JS-pleated sheets

X-ray diffraction analysis reveals the 3-D structure of both IL-1 molecules to be quite similar. Both are globular proteins, composed of six strands of anti-parallel jS-pleated sheet forming a barrel , which is closed at one end by a further series of jS-sheets. 

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. 

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-lielix or jS-pleated sheet, tertiary structure describes how the entire protein molecule coils into an overall three-dimensional shape, and quaternary structure describes how indwld-ual protein molecules aggregate into larger structures. 

Schroeder et al. (1955) comment on the high proline content of feathers and suggest that neither a-helical forms nor jS-pleated-sheet forms could be accommodated if the distribution of proline were approximately random. 

Secondary structure (Section 26.9) The level of protein substructure that involves organization of chain sections into ordered arrangements such as jS-pleated sheets or a helices. 

The adenosine moiety binds in a hydrophobic crevice, lined by residues from )8A, aB, 8B, and 8D, at the carboxyl end of the parallel jS-pleated sheet. Similar hydrophobic pockets are found in all the dehydrogenases, and equivalent residues are listed in Table VIII. The specificity of the hydrophobic pocket is for aromatic groups rather than for adenine in particular as is shown by the binding of 5-iodosalicylate to LADH (. 7) and the affinity of an iodofluorescein dye to LDH ( 8). The pocket in GAPDH is somewhat less hydrophobic in its interior because of the presence of Asp-6 and Asn-31. The adenine ring is situated between Phe-34 and Phe-99. 

Polypeptide chains are almost completely extended in the jS-pleated-sheet conformation (Fig. 20-2). There is a side-by-side alignment of adjacent polypeptide chains. Hydrogen bonding occurs between adjacent polypeptide chains—more precisely, between the C=0 oxygen of one chain and the N—H hydrogen of an adjacent chain. Each polypeptide chain is hydrogen-bonded to adjacent chains on each side of it, except for the chains at the two ends of the sheet. The sheets are pleated... 

The dimers are formed from monomers which associate in an antiparallel fashion, with the B21-30 region of each monomer lining up to form an antiparallel jS-pleated sheet structure. Four hydrogen bonds maintain this noncovalent structure together with hydrophobic interactions between B 12-valine and B24-phenylalanine in the adjoining molecule and B 16-tyrosine with B12-valine and B26-tyrosine. Nonpolar interactions are also the driving... 

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... 

As you study this section of jS-pleated sheet, note the following. 

The second type of secondary structure is a j8-pleated sheet. In a j8-pleated sheet, the polypeptide backbone is extended in a zigzag structure resembling a series of pleats. The hydrogen bonding in a j8-pleated sheet occurs between neighboring peptide chains, and these chains can run in the same direction or in opposite directions—called a parallel jS-pleated sheet and an antiparallel jS-pleated sheet (Figure 22.9). A j8-pleated sheet is almost fully extended the average two amino acid repeat distance is 7.0 A. 

The recognition of structural domains in proteins may be of importance for understanding the assembly of the structural elements of a molecule. A method based on the distance map, as proposed by Nishikawa et al (1972), Ooi and Nishikawa (1973), and Nishikawa and Ooi (1974), was used by Rossmann and Liljas (1974) and Liljas and Rossmann (1974) for comparison and detections of structural similarities of domains. For these maps, the distances between C atoms were plotted as a square matrix with the sequence numbers arranged horizontally and vertically. Contours were drawn for various C —C distances. Moreover, in such a plot the different ordered structures display typical representations. Helices are represented by short interactions along the diagonal. Parallel jS pleated sheets are represented by a line parallel to, but distant from the diagonal. Antiparallel p strands are shown as interactions in a line perpendicular to the diagonal. 

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