Imagine that you are looking at the amino acid sequences of the same protein from many different organisms. All of the organisms have a Cys at the same place in the protein sequence. What does that tell you about the role of that Cys in the protein? All of the organisms have a stretch of 3 amino acids that are Arg or Lys-Ser or Thr-Arg or Lys. What does that tell you about this part of the protein? Another part of the protein seems to have all of the possible amino acid sequences in the different organisms. What does that tell you about that part of the protein? Solution Answer: 1) Cysteine (Cys) and Methionine are the sulphur containng aminaocids. Cysteine presence in the teritary structure of a protein is a common thing due to formation of the disulphide linkage. Disulphide likages are formed by cysteine only but not methionine. The thiol group is oxidised to form disulphide linkage. 2) Arg or Lys-Ser, Thr-Arg or Lys From above sequence, it can be concluded that Basic group containing aminoacids such as Arginine (Arg) and Lysine (Lys) are forming bond with hydroxyl group containg aminoacids such as Serine (Ser) and Threonine(Thr). As basic aminoacids are insoluble in water, so they participate in protein structure by association with polar aminoacids (water loving) in the protein. 3) There are 20 aminacids with 64 codons which code for each aminoacid. Proteins seems to have all possible aminoacid sequences in different organisms. The aminoacids sequence of different protein vary with there functional properties, each protein function is based on the teritary arrangement of the polypeptide. Every gene has specific property and it codes for respective protein which is essential biological system, similarly, every enzyme has its owns aminoacid sequence to exhibit its catalytic activity..