A-helix content

Fig. 18. Hydration number and a-helix content of (Glu Na) as a function of solvent composition in mixtures of water with 1-PrOH (O), 2-PrOH (A) and t-BuOH (O) 1J<)...

In the reaction, it was essential to use an IL as a co-solvent. Lozano, Iborra and co-workers recently reported an interesting stabilizing effect of two types of water-immiscible ILs ([emim][TFSI] and [BuMe3N][TFSI]) for CAL-B-catalyzed transesterification of vinyl butyrate. The synthetic activity and the stability of the enzyme in these IL solvent systems were markedly enhanced as compared to those in hexane. CAL-B maintained its activity higher than 75% after 4 days of incubation in [emim][TFSI] solvent, while it showed an activity of only 25% when incubated in both water and hexane media at 50°C. Comparison of the ratio of a-helix and (3-sheet by CD spectra showed the activity was closely related with a-helix content which reduced to 31% immediately after lipase was added to hexane and had reached only 2% after 4 days in hexane. On the contrary, no significant reduction of a-helix content was... 

CD studies have recently been made on urea-isolated and renatured individual proteins from the small ribosomal subunit (Venyaminov and Gogia, 1982). In another study (Dijk et al., 1983a), many proteins obtained from both the small and large ribosomal subunits by a gentler salt extraction method were measured with the CD technique. It was found that, in general, the 30 S proteins are rich in a helix and contain a rather small amount of /3 sheet, whereas the 50 S proteins are more diverse, especially in their a helix content, and most are relatively rich in /3 sheet (Dijk et al., 1983a). 

Fig. 7. The crystal structure of the C-domain of hemopexin (PDB accession number IHXN) 128) showed a four-bladed p-propeller structure, which because of sequence similarity was also expected in the N-domain. The high degree of beta structure and limited a-helix content agrees with the earlier FTIR analysis.

The CD of the unordered form at 222 nm is an important parameter in the determination of a-helix content in peptides. The CD of short peptides (5-7 residues) has been used to evaluate the CD of the unordered form in a homologous series of peptides, fitting the temperature dependence to a linear function of temperature. For example, equation 8 has been used 160 ... 

Lyophilization is a very common method to prepare enzymes for use in organic media, but the procedure often results in preparations having low catalytic activity. The method can cause inactivation of the enzyme both in the freezing step and in the drying step [7]. FT-IR spectroscopy has been used to study secondary structure in enzyme preparations, and lyophilization has been shown to decrease the a-helix content and increase the (i-sheet content compared to native enzyme and... 

At pH 12, the disulfide and noncovalent bonds are both broken, and the monomer with a sedimentation constant of 1.45 Svedberg units is released. From frictional ratios, the monomer appears to exist as a coil with a diameter of 16 A and a length of 150 A. Analysis of the primary structure of K-casein (Loucheux-Lefebvre et al. 1978) suggests considerable secondary structure in the monomer. 23% a-helix, 31% /3-sheets, and 24% 0-turns. In contrast, other investigators, using several different approaches, obtained a-helix contents ranging from 0 to 20.8% (Bloomfield and Mead 1975). Circular dichroism spectra on the monomer indicated 14 and 31% for a-helix and / -sheet, respectively (Loucheux-Lefebvre et al 1978). An earlier study of the optical rotatory dispersion of the K-casein monomer yielded values for the a-helix content ranging from 2 to 16% (Herskovits 1966). 

Brash has also used CD to study eluted proteins and finds large changes in a-helix content of fibrinogen, perhaps due to enzymatic fragmentation produced by the surface-induced activation of plasminogen to plasmin11 18). 

Partial data on the optical rotation (57) and optical rotatory dispersion (55) of native, denatured, and renatured /3-lactamases of B. cereus suggest fairly strong hydrophobic interactions and compact folding. An a-helix content of 30%> has been proposed for the native and renatured enzyme of B. cereus 569 (55). 

The tail domain of nestin contains 22-residue (human) and 44-residue repeats (hamster and rat) (Parry and Steinert, 1999 Steinert et al., 1999b). The 44-residue repeat of hamster is E-E-D-Q-L/R-V-E-R/T-L-I—E—K-E- G-Q-E-S-L-S-S-P-E and E-E-D-Q-E-T-D-R-P-L-E-K-E-N-G-E-P-L-K-P-V-E and, as divided here, can easily be seen to consist of a pair of related but nonidentical 22-residue sequences. Interestingly, even these 22-residue repeats are quasi-halved, indicating an underlying 11-residue structure in all of the species studied. The secondary conformation is unknown, but there is an expectation of high a—helix content. 

If the intensity of the CD band at 222 nm, which can be considered a function of the a-helix content, is plotted as a function of methanol concentration, it is observed that addition of methanol induces a coil—>a-helix transition in the macromolecular conformation (Figure 7). However, the amount of methanol needed to induce the conformational transition is different for the sample irradiated at 340 (cis azo units) and that irradiated at 417 nm (trans azo units). Therefore, two separate curves are observed for the two samples. At solvent compositions in the range between the two curves, alternating irradiation at 340 and 417 nm gives rise to folding or unfolding of the macromolecular chains. The photoresponse occurs only in a selected and nar-... 

Fig. 7 Poly(Ne-p-phenylazobenzenesulfonyl-L-lysine) (VI) in HFP/MeOH solvent mixtures ellipticity at 222 nm and a-helix content percentage, as a function of methanol concentration, for samples irradiated at 417 (continuous line) and at 340 nm (dashed line).

For each mixture, maximum a-helix content was obtained at a characteristic ratio of amino acid residues to disaccharide repeating units (Table IV). 

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. 

The circular dichroism spectrum of E. coli tryptophanase18 showed that the secondary structure of this enzyme seems to be predominantly a-helical. The a-helix content was estimated to be about 50% (Y. Kawata, unpublished results) by the method of Greenfield and Fasman.19 ... 

Relative to secondary structure, viscosity, sedimentation velocity, ultraviolet difference spectra and optical rotatory dispersion studies (4,24,25) showed that glutenin appears to be an assymetric molecule with a low a-helix content (10-15%). Glutenin contained more a-helix structure in hydrochloric acid solutions and less in urea solutions. The amount of a-helix structure is also influenced by changes in ionic strength (26). 

However, a few proteins with an exceptionally high a-helix content (e.g., some of the myosins) and most of the synthetic polypeptides in the a-helical form, exhibit anomolous dispersion. 

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