Helix myoglobin

Fig. 9. Comparison of FTIR absorption spectra of four proteins in H20 (left, amide I + II) and D20 (right, amide F + IF). Comparison between protein spectra for dominant secondary structure contributions from a-helix (myoglobin, MYO, top), /Fsheet (immunoglobin, IMUN), from both helix and sheet (lactoferrin, LCF) and from no long-range order (o -casein, CAS, bottom). The comparisons emphasize the high similarity, differing mostly by small frequency shifts of the amide I with the changes in secondary structure.

As described in Chapter 2, the first complete protein structure to be determined was the globular protein myoglobin. However, the a helix that was recognized in this structure, and which has emerged as a persistent structural motif in the many hundreds of globular proteins determined subsequently, was first observed in x-ray diffraction studies of fibrous proteins. 

As noted, hemoglobin is an tetramer. Each of the four subunits has a conformation virtually identical to that of myoglobin. Two different types of subunits, a and /3, are necessary to achieve cooperative Oa-binding by Hb. The /3-chain at 146 amino acid residues is shorter than the myoglobin chain (153 residues), mainly because its final helical segment (the H helix) is shorter. The a-chain (141 residues) also has a shortened H helix and lacks the D helix as well (Figure 15.28). Max Perutz, who has devoted his life to elucidating the atomic structure of Hb, noted very early in his studies that the molecule was... 

The a-helical parts of myoglobin and other proteins stop whenever a proline residue is encountered in the chain. Why is proline never present in a protein o -helix ... 

Ribbon views of proteins with varying amounts of helices and pleated sheets. Immunoglobulin, an antibody, is made up almost entirely of pleated sheets (magenta). Myoglobin, which stores oxygen in muscle tissue, is composed of about 70% helix (blue). G-Actin, a component of muscle protein fibers, is a complex mixture of helices and pleated sheets. Regions with no specific secondaiy stmcture are shown in orange. 

Proteins are complex molecules, typically containing several thousand atoms. Although Pauling and Corey proposed the a helix and the 3 sheet as the main secondary structural elements of proteins in 1951, and the crystal structure of myoglobin was reported by John Kendrew in 1958,... 

Figures 9 and 10 represent a selected comparison of amide V and I+II FTIR and VCD for four proteins in D2O solution. Of these, myoglobin (MYO) has a very high fraction of a-helix, immunoglobulin (IMU) has substantial /1-sheet component, lactoferrin (LAF) has both a and j3 contributions, and a-casein (CAS) supposedly has no extended structure. The FTIR spectra of these proteins change little, the primary difference...

The CD of myoglobin at — 7°C, pH 3.72, indicates significant residual helix content, although by other criteria it is substantially unfolded (Privalov et al, 1986). 

Structure in solution Bombyx mori (Iizuka and Yang, 1966 Yao et al, 2004), MA, MI, FLAG, and CYL (Dicko et al., 2004b), Acinous and Pyriform (Fig. 8), Antheraea pemyi (Tsukada et al, 1994). The helix-like structure is loosely defined as a structure with a CD spectrum similar to myoglobin. /(-Spiral structure is defined as a super helical structure formed of straight sections and /(-turns. 

Figure 4.11 (a) Four helix bundle domain proteins, illustrated by myohaemerythrin. The oxygenbinding site is located at the di-iron centre within the hydrophobic core of the helical bundle, (b) The globin fold, represented here by myoglobin. (From Branden and Tooze, 1991. Reproduced by permission of Garland Publishing, Inc.)... 

Fig. 18. An example of the an conformation at the end of the A helix in myoglobin (residues 8-17). The normal a-helical hydrogen bonds are shown dotted, while the tighter a, bond is shown by crosses.

Alpha-helix content is not as high as myoglobin, but areas of beta sheeting are not unusual. 

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