Which part of the hemoglobin molecule is eventually metabolized to stercobilin, giving feces their characteristic brown color?

• A. The heme group
• B. The 2,3-bisphosphoglycerate (2,3-BPG) binding site
• C. The iron ion (Fe^{2+})
• D. The globin protein chains

Answer: (A)

Verified step by step guidance

1. Understand the structure of hemoglobin: Hemoglobin is a protein composed of four subunits, each containing a globin chain and a heme group. The heme group consists of a porphyrin ring with an iron ion (Fe^{2+}) at its center.
2. Learn the fate of the heme group during metabolism: When red blood cells are broken down, the heme group is separated from the globin chains. The globin chains are degraded into amino acids, while the heme group undergoes further processing.
3. Explore the breakdown of the heme group: The heme group is converted into biliverdin, which is then reduced to bilirubin. Bilirubin is transported to the liver, where it is conjugated and excreted into bile.
4. Understand the role of bilirubin in feces coloration: Conjugated bilirubin is further metabolized by intestinal bacteria into urobilinogen. Some urobilinogen is oxidized to stercobilin, which gives feces their characteristic brown color.
5. Identify the correct answer: Based on the metabolic pathway, the heme group is the part of the hemoglobin molecule that is eventually metabolized to stercobilin, contributing to the brown color of feces.