The
Molecular Characteristics:
Ovalbumin consists of (Nisbet et al. 1981). It is unique in that its signal sequence is in the middle of the polypeptide chain ) (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, (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).
Composition:
Ovalbumin has four cysteine residues and a single cystine disulfide bridge (Stevens 1991). Electrophoretic separation shows three ovalbumin bands (Lush 1961). These three bands correspond to the dephosphorylated, monophosphorylated, and diphosphorylated forms. 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).
Protein Accession Number: P01012
Molecular Weight: 85.5 kDa (Theoretical)
Isoelectric Point: 4.5 (Stevens 1991)
Extinction Coefficient:
61,180 cm-1M-1
E1%,280 = 7.16 (Theoretical)
Applications:
Medicinal characteristics:
In cases where poisoning by heavy metals (such as Iron) is suspected, ovalbumin may be administered. Ovalbumin chelates to heavy metals and traps the metal ions within the sulfhydryl bonds of the protein. Chelating prevents the absorption of the metals into the gastrointestinal tract and prevents poisoning.
