FUNCTIONAL HEALTH ATTRIBUTES OF DIFFERENT EGG WHITE PROTEINS
Authors: Arun Kharate, Vittal , Mamatha S.P. and Sarita
- Arun Kharate , Associate professor (I/C) Dept. Of VPH, Veterinary College Bidar
- Vittal , Senior Veterinary Officer , AHVS
- Mamatha, Assistant professor Dept. Of VPH, Veterinary College Bidar
- Sarita, Assistant professor, Dept. Of VPB, Veterinary College Bidar
Email: : kharatearunvet@gmail.com
msp89353@gmail.com Contact no- 6361126643
Saritapasara@gmail.com Contact no- 8884722058
INTRODUCTION
Scientific evidence and consumer awareness of the importance of diet in combination with a healthy lifestyle has led to an increased focus on the nutritional evaluation of certain foods such as eggs. For centuries, eggs have been a very important source of food owing to their high content of proteins and other essential nutrients, and this is still the case today. Eggs are an important part of the human diet both as shell eggs and as an ingredient in numerous prepared foods.
Egg white or albumen is deposited by the tubular glands around the developing oocyte during its passage through the oviduct, and this is followed by the deposition of shell by the shell gland. In a typical egg of the domestic fowl, the egg white makes up 58% by volume, and contains ~50% of the total egg protein. It consists of an aqueous protein solution, containing few minerals and carbohydrates. Proteins represent more than 90% of the dry matter of egg white, but until very recently, only the major ones had been identified. However, the recent and powerful techniques for separation and analysis have enabled the identification of many minor proteins. The egg white proteins are predominantly globular proteins, and acidic or neutral, except for lysozyme and avidin, which are highly alkaline proteins. All are glycosylated, excepting cystatin and the major form of lysozyme. Some of them are very heat-sensitive and/or sensitive to surface denaturation, explaining their noteworthy functional properties.
Ovalbumin
The major egg white protein (more than 50% of the total proteins) is ovalbumin, a 45 kDa globular and phosphorylated protein. Half of its amino acids are hydrophobic, and one-third are electrically charged, essentially negatively at physiologic pH. Ovalbumin possess six buried Cys residues, two being involved in a disulfide bridge (Cys73–Cys120). Ovalbumin is then the principal egg white protein with free thiol groups, capable of inducing some rearrangements with variations of storage conditions, pH and surface denaturation. It contains all nutritionally essential amino acid. The carbohydrate is mainly D- mannose. The native albumen is converted to a more heat stable S-ovalbumen during storage of eggs.
Conalbumin
This is a glycoprotein (does not contain phosphorus).The protein moieties of conalbumin and transferrin of chicken blood serum is identical hence, it is also called transferrin. It has ability to bind di- or tri valent metal ions, particularly iron, Fe 3+ and Al 3+, Cu 2+, Zn 2+ and render them nutritionally unavailable to microorganism. Hence it acts as antibacterial factor.
Ovotransferrin
This is a protein with a molecular weight around 78 kDa. The function of ovotransferrin is generally accepted as that of iron transport. The transferrin molecule interacts with cell surface receptors to transfer iron into cells. This protein consists of two lobes, each containing a specific binding site for iron (or copper, zinc, aluminum). It is the most heat-sensitive egg white protein, but the complexation of iron or aluminium significantly increases its heat stability.
Ovomucin
It is also a highly glycosylated protein, with a very high molecular weight (104 kDa). It is water insoluble, fibrous (microscopic fibres) glycoprotein (muco protein) and is responsible for jelly like structure of thick albumen. Ovomucin makes up 1.5% of the total protein of egg white. It is very largely responsible for conferring the high viscosity of egg white. It therefore plays a role in maintaining the structure of egg white, and it has been shown that the thick egg white, which makes up the outer 50% of the volume has a higher ovomucin content than that of the thin egg white. As the pH of egg albumen approaches the isoelectric pH (10.7) of lysozyme, the ovomucin-lysozyme complex formation decreases, leading to thinning of thick white.
Lysozyme
It is a small (14 kDa) globular, and strongly basic protein. It is an egg white protein having enzymatic property. Its structure is very rigid, stabilized by four disulfide bridges. It causes destruction of mainly Gram +ve bacteria by splitting β(1-4) linkages between N- acetyle-muramic acid and N- acetyl glucose amine, which are component making up bacterial cell wall. However, lytic action of this enzyme on Gram –ve bacteria likes E. coli (enteripathogenic), Salmonella, Aeromonas and Pseudomonas is limited extent. Gram +ve bacteria is sensitive to lytic action of lysozyme belonging to genera Bacillus, Micrococcus, Lactobacillus, Serratia,Corynebacterium and Staphylococcus. However, the cell wall of Staphylococcus is more resistant.
Avidin
It binds with biotin in raw egg and renders biotin (B complex vitamin) unavailable. About 3 moles of biotin are bound to one mole of avidin. Cooking the egg breaks down avidin-biotin complex and makes the latter nutritionally available.
Ovoglobulin
G2 and G3 globulin fractions are called ovoglobulins and are distinct from G1 globulin (called lysozyme).These are excellent foaming agents.
PROTEASE INHIBITORS
A major group of proteins present in egg white are proteinase inhibitors. Of these, ovomucoid is the most abundant and most extensively studied, but recently other less abundant inhibitors have been studied. Although the functions of these are not known for certain, it is believed they may have a protective role against bacterial proteinases. Proteinases can be classified according to the nature of their catalytic site into serine proteinases, thiol proteinases, acid proteinases (having aspartic acid at the active site) and proteinases requiring metal ions. Of these the serine proteinases are most widespread. Ovomucoid and ovoinhibitor are both inhibitots of serine proteinases, cystatin is an inhibitor of thiol proteinases, and ovostatin inhibits a variety of proteinases.
Ovomucoide
It is a highly glycosylated protein (up to 25% carbohydrates, w/w) of 28 kDa. At pH 7, its denaturation temperature is around 77 °C, but this protein is much more heat resistant at acidic pH. Ovomucoid is trypsin inhibitor i.e. less than one molecule of ovomucoid can reduce the activity of one molecule of digestive trypsin enzyme by 50%. Hence it is antibacterial factor in egg.
Ovoinhibitor
Ovoinhibitor is also an inhibitor of serine proteinases, and is similar to ovomucoid in its properties. It is larger than ovomucoid, having an M, value of 49,000, and comprising seven domains, each having a similar arrangement of disulphide bridges to that of ovomucoid. Ovoinhibitor from domestic fowl is able to inhibit trypsin, ~-chymotrypsin, subtilisin and Aspergillus oryzae alkaline proteinase. One molecule of ovoinhibitor is able to inhibit two molecules of trypsin and two of chymotrypsin, each binding to different domains. A possible advantage of having a number of domains, each of which inhibits proteinases is that a wider range of proteinases may be inhibited, and so afford a greater measure of protection from microorganisms. It is a proteolytic enzyme inhibitor which is distinct from ovomucoid. Ovoinhibitor is capable of inhibiting the activity of trypsin, chymotrypsin, digestive enzymes as well as fungal and bacterial proteases. Hence it acts as antibacterial agent.
Cystatin
Cystatin was first isolated in small quantities from egg white and known as ficin inhibitor on account of its properties. Cystatin inhibits a number of cysteine proteinases including ficin, papain, cathepsin B, cathepsin H, cathepsin L and dipeptidyl peptidase I, but not clostipain or streptococcal proteinase, and it only weakly inhibits bromelain.
Ovostatin
Ovostatin (formerly known as ovomacroglobulin) is a large molecule having a tetrameric structure (M, 780,000 = 4 × 195,000). It inhibits a wide range of endoproteinases including thermolysin (a metal-ion requiring proteinase) and collagenase. Its structure and mechanism of action is like that of the serum proteinase inhibitor, ~2-macroglobulin. The proteinases first cleave a bond within ovostatin, which then undergoes a conformational change so as to hinder the access of large, but not small substrate molecules to the catalytic site. Ovostatin from both the domestic fowl and the duck have been studied, the latter inhibits both metalloproteinases and serine proteinases, whereas the former inhibits metallo proteinases only.
Flavoprotein
It binds all the riboflavin of egg albumen. Albumen contains about 1% in equal amount of flavoprotein and apoprotein (non-riboflavin containing protein).Thus flavoprotein is an antimicrobial agent due to its ability to bind all the riboflavin. Although apoprotein does not contain riboflavin but it can bind very tightly riboflavin to transfer it for the blood serum to the albumen.
BIOACTIVE PROPERTIES OF EGGS
Antimicrobial activities
Egg antimicrobials in edible parts are essentially concentrated in egg white and the vitelline membrane. Depending on the protein considered, these antimicrobials may exhibit antibacterial, antiviral, antifungal, or antiparasitic activities. Their antibacterial effect relies on several bactericidal or bacteriostatic mechanisms. Some of them have a powerful activity via interaction with bacterial walls that further triggers permeabilization and bacterial death (lysozyme). The effects of the other molecules are rather indirect by decreasing the bioavailability of iron (ovotransferrin) and vitamins (avidin) that are required for some microbial growth, and by inhibiting microbial proteases that are virulent factors of infection (ovoinhibitor, cystatin). In addition to these egg proteins and peptides, there are increasing data reporting the antimicrobial activity of egg-derived peptides that may be released after partial hydrolysis by exogenous proteases. Such hydrolytic peptides obtained from lysozyme, from ovotransferrin, from ovomucin, and from cystatin have shown a broad range of antibacterial activities.
Among egg white proteins, lysozyme and ovotransferrin have been the most studied for this type of application. Lysozyme from egg albumen catalyzes the hydrolysis of the glycosidic ß-(1–4) linkage between N-acetylmuramic acid and N-acetylglucosamine of the bacterial peptidoglycan, which is the structural component of Grampositive bacterial cell walls. The peptidoglycan can be thought to be a strong, woven mesh that allows solutes to pass through but maintains the cell shape and protects cell from osmotic lysis. Without a wall or if it is attacked by lysozyme the cell would swell and burst. Cell death occurs by the lytic action of lysozyme on peptidoglycan only when in low osmotic-strength media, or when the rate of the synthesis and polymerization process for new peptidoglycan formation is slower than the lysozyme catalyzed degradation. Gram-negative bacteria are resistant to the lytic action of lysozyme because they possess an additional outer membrane. Their cell wall includes the outer membrane, lipoproteins, lipopolysaccharides and peptidoglycan layers, protected from the lytic action of lysozyme because the outermost surface acts as a permeability barrier.
Lysozyme is particularly effective against oral affections, due to the cariogenic bacteria Streptococcus mutans or against periodontitis-associated bacteria. Given orally, lysozyme alleviates the reduced endogenous production of saliva lysozyme during parodontopathy. Lysozyme is also used extensively as a food preservative. It shows high activity against mesophilic and thermophilic spore-forming bacteria such as Bacillus staerothermophilus, Clostridium thermosaccharolyticum and Clostridium tyrobutyricum. It is used in cheese to prevent contamination because it does not inhibit on starter and secondary cultures required for the ripening of cheeses. Lysozyme also prevents the growth of pathogenic bacteria on refrigerated foods: Listeria monocytogenes, Clostridium botulinum, Clostridium jejuni and Yersinia enterocolitica are susceptible to the lytic activity of lysozyme (Johnson, 1994). Given orally, after local application, or when combined with specific immunotherapy, lysozyme is effective in a wide range of viral skin diseases: herpetic lesions, verruca vulgaris and plantaris, aphtous stomatitis, polymorphous exudative erythema, molluscum contagiosum. Moreover, when combined with specific immunotherapy, lysozyme cures viral sinusitis or bronchitis. The antiviral action of lysozyme has partly been explained by its role on the precipitation of viral particles and by its immune-enhancing action on the host together with its interaction with the pathogens.
Ovotransferrin from egg albumen inhibits Gram-negative bacteria by depriving bacteria of iron that is essential for their growth. Iron participates in many major biological processes and some bacteria deposit intracellular reserves that can then be used to enhance growth when external iron supplies are restricted. Ovotransferrin belongs to the family of transferrins, a metal-binding transport protein family with an in vivo preference for iron, and widely distributed in physiological fluids. It is proposed that ovotransferrin antimicrobial activity can result from a direct effect on the membranes: interaction of the cationic ovotransferrin with the anionic outer membrane of Gram-negative bacteria. Some suggested that ovotransferrin antibacterial activity can result from a direct effect on the membranes of Candida albicans or Salmonella Enteritidis. Its strong bactericidal activity against both Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) strains led the authors to envisage therapeutical applications for this natural peptide. More recently studies showed that transferrins are able to permeate the outer membrane of E. coli and to access the inner membrane where they cause permeation of ions in a selective manner. Ovotransferrin has therefore been proposed for infant nutrition and for the treatment of infants with acute diarrheas. Recently, the iron chelating activity of ovotransferrin has been shown to increase the stimulation (by inhibition of AMPc b-lactamase) of some antibiotics which are efficient against most b-lactamase-producing bacteria. Ovotransferrin therefore appears to be a key factor for drug associations able to overcome cephalosporin resistance.
Furthermore other proteins belonging to egg white and possessing antiprotease activities could be involved in the antibacterial defence of eggs mainly by inhibiting bacterial proteases that are secreted by pathogens during host colonization process. Among these proteins we can cite ovostatin, ovomucoid and cystatin. Finally other proteins binding vitamins such as riboflavin-binding protein, avidin and thiamin binding protein have antimicrobial properties by chelating riboflavin, biotin and thiamine respectively. All these proteins can have a synergistic effect on microbial development and strategies to purify them individually have to be linked for best efficiency for use in pharmaceutical, cosmetic or food industries.
Antihypertensive activities
Considering the prevalence and importance of hypertension worldwide, there is increasing ongoing research to find ways to regulate this multifactorial disease. At the population level, the most important factors of long-term control of blood pressure are sodium and potassium intakes and the importance of the renin-angiotensin-aldosterone system. Eggs are generally recognized as a very important source of components possessing blood pressure lowering effects. In this area, a wide range of peptides derived from food proteins are useful in the prevention and/or treatment of hypertension. Most of these peptides acts as inhibitors of angiotensin converting enzyme (ACE), an exopeptidase that cleaves dipeptides from the C-terminal side of various oligopeptides. ACE hydrolyzes angiotensin I to the potent vasoconstrictor angiotensin II. ACE also takes part in the kinin–kallikrein system, as it hydrolyzes bradykinin, which has a vasodilator action.
Concerning egg proteins, two peptides derived from ovalbumin are effective in preventing hypertension. The first one, named ovokinin, was purified from a peptic digest of ovalbumin (residues 358–365). It is a weak bradykinin B1 agonist peptide, which lowers the systolic blood pressure in spontaneously hypertensive rats when administered as an emulsion in egg yolk. The second one, ovokinin (2–7) is obtained by chymotryptic digestion of ovalbumin (residues 359–364). This hexapeptide corresponds to the 2–7 fragment of ovokinin (RADHPF) was purified from a chymotrypsin digest of ovalbumin. When orally administered, this vasorelaxing peptide reduces the blood pressure of spontaneously hypertensive rats in a dose-dependent manner. Ovokinin (2–7) has also been used for designing peptide derivatives with activity comparable to that of synthetic anti-hypertensive drugs used clinically. The vasodilation due to ovokinin (2–7) is mediated by nitric oxide and an unknown receptor. Ovokinin (2–7) exerted a dose-dependent vasodilation in mesenteric arteries from hypertensive rates and exhibited hypotensive activities after the oral administration of 10 mg/kg to SHR.
Antioxidant properties
Long-term oxidative stress in the gastrointestinal tract can lead to chronic intestinal disorders and there is increasing interest in investigating the potential of food-derived antioxidants, including egg antioxidants, in intestinal health. Chicken egg contains many antioxidant compounds that encompass vitamins, carotenoids, minerals, and trace elements but also major egg-white proteins such as ovotransferrin, in its native form or as hydrolytic peptides, ovomucoid and ovomucoid hydrolysates, ovomucin hydrolysates and derived peptides. Ovotransferrin is an excellent metal ion-binding protein. These binding properties allow biological properties, particularly antioxidant and antibacterial activities. The biological role of ovotransferrin is generally accepted as iron transport but it can bind other metal ions such as aluminum. Few studies are concerned with ovotransferrin antioxidant activity although some reported that ovotransferrin iron-binding ability may have an indirect role in prevention of lipid oxidation.
Anti-Cancerous Molecules
There are only few data showing that food-derived proteins and peptides can also be beneficial to prevent and to cure cancer diseases. Several studies have confirmed the tumor-inhibitory activity of egg white lysozyme using experimental tumors. Its effect essentially relies on immunopotentiation. Ovomucin (beta subunit) and ovomucin-derived peptides also showed anti-tumor activities via cytotoxic effects and activation of the immune system. The anticancerous effect of egg tripeptides and hydrolytic peptides from ovotransferrin have also been published. Information in this field is quite scarce, but it may be worth continuing to investigate such activities. Some interesting data may arise from studies on egg protease inhibitors since similar molecules existing in other food product, including legumes like pea, have been described as potential colorectal chemopreventive agents.
Immunomodulatory Activities
Several egg proteins have potential immunomodulatory activities. Among these, egg-white lysozyme is a promising agent for the treatment of inflammatory bowel disease. In a colitis porcine model, lysozyme was demonstrated to significantly protect animals from colitis and reduce the local expression of pro-inflammatory cytokines while increasing the expression of the anti-inflammatory mediators. Sulfated glycopeptides generated by proteolysis from ovomucin, chalazae, and yolk membrane can exhibit macrophage-stimulating activities in vitro. Cytokines, such as egg-white pleiotrophin, play a pivotal role in the generation and resolution of inflammatory responses. In human, pleiotrophin have been shown to promote lymphocyte survival, and to drive immune cell chemotaxis. But, the biological significance of the potential immunomodulatory activity of egg white pleiotrophin in human intestine remains very speculative. In contrast, some valuable immunomodulatory activities might emerge from ovotransferrin and egg yolk vitellogenin hydrolysates after partial degradation by digestive proteases.
CONCLUSION
Egg is rich in proteins which have a tremendous potential to nutritiously assist human beings. However, only a part of the treasures have been uncovered till now. The remaining is yet to follow with the help of advanced techniques. It is highly imperative to consider egg as an essential part of a balanced diet.
REFERENCES
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Miranda, J.M., Anton, X., Redondo-Valbuena, C., Roca-Saavedra, P., Rodriguez, J.A., Lamas, A., Franco, C.M. and Cepeda, A., 2015. Egg and egg-derived foods: effects on human health and use as functional foods. Nutrients, 7(1), pp.706-729.
Réhault-Godbert, S., Guyot, N. and Nys, Y., 2019. The golden egg: nutritional value, bioactivities, and emerging benefits for human health. Nutrients, 11(3), p.684.
Seuss-Baum, I. and Nau, F., 2011. The nutritional quality of eggs. In Improving the safety and quality of eggs and egg products (pp. 201-236). Woodhead Publishing.
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