Date Published: November 01, 2017
Publisher: International Union of Crystallography
Author(s): David Brent Langley, Ben Crossett, Peter Schofield, Jenny Jackson, Mahdi Zeraati, David Maltby, Mary Christie, Deborah Burnett, Robert Brink, Christopher Goodnow, Daniel Christ.
Lysozyme isoforms purified from duck eggs have been characterized by crystallography, mass spectrometry and their capacity to bind landmark antibodies.
Lysozymes purified from bird eggs are one of the best biologically, immunologically and structurally characterized families of proteins. The Protein Data Bank (PDB) currently contains approximately 650 entries containing chicken-type (C-type) hen egg-white lysozyme (HEL), HEL variants or complexes involving the HEL entity, in some cases at extremely high resolution. A far more limited number of PDB entries contain C-type lysozymes derived from a diverse range of other species, including turkey (Sarma & Bott, 1977 ▸), trout (Karlsen et al., 1995 ▸), human (Artymiuk & Blake, 1981 ▸) and even echidna (Guss et al., 1997 ▸). The C-type lysozyme is distinct at the sequence, structural and immunological levels from two other classes of lysozyme: the form purified from goose eggs (‘G-type’; Canfield & McMurry, 1967 ▸; Prager et al., 1974 ▸) and that encoded by bacteriophages (T4-type; Weaver & Matthews, 1987 ▸), neither of which will be discussed at length in this report. Surprisingly, despite the enzyme purified from duck eggs being extensively studied for its biochemical and antibody-recognition properties, until now no PDB entries have existed for C-type lysozyme derived from ducks.
The crystal structures of DEL isoforms I and III, purified from Pekin duck eggs and separated from each other using ion-exchange resin (Fig. 1 ▸), were solved (Tables 1 ▸ and 2 ▸). The resolutions of these structures were sufficient to unambiguously observe electron density for most amino acids, including their side chains. This allowed direct comparison with amino-acid sequences described in the literature which were obtained from a combination of amino-acid sequencing [as performed by Kondo et al. (1982 ▸), who describe all three isoforms], mass spectrometry [as performed by Takao et al. (1984 ▸), who also describe all three isoforms] and DNA sequencing [as performed by Huang et al. (2013 ▸), where just the one isoform is described]. Additional comparison could be made with the primary sequences of other duck species for which amino-acid sequences have been determined, including Egyptian goose (Alopochen aegypticus, actually a shelduck) and American wood duck (Aix sponsa) (UniProt entries P84496 and Q7LZQ2, respectively; Fig. 2 ▸).
We have solved the crystal structures of two of the three isoforms of lysozyme purified from Pekin duck eggs (DEL-I and DEL-III). The duck egg lysozyme structures have the basic fold of the C-type lysozyme characteristic of HEL. When lysozyme from duck eggs was first analyzed, it was noted that the most striking differences between the enzymes from ducks and chickens, apart from ducks having multiple isoforms, were the complete absence of any histidine residues in the duck variants (which at one stage was incorrectly suspected of being a requisite for catalysis) and the increased amounts of arginine relative to the chicken counterpart. These findings have been verified by our structures of DEL-I and DEL-III. Both contain a leucine residue in place of histidine at position 15, and whilst the enzyme from chicken contains 11 arginine residues, DEL-I contains 13, DEL-II contains 14 and DEL-III contains 16 (not 15 as expected). The single difference in charge between DEL-I and DEL-II, and an additional two positive charges between DEL-II and DEL-III, perhaps better explains the elution profile observed in response to increasing salt from CM ion-exchange resin; the peaks for DEL-I and DEL-II partially overlap, while clear separation exists between the peaks containing DEL-II and DEL-III (Fig. 1 ▸).