Which pair of statements regarding the biosynthesis origins of alkaloids is correct?

• A. Tropane alkaloids are derived from threonine; Quinoline alkaloids are derived from tyrosine
• B. Isoquinoline alkaloids are derived from tyrosine; Indole alkaloids are derived from tryptophan
• C. Tropane alkaloids are derived from threonine; Indole alkaloids are derived from tryptophan
• D. Quinoline alkaloids are derived from tyrosine; Isoquinoline alkaloids are derived from tyrosine

Answer: (B)

The main idea is that alkaloids come from specific amino acids, and those starting points steer the type of nitrogen-containing scaffold they form. Isoquinoline alkaloids, like morphine and related compounds, originate from tyrosine as the starting amino acid, going through intermediates such as tyramine and dopamine to build the benzylisoquinoline framework. Indole alkaloids, including many well-known alkaloids, derive from tryptophan via tryptamine and the monoterpenoid indole alkaloid pathway to form the characteristic indole ring system. Tropane alkaloids actually come from ornithine-derived pathways, not threonine, and quinoline alkaloids do not universally originate from tyrosine. So the statement that isoquinoline alkaloids come from tyrosine and indole alkaloids come from tryptophan correctly pairs the biosynthetic origins with the respective alkaloid classes.