Antiparallel p-pleated sheets

X-ray diffraction analysis reveals the three-dimensional structure of both IL-l molecules to be quite similar. Both are globular proteins, composed of six strands of antiparallel P pleated sheet forming a barrel that is closed at one end by a further series of P sheets. 

Monomeric TNF is biologically inactive the active form is a homotrimer in which the three monomers associate non-covalently about a threefold axis of symmetry, forming a compact bellshaped structure. X-ray crystallographic studies reveal that each monomer is elongated and characterized by a large content of antiparallel P pleated sheet, which closely resembles subunit proteins of many viral caspids (Figure 9.4). 

Figure 11.2 Polypeptide chains held together by hydrogen bonds, indicated by dashed lines, in a configuration called a P sheet, (a) The antiparallel P pleated sheet, (b) The parallel P pleated sheet. (Illustration, Irving Geis/Geis Archive Trust. Copyright Howard Hughes Medical Institute. Reproduced with permission.)...

Secondary Structure. The silkworm cocoon and spider dragline silks are characterized as an antiparallel p-pleated sheet wherein the polymer chain axis is parallel to the fiber axis. Other silks are known to form a-helical (bees, wasps, ants) or cross- p-sheet (many insects) structures. The cross-p-sheets are characterized by a polymer chain axis perpendicular to the fiber axis and a higher serine content. Most silks assume a range of different secondary structures during processing from soluble protein in the glands to insoluble spun fibers. 

In an antiparallel p-pleated sheet, the strands run in the opposite direction. 

Suggest a reason why antiparallel p-pleated sheets are generally more stable than parallel p-pleated sheets. 

Consider two molecules of a tetrapeptide composed only of alanine residues. Draw the hydrogen bonding interactions that result when these two peptides adopt a parallel p-pleated sheet arrangement. Answer this same question for the antiparallel p-pleated sheet arrangement. 

There are two ways in which proteins chains can form the pleated sheet structure. One is with the chains running in the same direction i.e. the -COOH or NH2 ends of the polypeptide chains lying all at the top or all at the bottom of the sheet. This is called parallel pleated-sheet structure. In another type, known as antiparallel p-pleated sheet structure, the polypeptide chains alternate in such a way that the -COOH end of the one polypeptide is next to the -NH2 end of the other i.e., polypeptide chains run in opposite directions. 

The second common secondary structure in proteins resembles the pleated folds of drapery and is known as p-pleated sheet (Figure 19.8). All of the carbonyl oxygens and amide hydrogens in a p-pleated sheet are involved in hydrogen bonds, and the polypeptide chain is nearly completely extended. The polypeptide chains in a P-pleated sheet can have two orientations. If the N-termini are head to head, the structure is known as a parallel p-pleated sheet. And if the N-terminus of one chain is aligned with the C-terminus of a second chain (head to tail), the structure is known as an antiparallel p-pleated sheet. 

Some fibrous proteins are composed of p-pleated sheets. For example, the silkworm produces silk fibroin, a protein whose structure is an antiparallel p-pleated sheet (Figure 19.9). The polypeptide chains of a p-pleated sheet are almost completely extended, and silk does not stretch easily. Glycine accounts for nearly half of the amino acids of silk fibroin. Alanine and serine account for most of the others. The methyl groups of alanines and the hydroxymethyl groups of serines lie on opposite sides of the sheet. Thus the stacked sheets nestle comfortably, like sheets of corrugated cardboard, because the R groups are small enough to allow the stacked-sheet superstructure. 

The structure of silk fibroin is almost entirely antiparallel p-pleated sheet. 

We have used this mimetic system to explore SAF among molecules of the immunoglobulin gene super-family. ° Immunoglobulins are constructed from a series of antiparallel P-pleated sheets connected by loops. ° The specificity of these molecules is determined by the sequence and size of the canonical hypervariable complementaritydetermining regions (CDRs). ° ... 

Wool and hair are elastic both are a-keratins, which contain long polypeptide chains composed of a helices twisted about each other to form cablelike assemblies with crosslinks involving Cys residues. Silk, on the other hand, is rigid and resists stretching it is composed primarily of antiparallel P pleated sheets, which are often stacked and interlocked. Briefly explain these observations in terms of the characteristics of the secondary structures of these proteins. 

Most silkworm cocoon and spider dragline silk fibers contain assembled antiparallel p-pleated-sheet crystalline structures [33-35]. Silks are considered semicrystalline materials with 30-50% crystallinity in spider silks, 62-65% in cocoon silk fibroin from the silkworm B. mori, and 50-63% in wild-type silkworm cocoons. In the -sheet erystals the polymer chain axis is parallel to the fiber axis. The extent to which these struetures form, as well as their orientation and size, directly impact the mechanical features of silk fibers. Furthermore, the polyalanine repeats or... 

FIGURE 2 Panel A shows the secondary structural Greek Key motif of CC chemokines, as typified by CCL2. Three antiparallel p-pleated sheets overlay a C-terminal, a-helical domain. Panel B shows the elongated CCL2 dimer, which is facilitated by interactions of the chemokine N-termini. The CCL2 monomers making up the dimer are depicted in blue and red. 

Many efforts were made to refine the methodology of the prediction algorithms. The aid of computers became indisp isable when known parameters were supplemented by values obtained from the statistical study of di- and tripeptide units Lifson and Sander discriminated between parallel and antiparallel P-pleated sheets and Geisow and Roberts demonstrated that conformational preference parameters vary with the protein class Despite these refinements the improvement of the prediction accuracy must be considered minor. The upper limit of exclusively statistical algorithms appears to be in the order of 60-70%... 

---