Detailed Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Problem:Passage: Hemoglobin is a compound in the class of compounds called proteins. Proteins are one of four classes of biochemical compounds, which are compounds in living things. (The other three classes are carbohydrates, lipids, and nucleic acids. ) Proteins contain carbon, hydrogen, oxygen, nitrogen, and sulfur. Protein molecules consist of one or more chains of small molecules called amino acids. Answer: four
Solution:
How many classes of biochemical compounds are there?