Helical proteins

Most proteins contain some helix. In a significant number of proteins, the secondary structure is limited to a-helices. For example  

For individual pairs of interacting helices, certain regularities in the patterns of tertiary-structural interactions between pairs of helices (e.g. orthogonal or parallel arrangements) have been observed. 

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Martensite transformations are not limited just to metals. Some ceramics, like zirconia, have them and even the obscure system of (argon + 40 atom% nitrogen) forms martensite when it is cooled below 30 K. Helical protein crystals in some bacteria undergo a martensitic transformation and the shape change helps the bacteria to burrow into the skins of animals and people ... 

Alpha helices are sufficiently versatile to produce many very different classes of structures. In membrane-bound proteins, the regions inside the membranes are frequently a helices whose surfaces are covered by hydrophobic side chains suitable for the hydrophobic environment inside the membranes. Membrane-bound proteins are described in Chapter 12. Alpha helices are also frequently used to produce structural and motile proteins with various different properties and functions. These can be typical fibrous proteins such as keratin, which is present in skin, hair, and feathers, or parts of the cellular machinery such as fibrinogen or the muscle proteins myosin and dystrophin. These a-helical proteins will be discussed in Chapter 14. 

Weber, PC., Salemme, ER. Structural and functional diversity in 4-a-helical proteins. Nature 287 82-84, 1980. 

Cohen, C., Parry, D.A.D. a-Helical coiled coils and bundles how to design an a-helical protein. 

Figure 17.16 Ribbon diagram representations of the structures of domain B1 from protein G (blue) and the dimer of Rop (red). The fold of B1 has been converted to an a-helical protein like Rop by changing 50% of its amino acids sequence. (Adapted from S. Dalai et al.,...

Regan, L., DeGrado, W.F. Characterization of a helical protein designed from first principles. Science 241 976-978, 1988. 

The so-called globin proteins are an important group of a-helical proteins. These include hemoglobins and myoglobins from many species. The globin structure can be viewed as two layers of helices, with one of these layers perpendicular to the other and the polypeptide chain moving back and forth between the layers. 

Although their medium-resolution model was successful for a-helical proteins, folding P-hairpin structures have been difficult. In general, many off-lattice approaches have been tested, and although definitive proof does not exist in most cases, there appears to be a growing consensus that such off-lattice models are not sufficient. 

Fig. 4. Backscattered Raman and ROA spectra of the n-helical protein human serum albumin in H20 (top pair) and the /3-sheet protein jack bean concanavalin A in acetate buffer solution at pH 5.4, together with MOLSCRIPT diagrams (Kraulis, 1991) of their X-ray crystal structures (PDB codes lao6 and 2cna). 

The gross fold type identification is established such that highly helical proteins have an amide I at 1650 cm-1 that is relatively sharp in the... 

Fe in haem is bound in a mostly a-helical protein usually by histidine but sometimes by methionine, cysteine or tyrosine. Several such proteins are to be found in membranes. 

Ca is bound usually in 7 coordination with carboxylate and neutral O-donors in helical proteins. 

The iron storage protein ferritin is a small 20 kDa a-helical protein that spontaneously assembles into a hollow ball-like homo-24-mer. The outer diameter of the sphere is circa 12 nm and the inner diameter, or core diameter, is circa 8 nm. A smaller version, known as miniferritin or Dps protein (Dps = DNA protecting... 

Traditional methods for fabricating nano-scaled arrays are usually based on lithographic techniques. Alternative new approaches rely on the use of self-organizing templates. Due to their intrinsic ability to adopt complex and flexible conformations, proteins have been used to control the size and shape, and also to form ordered two-dimensional arrays of nanopartides. The following examples focus on the use of helical protein templates, such as gelatin and collagen, and protein cages such as ferritin-based molecules. 

Tropomyosin and troponin are proteins located in the thin filaments, and together with Ca2+, they regulate the interaction of actin and myosin (Fig. 43-3) [5]. Tropomyosin is an a-helical protein consisting of two polypeptide chains its structure is similar to that of the rod portion of myosin. Troponin is a complex of three proteins. If the tropomyosin-troponin complex is present, actin cannot stimulate the ATPase activity of myosin unless the concentration of free Ca2+ increases substantially, while a system consisting solely of purified actin and myosin does not exhibit any Ca2+ dependence. Thus, the actin-myosin interaction is controlled by Ca2+ in the presence of the regulatory troponin-tropomyosin complex [6]. 

Based on the same principle, there are monomeric / -helical proteins that carry at their extremities a cluster of helical or nonrepetitive structures that could act as a capping element covering their exposed ends (Emsley et al., 1996 Lietzke et al, 1994 Petersen et al, 1997 Steinbacher et al, 1994). For example, the last 40 residues of pectate lyase C form a large loop that partially covers the surface of the /Hielix (Yoder et al, 1993). The fibrous (or otherwise elongated) domain of these natural /f-stranded proteins is not stable in isolation, as for example in the case of the P22 tailspike where bacterially expressed isolated /Hielix domain, at high concentrations, forms fibrous aggregates that bind Congo red (Schuler et al, 1999). 

This review describes designed and folded helical proteins, )5-sheet proteins, a )5)5a-motif and TASP proteins that are targets for functionalization. The functionalization of folded polypeptides using natural amino acids to form catalysts... 

Our own experience has shown that the pattern of recognition for positively charged amine moieties is relatively universal at least in helical protein environments and in most cases there is both a cation-Jt as well as an electrostatic component operating. 

The two major secondary structures found in nature— the helix and the sheet—are also major secondary structures found in synthetic polymers. The helix takes advantage of both the formation of intermolecular secondary bonding and relief of steric constraints. Some materials utilize a combination of helix and sheet structures such as wool, which consists of helical protein chains connected to give a pleated sheet. 

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