Cytochrome C of the bacteria has 50% identity of amino acid sequence with that of human. Which of the following is the most conserved parameter in these 2 proteins

Cytochrome c occurs only in eukaryotes, but prokaryotes contain proteins, known as c-type cytochromes, which perform the same general function (that of an electron carrier). The major structural differences among c-type cytochromes lie in the various polypeptide loops on their surfaces. The sequences of the c-type cytochromes have diverged so far from one another that, in the absence of their X-ray structures, they can be properly aligned only through the use of mathematically sophisticated computer programs. Thus, it appears that the essential structural and functional elements of proteins, rather than their amino acid residues, are conserved during evolution. Ref : Voet Fundamentals of Biochemistry 4th Edition Pg no 151-152.