The beta-pleated sheet

The secondary structure of proteins consists of regions of ordered structures taken up by the protein chain. There are two main structures—the alpha helix and the beta-pleated sheet. 

The beta-pleated sheet is a layering of protein chains, one on top of another as shown in Fig. 3.4. Here too, the structure is held together by hydrogen bonds between the peptide links. The residues are situated at right angles to the sheets, once again to reduce steric interactions. 

In structural proteins such as wool and silk, secondary structures are extensive and determine the overall shape and properties of such proteins. 

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The beta pleated sheet, or simply beta sheet, structure is illustrated in Figure 12.53. The carbon backbone in the beta sheet is fully extended, and adjacent chains are held together by a large number of hydrogen bonds. 

Long, thin, resilient proteins (such as hair) typically contain elongated, elastic a-helical protein molecules. Other proteins (such as silk) that form sheets or plates typically contain protein molecules having the beta pleated-sheet structure. Proteins without a structural function in the body (such as hemoglobin) typically have a globular structure. 

In another secondary structure known as the beta-pleated sheet -pleated sheet), hydrogen bonds hold polypeptide chains together side by side. The hydrogen bonds holding the sheets tightly in place account for the strength and durability of proteins such as silk (see Figure 18.11). 

The secondary structure of a protein describes the type of structure that forms when amino acids form hydrogen bonds within a polypeptide or between polypeptide chains. The three most common types of secondary structure are the alpha helix, the beta-pleated sheet, and the triple helix. 

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... 

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.

Can a polypeptide chain fold into a regularly repeating structure In 1951, Linus Pauling and Robert Corey proposed two periodic structures called the a helix (alpha helix) and the p pleated sheet (beta pleated sheet). Subsequently, other structures such as the P turn and omega ( Q) loop were identified. Although not periodic, these common turn or loop structures are well defined and contribute with a helices and P sheets to form the final protein structure. 

Which of the following statements about beta-pleated sheets (b-sheets) is false ... 

Different amino acids favor the formation of alpha helices, beta pleated sheets, or loops. The primary sequences and secondary structures are known for over 1,000 different proteins. Correlation of these sequences and structures revealed that some amino acids are found more often in alpha helices, beta sheets, or neither. Helix formers include alanine, cysteine, leucine, methionine, glutamic acid, glutamine, histidine, and lysine. Beta formers include valine, isoleucine, phenylalanine, tyrosine, tryptophan, and threonine. Serine, glycine, aspartic acid, asparagine, and proline are found most often in turns. 

If the only structural characteristics of proteins were their amino acid sequences (primary structures), all protein molecules would consist of long chains arranged in random fashion. However, protein chains fold and become aligned in such a way that certain orderly patterns result. These orderly patterns, referred to as secondary structures, result from hydrogen bonding and include the a-helix (alpha-helix) and the P-pleated (beta-pleated) sheet. 

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