Philosopher AI
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Think about amino acids and enzymes.
To begin with, I am not aware of any enzyme that is an amino acid. Enzymes, the molecules that catalyze chemical reactions in living organisms, are typically proteins or RNA-like nucleic acids. Proteins are large macromolecules consisting primarily of amino acids linked together by peptide bonds; but enzymes themselves do not use this bond for catalysis and may in fact lack it entirely.
In fact, I am not aware of any enzyme that is a typical amino acid. For example, the most common catalytic type for amino acids in proteins are serine and cysteine; these contain an active group attached to a carbon atom with two hydrogen atoms (a thiol), whereas all other amino acids have one or more C-H bonds.
I am aware of carboxypeptidases, which are a sub-type of proteases (enzymes that break peptide bonds). It is true that these enzymes catalyze the cleavage of C-terminal amino acids from proteins. However, since they do not use any active groups for catalysis and instead rely on the degradative removal of C-H bonds by water molecules, it would be more accurate to call them "peptidases". Peptides without an attached N-terminus are referred to as free oligopeptides.
A more interesting question, however, is why your cells have this fixation on catalysing reactions by peptides and proteins. In fact, it appears that RNA-based catalysts are in general far more efficient at these jobs.
This is because the peptide bond, which has a double-bond character and thus breaks relatively easily under catalysis, is generally less stable than typical C-H bonds. This means that it takes more energy input to break a peptide bond than to remove an H atom from an alkane.
When it comes to the ability of enzymes to catalyze reactions, RNA-based catalysis is also more efficient. This is because it relies on base pairs in a double helix structure, which are more stable and thus have less energy disruption than peptide bonds.