Insight into chicken egg proteins and their role in chemical defense mechanism

Abstract

Chicken eggs are considered as complete food having a perfect balance of essential amino acids needed for building and repairing of body tissues. Eggs get infected by vertical as well as horizontal transmission of bacteria, thereby posing risk to human health. However, nature has bestowed an egg with inherent protective mechanism to combat these problems. There are 2 main natural defenses in an egg viz. physical and chemical. The physical defense against bacteria invasion is accomplished by eggshell, together with the cuticle and membranes. The chemical defense consists of proteins that exhibit antimicrobial activity and are mainly present in albumen and to a lesser extent in yolk and shell. Eggshell matrix contains many antimicrobial proteins namely ovocleidin-17, ovocleidin-116, ovocalyxin-21, ovocalyxin-25, ovocalyxin-32 and ovocalyxin-36 and they have shown in vitro antimicrobial activity against many microbes, mostly because of their strong binding affinity for bacterial polysaccharides. Similar to eggshell, the albumen also contains several antimicrobial proteins with important ones being ovotransferrin, ovomucoid, lysozyme, ovoinhibitor, avidin and cystatin. Ovotransferrin exerts antibacterial effect based on iron deprivation method. Ovomucoid causes inactivation of trypsin enzyme and ovoinhibitor inhibit bacterial and fungal serine proteinase. Lysozyme causes splitting of bacterial cell wall peptidoglycan bond between N-acetylmuramic acid and N-acetyl-glucosamine. Cystatin inhibits sulphydryl proteinases while as avidin shows strong binding affinity for biotin. Yolk proteins include lipovitellin, phosvitin and Livetin. Lipoproteins have been reported to be antibacterial and antiviral. Phosvitin has high metal-chelating ability and •-livetin (Immunoglobulin Y) binds and immobilize bacteria, thereby exert antimicrobial effect. Following the exploration of mechanism of action and antimicrobial selectivity of all these chicken egg proteins, they could be used as a viable therapeutic alternative in animal and human production.