A-helical content

For A/ i 4o and a-synuclein, physicochemical studies documented that the interaction with a membrane was dependent on the presence of acidic lipids and involved a conformational change of the peptide yielding an increase in its a-helical content (Davidson et al., 1998 Hertel et al., 1997 Terzi et al., 1995, 1997). 

Although the histone fold was first described from the structure of the histone octamer core of the nucleosome [17], the high a-helical content was predicted much earlier [43]. The core histones possess three functional domains (1) the histone fold domain, (2) an N-terminal tail domain, and (3) various accessory helices and less structured regions. The N-terminal tail domains of the core histones are currently the focus of intense research. Covalent modifications of residues in these unstructured domains appear to modify local chromatin structure, either directly or... 

To design photo-induced electron transfer devices, the group of Nishino [69] reported the preparation of a bis-a-helical nanostructure, 74. The peptidic framework was designed to orient rigidly in space the redox triad, composed of a Ru2 + complex, an anthraquinone, and two propylviologens, when incorporated in a lipid bilayer (Fig. 28). Although the compound exhibited a strong a-helical content in methanol, its conformation in a vesicle bilayer was different and undetermi ned. Nevertheless, irradiation of the Ru(II) complex of 74 resulted in a slow electron transfer. 

Lys-Glu) showed considerable disruption of a-helical content relative to the linear peptide control (linear EK) as well as the EK (j, i+4) and 2EK (/, i+4) analogues. [U71... 

The n-jt transition (222 nm CD band) is responsive to the a-helical content. The n n excitation band at 208 nm polarizes parallel to the helix axis and is sensitive to whether the a-helix is single-stranded or is an interacting helix, as in the case of two-stranded coiled coils. 31,131 The decrease in parallel band intensity, coupled with the red shift in the parallel band maximum, corresponds to the conversion of a rigid single-stranded a-helix to an a-helical coiled coil.1 31 The maximum ellipticity at 208 nm in benign medium shifts to 206 nm in 50% TFE[861 and the ratio changes from >1.0 to <1.0 (range ca. 0.85-0.90 in 50%... 

Circular dichroism (CD) studies on a-elastin (Tamburro et al., 1977), K-elastin, bovine, and human tropoelastin (Debelle et al., 1995 Vrhovski et al., 1997) have demonstrated a conformational transition to increased a-helical content with increasing temperature. The a-helical content predicted for tropoelastin is probably confined to the crosslinking domains, as the rest of the molecule is rich in helix breaking proline residues (Muiznieks et al., 2003). 

Far-UV circular dichroism spectral analyses show that on inactivation at low salt concentration the a-helical fraction is decreased from 30 to 0% (Pundak et al., 1981 Hecht and Jaenicke, 1989a). The kinetics of the decrease are not simple. A sharp decrease in the a-helix content of the enzyme is followed by a prolonged moderate decrease of the helical structure, resulting, finally, in the complete disruption of all organized structures. It is possible that the initial decrease in the a-helical content corresponds to the transition D — M, which is followed by the slow transition M — M as discussed above. 

Analysis of the CD spectrum has yielded values of 14% a helix and 31 % p strand, with a possible increase in helix content observed with increase of temperature (Loucheaux-Lefebvre et al., 1978). In a more recent study (Ono et al., 1987), a lower fraction of a helix was calculated, but the results vary with the method of calculation. Structure prediction methods have also been applied to this protein and have given results that encourage the view that K-casein has a number of stable conformational features. Loucheaux-Lefebvre et al. (1978) applied the Chou and Fasman (1974) method and predicted an a-helical content of 23%, with 31% P strand and 10% p turns. Raap et al. (1983) preferred the method of Lim (1974) to predict a-helix and P-strand content, because the method of Chou and Fasman, as published in 1974, was considered to overpredict these elements (Lenstra, 1977). They also tested their predictions for the structure about the chymosin-sensitive bond using the later boundary analysis method... 

Most of HSQC peaks assigned to the N-terminal region disappear upon the formation of PYPm [33,34]. The loss of a-helical content is also observed by CD [35]. However the controversial conclusion was obtained by the fragmentation and H/D exchange mass spectroscopic analysis [36]. Therefore, detailed structural information about PYPm in solution is required to clarify the mechanism underlying the phototransduction. 

Myosin. Rabbit muscle myosin is a long, thin molecule (VI400 X 20-50 A) with a molecular weight of 5 X 10. It is composed of two heavy chains and four light chains as demonstrated by SDS-polyacrylamide disc gel electrophoresis. On tryptic digestion, myosin is split into the subunits, H-meromyosin (HMM) and L-mero-myosin (LMM). HMM is further split into S-l and S-2 subunits. While LMM is a rod of V)0% a-helical content, the a-helical content for HMM, S-l and S-2 fragments is 46%, 33% and 87%, respectively. The ATPase activity is localized in the S-l subunit (33,34). Although fish myosins appear to have the same structural profile (10,22,35-40) and similar amino acid composition as rabbit myosin (39,41,42), fish myosin is different from rabbit myosin in physicochemical properties such as solubility, viscosity and stability (10,22,35-40). 

Yamniuk etal. report the use of CD spectroscopy to measure the structural changes in calcium- and integrin-binding protein upon addition of the divalent metal ions Ca + and Mg + to the metal-lfee apoprotein. They found increases in the a-helical content of the protein of over 30% when either of the metal ions was added to the protein. Futaki et al. showed that the a-hehcal character of a model peptide could be destabihzedby the addition of ferric ion. This destabihzation of the secondary structure was indicated by suppression of the 222 inn signal upon addition of Fe +. The authors concluded a possible ferric/ferrous redox control of the hehcal stmcture of the peptide. 

Karlin-Neumann et al. [31] have presented a model for the main apoprotein of LHC II which has 3 a-helices (Chapter 11), consistent with the direct determination of the a-helical content of LHC II [32]. There is a large domain of surface-exposed protein (= 48%) this is also consistent with the electron microscopic pictures of reconstituted LHC II [18,19], While most of the Chi molecules are thought to reside in the hydrophobic membrane interior, there are insufficient histidine residues present for the co-ordination of all Chi a molecules, and the ligand for Chi b has not been recognized yet. The location of the carotenoids, so often ignored, but always a constituent of all Chl-proteins, is not established [30],... 

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