Ovalbumin is a glycoprotein that comprises 54% of the total proteins of egg white.
History
Ovalbumin and albumin were some of the very first proteins to be studied. Ovalbumin was first crystallized in 1890 by Hofmeister. In 1938, Neuberger reported that the carbohydrate moiety contained two moles of hexosamine, four moles of mannose, and some unidentified nitrogeneous material.
In the 1930s to 1940s, the electrophoretic differences of the three components were discovered (Young 1939, Longsworth 1940, and Lindterstrøm-Lang and Otteseen 1947). In the 1950s, Perlmann determined that the electrophoretic separation was due to differences in ovalbumin’s phosphorous content. The first component having two atoms of phosphorous per mole protein, the second, one atom, and the third minor fraction no phosphorous (Perlmann 1952). The C-terminal amino acid residue was determined in 1955 by Niu and Fraenkel-Contrat, and Narita and Ishii determined portions of the N-terminal sequence in 1962.
In the 1960s, Nuenke and Cunningham believed that the carbohydrate is linked to the protein through an aspartic acid carboxyl group, and they were later able to purify the glycopeptides (Nuenke and Cunningham 1961). It was later determined that an asparagine residue was the site of glycosylation. Because ovalbumin was found to be unstable in aqueous solutions, especially at room temperature, physical studies were difficult. To better study the physical properties and electrophoretic behavior, Winzor and Creeth modified the cysteines residues by reacting the thiol groups with iodine (Creeth and Winzor 1962, and Winzor and Creeth 1962).
In the 1970s, the ovalbumin gene was the first split gene to be discovered (Breathnach et al. 1977). Preliminary X-ray diffraction studies on ovalbumin crystals were performed in the 1980s (Miller et al. 1982).
Recent work has used ovalbumin in the development of a microbicide to prevent the sexual transmission of HIV and other sexually transmissible viruses (Li et al. 2011). Researchers also continue to investigate the mechanisms for regulation of the ovalbumin gene (Dougherty et al. 2009).
Composition
Ovalbumin has four cysteine residues and a single cystine disulfide bridge (Stevens 1991). It is a monomeric protein; however, at high concentrations dimeric and, to a lesser extent, trimeric forms are observed. These three bands correspond to the dephosphorylated, monophosphorylated, and diphosphorylated forms. Electrophoretic separation shows three ovalbumin bands (Lush 1961). The phosphorylation sites are Ser68 and Ser344. A carbohydrate moiety is linked through Asn292. The N-terminus is acetylated. Two polymorphic forms of ovalbumin are known (ovalbumin A and ovalbumin B). Ovalbumin A has an asparagine at position 311, while ovalbumin B has an aspartic acid (Stevens 1991).
Molecular Characteristics
Ovalbumin consists of 385 amino acid residues (Nisbet et al. 1981). It is unique in that its signal sequence is in the middle of the polypeptide chain (residues 234-252) (Lingappa et al. 1979). Interestingly, ovalbumin has been found to have sequence homology with a group of proteinase inhibitors called serpins (30% homology with the archetype member of the family, alpha1-antitrypsin) (Hunt and Dayhoff 1980). However, it differs from this group of inhibitors in that it does not undergo a conformational change upon proteolytic cleavage. Upon proteolytic cleavage, serpins are converted from the S (stressed) to R (relaxed) conformation, and each conformation exhibits different heat stabilities. Ovalbumin does not exhibit these structural changes or differences in heat stability (Stein et al. 1989).
The synthesis of ovalbumin is hormonally induced in the oviduct by the hormone oestrogens (O’Malley et al. 1979). The ovalbumin gene (ov) comprises eight exons and seven introns (McReynolds et al. 1978). Two genes under steroid hormone control have been found within a 46 kb region that also includes the ovalbumin gene. These genes, gene X and Y of unknown function, also have seven introns but are transcribed at a much lower level than ovalbumin mRNA (Royal et al. 1979, and LeMeur et al. 1981). The ovalbumin gene has been a model to study tissue-specific, steroid hormone-induced gene expression for decades; however, the regulation mechanisms of this gene are yet to be determined (Dougherty et al. 2009).