Proteins a helix

Brazzein (54 aa, 6kDa protein a-helix, short a-helix, 2 antiparallel (3-strands 8 Cys, 4 S-S, heat-stable) Pentadiplandra brazzpana (Pentadiplandraceae) [fruit] Sweet (2000x sucrose)... 

T he a-helical parts of myoglobin and other proteins stop whenever a proline residue is encour tered in the chain. Why is proline never present in a protein a-helix ... 

FIGURE 3-22 Hydrogen -Bonded Protein Structures, (a) A protein a helix. Peptide carbonyls and N—H hydrogens on adjacent turns of the helix are hydrogen-bonded. (From T. L. Brown and H. E. LeMay, Jr., Chemistry, the Central Science, Prentice Hall, Englewood Clifts,... 

The description by Pauling and his colleagues, Corey and Branson, in 1951 (161) of the protein a-helix (3.6 -helix) with multiple thirteen-membered hydrogen bonds between the carbonyls of residues i and the amide hydrogens of residues i + 4... 

See also Fibrous Proteins, a-Helix, / -Sheet, Cysteine... 

Early attempts by Greenfield and Fasman (6) centered around the derivation of [0]x from model polypeptides, and by assuming only three conformations common to polypeptides and proteins (a-helix, a (3-sheet, and coil), it was possible to obtain structural parameters relatively consistent with x-ray data using ellipticity values at two wavelengths ... 

Discrimination between stereoisomers in biological environments should in fact not be unexpected, as at a molecular level such environments are composed of handed macromolecules, i.e., proteins, glycolipids and nucleic acids, from the chiral precursors of L-amino acids and D-carbo-hydrates. In addition, the macromolecular structures of these biopolymers also give rise to chirality as a result of helicity, e.g., the protein a-helix and DNA double helix. Such helical structures may have either a left or right handed turn in the same way that a spiral staircase may be left or right handed. In the case of the DNA double helix and the protein ot-helix, the biopolymers have a right-handed turn. 

Proteins and nucleic acids experienced their first intimacy within the nucleation event. Later, they turned out to be the driving forces of the "grand game". It was representatives of the ordered patterns of these two partners in our life process that emerged first from our modelbuilding, X-ray assisted investigation as the two "hollow" structures the protein a-helix and the DNA double helix. 

Hydrogen bonds form four units between the CO group and the NH group further along the peptide chain (Figure 9.5). These hydrogen bonds determine the secondary structure of the protein (a-helix or p-sheet). Figure 9.5 shows an a-helix. 

The protein a-helix is characterized by a 3.6 amino acid/tum conformation that places the i,i +A,i +1, and... 

In organic polymers, macromolecules can form regular crystal network type arrangements. These arrangements have a marked effect on the properties of polymers, especially their mechanical strength. For proteins, a-helix or (3-sheet secondary structures are highly... 

It is hoped that the structure of the protein a-helix can be visualised from the description of its origin which has been given and from Fig. 3.3(B). The value of using a modem set of atom models to build up such a structure cannot, however, be overemphasised. This is the only really satisfactory introduction to thinking about molecular architecture in three dimensions. 

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