Ovalbumin: Structure, Functions, Denaturation

The ovalbumin is the most abundant protein in the “clear” eggs of birds. It belongs to the family of proteins known as “serpin” or “serine protease inhibitor”, which is a highly diverse group of eukaryotic proteins (includes more than 300 homologous proteins).

It was one of the first proteins isolated with great purity and, thanks to its surprising abundance in the reproductive structures of birds, it is widely used as a “model” in the preparation of “standards” for the study of the structure, the properties, the synthesis and secretion of many proteins.

In percentage terms, ovalbumin comprises between 60 and 65% of the total protein content of egg white, but unlike the other members of the Serpin protein family, it has no activity as a protease inhibitor.

The white of chicken eggs also has other proteins:

– Ovotransferrin, also called conalbumin, which represents 13% of the total protein content of the white

– Ovomucoid, a glycoprotein that comprises 11% of the total

– Ovomucin, another sulfated glycoprotein that represents 3.5%

– Lysozyme or muramidase, which also comprises 3.5% of the total protein of the white

– Globulins, which represent 4%

The synthesis of ovalbumin occurs from a mixture of intermediate peptides during the transit of the egg through the oviduct of birds and there are reports that the transcription of the genes involved only occurs in response to the presence of estrogen, a sex hormone.

Structure

Ovalbumin is a monomeric phosphoglycoprotein with a molecular weight of around 45 kDa and an isoelectric point close to 4.5. In its structure, therefore, there are numerous sites for phosphorylation and glycosylation, which are very common post-translational modifications in proteins.

This protein is encoded by a 7,700 base pair gene, characterized by the presence of 8 exons interspersed with 7 introns, so it is suspected that its messenger undergoes multiple post-transcriptional modifications to yield the mature protein.

The ovalbumin of chicken eggs has 386 amino acid residues and it has been shown that the pure form of this protein consists of three subclasses known as A1, A2 and A3, characterized by containing two, one and no phosphate groups, respectively.

Regarding the tertiary structure, the amino acid sequence of ovalbumin reveals the presence of 6 cysteine ​​residues, between which four disulfide bridges are formed. Furthermore, some structural studies have shown that the N-terminal end of this protein is acetylated.

S- ovalbumin

When the eggs are stored, the structure of ovalbumin is modified, forming what is known in the literature as S- ovalbumin, which is a more stable form against heat and which is formed due to exchange mechanisms between disulfides and sulfhydryls.

In addition to storage temperature, this “form” of ovalbumin is also formed depending on the internal pH of the eggs, which can be expected in any type of protein in nature.

The S- ovalbumin, then, to which is attributed some hypersensitivity reactions experienced by some people after eating eggs.

Features

Despite the fact that ovalbumin belongs to a family of proteins characterized by their activity as protease inhibitors, it does not have inhibitory activity and its function has not been fully elucidated.

However, it has been hypothesized that a potential function of this enzyme is the transport and storage of metal ions to and from the embryo. Other authors have proposed that it also functions as a nutritional source for the embryo during its growth.

From an experimental point of view, ovalbumin represents one of the main “model” proteins for various structural, functional, synthesis and protein secretion study systems, which is why it has been very important for progress in scientific matters .

Functions for the food industry

In addition, since it is one of the most abundant proteins in chicken egg white, this is an extremely important protein for the nutrition of humans and other animals that eat the eggs of different birds.

In the culinary aspect, ovalbumin, as well as the rest of the proteins in egg white, are used for their functional properties, especially for the ability to foam, a process during which the polypeptides are denatured, forming the air interface / stable liquid characteristic of said state of dispersion.

Denaturation

Since ovalbumin has numerous sulfhydryl groups, it is a fairly reactive and easily denatured protein.

The denaturation temperature of ovalbumin is between 84 and 93 ° C, 93 being the one that characterizes the S- ovalbumin form , which is more stable at higher temperatures. Denaturation of ovalbumin by heat results in the formation of the characteristic off-white “gels” seen during egg cooking.

PH is also an important factor when considering the denaturation of this protein, as well as the type and concentration of salts. For ovalbumin, the denaturation pH is around 6.6.

Under different denaturation conditions, ovalbumin molecules have a high tendency to aggregate, a process that can usually be accelerated with the addition of salts and increasing the temperature.

The ability of ovalbumin and other egg white proteins to form gel-like structures when heated, as well as their ability to bind to water molecules and function as emulsifiers, are what give them their most important functional characteristics and which is why they are so exploited especially in the food industry.

The denaturation process of this protein has been very useful for investigating the transition mechanisms between solid and gel states, as well as for studying the effect that different types of salts have at different concentrations (ionic strength) on integrity of proteins.

References

  1. Huntington, JA, & Stein, PE (2001). Structure and properties of ovalbumin. Journal of Chromatography B: Biomedical Sciences and Applications, 756 (1-2), 189-198.
  2. Koseki, T., Kitabatake, N., & Doi, E. (1989). Irreversible thermal denaturation and formation of linear aggregates of ovalbumin. Food Hydrocolloids, 3 (2), 123-134.
  3. Nisbet, AD, SAUNDRY, RH, Moir, AJ, Fothergill, LA, & Fothergill, JE (1981). The complete amino-acid sequence of hen ovalbumin. European Journal of Biochemistry, 115 (2), 335-345.
  4. Phillips, GO, & Williams, PA (Eds.). (2011). Handbook of food proteins. Elsevier.
  5. Remold-O’Donnell, E. (1993). The ovalbumin family of serpin proteins. FEBS letters, 315 (2), 105-108.
  6. Sankar, DS, & Theis, HW (1959). Biosynthesis of ovalbumin. Nature, 183 (4667), 1057.
  7. Sharif, MK, Saleem, M., & Javed, K. (2018). Food Materials Science in Egg Powder Industry. In Role of Materials Science in Food Bioengineering (pp. 505-537). Academic Press.
  8. Weijers, M., Barneveld, PA, Cohen Stuart, MA, & Visschers, RW (2003). Heat-induced denaturation and aggregation of ovalbumin at neutral pH described by irreversible first-order kinetics. Protein science: a publication of the Protein Society, 12 (12), 2693–2703.

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