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

Pseudomonas (bacterial) cytochrome C differs from eukaryotic (human) cytochrome in the total number of amino acid residues, in composition and in the order of residues in the large segment of amino acid sequence' Comparison of primary protein structure with the predicted tertiary structure revealed that functionally crucial residues fold closely together in 3D space (tertiary structure), even though some of these residues were distant to each other in the primary structure. The tertiary structure of cytochrome C's are highly conserved and explain for functional plausibility.