How many different amino acids are commonly found in proteins?

Proof that an imine was formed between aldolase and its substrate was obtained by using D-fructose-1,6-bisphosphate labeled at the C-2 position with ¹⁴C as the substrate. NaBH₄ was added to the reaction mixture. A radioactive product was isolated from the reaction mixture and hydrolyzed in an acidic solution. Draw the structure of the radioactive product obtained from the acidic solution. (Hint: NaBH₄ reduces an imine linkage.)

Draw the product obtained when a lysine side chain in a polypeptide reacts with maleic anhydride.

Draw the tetrapeptide Ala-Thr-Asp-Asn and indicate the peptide bonds.