Amino Acids

Amino acids are linked together by peptide bonds (co-NH) forming a polypeptide chain, which is the building block of protein.
In addition to this role amino acids when freed of the bond serve as precursors to many important small molecular compounds, including nucleotides*, porphyrins, parts of lipid molecules, several co-enzymes and other Biomolecules.
*Nucleotides-Ester of Nucleoside and phosphoric acid, structural unit of nucleic acid contains sugar, purine and pyrimidine.
As an alternative energy source, amino acids can be degraded in ways that regenerate ATP from Adenosine Diphosphate=ADP or Adenosine Monophosphate=AMP.