Separation of Contributions of Tryptophans and Tyrosines to the Ultraviolet Circular Dichroism Spectrum of Hen Egg‐White Lysozyme

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Abstract
Ultraviolet circular dichroism measurements over the wavelength range of 190–300 nm have been carried out on hen egg‐white lysozyme and on an iodine‐oxidized derivative of lysozyme in which Tryptophan‐108 has been selectively modified. The contribution of Tryptophan‐108 to the aromatic circular dichroism profile has been ascertained from a difference spectrum generated between the native and the modified protein. This tryptophyl residue is exceptional in its properties, as it is the principal contributor to the circular dichroism pattern in this wavelength range. The chemically modified protein shows only minor differences from the native molecule in terms of the magnitude of the conformation‐dependent ellipticity bands at 209 and 192 nm. Separation of the partial contributions of tyrosines was obtained from circular dichroism difference spectra constructed from circular dichroism profiles of both the native and iodine oxidized proteins at pH 7 and 11.2. Oligomers of N‐acetyl‐d‐glucosamine, which are inhibitors of lysozyme, markedly affect the circular dichroism spectrum of the enzyme at the aromatic region. It is postulated that this effect on the spectrum is a result of a change in orientation of Tryptophan‐108, which occurs on binding of the inhibitors to the active site of the enzyme.These results are discussed with respect to the proposed three‐dimensional model of lysozyme.

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