 Quaternary structure : the structure formed by several protein molecules ( polypeptide chains ) , usually called protein subunits in this context , which function as a single protein complex . 
 Quaternary structure : the structure formed by several protein molecules ( polypeptide chains ) , usually called protein subunits in this context , which function as a single protein complex . 
Quaternary structure : the structure formed by several protein molecules ( polypeptide chains ) , usually called protein subunits in this context , which function as a single protein complex .