Please learn the 3 letter code for the amino acids. Please consult the Jmol site below dealing with amino acids. Hence the diversity of possible proteins is enormous. Compare this to just 65536 different oligonucleotides (4 different monomeric deoxynucleotides) of 8 monomeric units, an 8mer (84). Just for an octapeptide, there are over 25 billion different possible arrangement of amino acids (820). In these cases, the codes asx (B) and glx (Z) are respectively used. In some cases, distinguishing between asparagine/aspartic acid and glutamine/glutamic acid is difficult. Both the sequence of a protein and it's total length differentiate one protein from another. Selenocysteine is often referred to as the 21st amino acid, but is encoded in a special manner. In contrast, nucleic acids are polymers of just 4 different monomeric nucleotides. (Note: above picture represents the amino acid in an unlikely protonation state with the weak acid protonated and the weak base deprotonated for simplicity in showing removal of water on peptide bond formation and the hydrolysis reaction.) Proteins are polymers of twenty naturally occurring amino acids.
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