The alpha helix

FIGURE 6.24 (a) The alpha helix consisting of residues 153-166 (red) in flavodoxin from Anahaena is a surface helix and is amphipathic. (b) The two helices (yellow and blue) in the interior of the citrate synthase dimer (residues 260-270 in each monomer) are mostly hydrophobic, (c) The exposed helix (residues 74-87—red) of calmodulin is entirely accessible to solvent and consists mainly of polar and charged residues. 

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... 

The Alpha Helix Expedition to the Amazon for the Study of Fish Bloods and Hemoglobins. Comp. Biochem. Physiol. 624 (1) (1979). 

Concerning the helical stmcture of biological macromolecules in general, they relied on Linus Pauling s discovery of the alpha-helix stmcture of proteins and concerning the helical stmcture of DNA, they relied on Rosalind Franklin s X-ray diffraction pattern of DNA. 

Secondary structure those protein structures that result from hydrogen bond formation between the amino and carbonyl groups of peptide bonds. The most important ones are the alpha helix and beta sheets. 

The alpha helix and beta sheets are two types of secondary structures exhibited by polypeptides and proteins. (Rae Dejur)... 

Via. 12-18.—A drawing showing two possible forms of the alpha helix the one on the left is a left-handed helix, and the one on the right is a right-handed helix. The amino acid residues have the configuration in each case. 

The alpha helix represents the second major structural element of soluble proteins108 128 and is also found in many fibrous proteins, including those of muscle... 

An especially favorable conformation of a polypeptide chain that was originally deduced by Pauling and Corey is the alpha helix (Figure 25-11). The principal feature of the a helix is the coiling of the polypeptide chain in... 

Secondary Structure of Proteins The secondary structure of a protein is how the polypeptide chain is twisted. There are two common types of secondary structure the alpha helix and the beta pleated sheet. 

Figure 3.3 The alpha helix is one of the most common secondary structure patterns. Pictured here are (a) the molecular structure of the alpha helix and (b) a protein consisting of several spiraling alpha helices.

Biological macromolecules are often distinguished by their helical structures to which one-dimensional space-group symmetries are applicable. Figure 8-15a shows Linus Pauling s sketch of a polypeptide chain, which he drew while he was looking for the structure of alpha-keratin. When he decided to fold the paper, he arrived at the alpha-helix. The solution may have come in a sudden moment,... 

Figure 8-15. (a) Linus Pauling s sketch of the polypeptide chain in 1948. The alpha-helix came together eventually when he folded the paper along the creases [16] (b) Computer-drawing of the alpha-helix (courtesy of Ilya Yanov, Jackson, Mississippi). 

L. Pauling, The Discovery of the Alpha Helix. Chem. Intell. 1996, 2(1), 32-38 (posthumous publication). 

At the time Pauling was working on the structure of proteins culminating eventually in his discovery of the alpha-helix. Yet this declaration sounds as if he were anticipating the mechanism of DNA replication via the double helix. It came, however, only in 1953, and it was not Pauling, but Watson and Crick, who discovered it. 

And yet Pauling had not solved the mystery of life. His alpha helix was an important structural feature of many proteins, but it could not explain how the important ones worked. Apart from hair and horn, Pauling s alpha helix seemed to explain nothing about protein activity. To make an antibody, for instance, sections of alpha helix would have to bend and twist to create an area complementary in shape to the target molecule. Pauling s model did not account for bends, nor did it predict in any way how the alpha helix could create the fantastic variety of shapes necessary for proteins. 

And he decided to publish. Only a month had passed since he had first sat down to take a serious look at the structure of DNA. There were still many questions to answer, such as why the phosphates, which would probably carry a negative charge, wouldn t repel each other and blow the structure apart. Pauling decided to ignore this question for the moment, as he had ignored the 510 picometer repeat with the alpha helix. 

A ball-and-stick model of a protein segment illustrating the alpha helix. 

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