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Medientyp:
E-Artikel
Titel:
Secondary Structure of the lac Repressor Headpiece : Possibilities and Limitations of a Joint Infrared and Circular Dichroism Study
:
Possibilities and Limitations of a Joint Infrared and Circular Dichroism Study
Beschreibung:
<jats:p>The secondary structure of the short tryptic headpiece of the <jats:italic>lac</jats:italic> repressor has been investigated by the analysis of its infrared and circular dichroic spectra. For the latter we used the method of Provencher and Glöckner [<jats:italic>Biochemistry</jats:italic> (1981) <jats:italic>20</jats:italic>, 33–37], which seems to be at present the most successful for the determination of the β content of proteins. Nevertheless our results indicate that in the case of the <jats:italic>lac</jats:italic> repressor headpiece this method overestimates the amount of β structure. We find that the headpiece contains an important helical content of about 50%, depending slightly on the ionic strength. A decomposition of the infrared spectrum in a sum of Gaussian curves reveals clearly the absence of a vibrational band around 1630 cm<jats:sup>−1</jats:sup> excluding thus the presence of a multi‐stranded β‐pleated sheet. The only β structure compatible with the infrared results seems to be a two‐stranded antiparallel β sheet, as judged from our results on the β‐sheet model‐compound gramicidin S. The unusually strong intensity of the amide I' band is in favour of the existence of such a structure. The quantitative analysis of both infrared and circular dichroism spectra indicates the presence of a certain (but different) amount of β structure. Comparing these results with several secondary structure predictions, part of the helical residues should be located between Leu‐45 and (at least) Arg‐35, and an eventual two‐stranded β sheet should be situated in the N‐terminal part of the headpiece.</jats:p>