (p. 970) amino acid residue (p. 959) anion-exchange resin (p. 970) antiparallel )S-pleated sheet (p. 990) automated solid-phase peptide synthesis (p. 980)... 

FIGURE 15.14 /S-Pleated sheet protein strueture. Hydrogen bonds are shown as dotted lines. 

As you study this section of /S-pleated sheet in Figure 18.12 note the following ... 

Figure 16.22 Hydrogen bonds (a) in a parallel S-pleated sheet structure, in which all the polypeptide chains are oriented in the same direction, and (b) in an antiparallel /S-pleated sheet, in which adjacent polypeptide chains run in opposite directions. For color key, see Figure 16.19.

Like their CXC counterparts, CC chemokines share a common protein fold, known as a Greek key motif, in which three antiparallel jS-pleated sheets are overlaid by a C-terminal a-helix (Fig. 2A). Following the first pair of cysteine residues is a ten-residue loop known as the N-loop, and then a succession of three jS-strands and a C-terminal a-helix. The three jS-strands are positioned antiparallel to each other and form a )S-pleated sheet that is overlaid at an angle of approximately 75° by the C-terminal a-helix. As might be expected, the existence of four conserved amino-terminal cysteine residues within CC chemokines has structural implications. Using their... 

Proteins have four levels of structure. Primary structure describes a protein s amino acid sequence secondary structure describes how segments of the protein chain orient into regular patterns—either a-helix or /3-pleated sheet tertiary structure describes how the entire protein molecule coils into an overall three-dimensional shape and quaternary structure describes how individual protein molecules aggregate into larger structures. 

Arnott, S., Dover, S. D., and Elliott, A. (1967). Structure of / -poly-L-alanine Refined atomic co-ordinates for an anti-parallel /(-pleated sheet./. Mol. Biol. 30, 201-208. 

D. Seebach, S. Abele, K. Gademann, B. Jaun, Pleated Sheets and Turns of /3-Peptides with Proteinogenic Side Chains , Angew. Chem., Int. Ed. 1999, 38, 1595- 1597. 

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. 

Crystal structure(s) of ACTH-(1-39) or 1-24 are not known. Suitable crystals for X-ray diffraction experiments could be obtained however, for the heptapeptide 4-10 (54, 55) and the smaller tetrapeptide 4-7 (54, 56). In the former case, an anti--parallel p-pleated sheet structure of the backbone was found with clustering of hydrophobic (Met, PheandTrp) and hydrophilic (Glu, His, Arg) side-chains as remarkable features. ACTH-(4-7)... 

A protein s primary structure is its amino acid sequence. Its secondary structure is the orientation of segments of the protein chain into a regular pattern, such as an a-helix or a P-pleated sheet. Its tertiary structure is the three-dimensional shape into which the entire protein molecule is coiled. 

Two of the most common secondary structures found in proteins are helical and pleated-sheet conformations, shown in the diagram above. One might compare the helical structure of a protein, for example, with the spiral-shaped cord found on many home telephones. These structures form when atoms, ions, or other chemical species in one part of the protein s primary structure are attracted to other atoms, ions, or chemical species with opposite electrical charges in another part of the structure. 

Both theory (CASP6 blind prediction, Fig. 5.3a) and experiment (carried out within CASP6 as well, Fig. 5.3b) give the target molecule containing five a-helices and two fi-pleated sheets (wide arrows). These secondary structure elements interact and form the unique (native) tertiary structure, which is able to perform its biological function. Both structures in atomic resolution differ by the r.m.s. equal to 2.9 A, which is a rather small deviation. 

Many proteins can be made to clump into fibrous amyloid deposits like those seen in Alzheimer s disease, Creutzfeldt-Jakob disease (the human counterpart of mad cow disease), and other serious ailments. To help prove this point, a natural enzyme to convert to amyloid fibrils—insoluble protein aggregates with a /3-pleated sheet structure—simply by maintaining protein for some time in the unfolded state. Until now, scientists have generally believed that only specific proteins such as amyloid /3-protein and prions are capable of being converted into amyloid fibrils.11 A variety of spectroscopic techniques have been used to confirm the gradual development of amyloid fibrils and to verify the fibrils predominant /3-pleated sheet structure. In the partially unfolded intermediates that form under denaturing conditions, hydrophobic amino acid residues and polypeptide backbone normally buried inside fully folded structures become exposed. Further work is needed to confirm and advance these findings. 

Just as main-chain NH 0=C hydrogen bonds are important for the stabilization of the a-helix and / -pleated sheet secondary structures of the proteins, the Watson-Crick hydrogen bonds between the bases, which are the side-chains of the nucleic acids, are fundamental to the stabilization of the double helix secondary structure. In the tertiary structure of tRNA and of the much larger ribosomal RNA s, both Watson-Crick and non-Watson-Crick base pairs and base triplets play a role. These are also found in the two-, three-, and four-stranded helices of synthetic polynucleotides (Sect. 20.5, see Part II, Chap. 16). 

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