Possible Clues:

• Essential-acid link
• With 48 Down, protein building block
• Compound containing an NH2 group, informally
• Protein builder, informally
• With 113 Down, compound produced through catabolism
• __ acid (bit of protein)
• ___ acid sequence
• Important acid type
• Protein-building acid type
• With 22-Across, protein building block