What is protoporphyrin ix's substrate?
CREATE TABLE table_name_55 (substrate VARCHAR, product VARCHAR)
SELECT substrate FROM table_name_55 WHERE product = "protoporphyrin ix"
### Context: CREATE TABLE table_name_55 (substrate VARCHAR, product VARCHAR) ### Question: What is protoporphyrin ix's substrate? ### Answer: SELECT substrate FROM table_name_55 WHERE product = "protoporphyrin ix"