When hemoglobin is recycled, what happens to the heme group?

Question

When hemoglobin is recycled, what happens to the heme group?

Answer 1

The heme group is converted to biliverdin and then to bilirubin.  The bilirubin is excreted in bile by the liver.

Answer 2

The heme is converted to bilirubin and excreted in the f***s.

These are the steps:

In the first step, heme is converted to biliverdin by the enzyme heme oxygenase (HOXG). NADPH is used as the reducing agent, molecular oxygen enters the reaction, carbon monoxide is produced and the iron is released from the molecule as the ferric ion (Fe3+).

                         HOXG

                heme --------------> biliverdin + Fe3+

                      /          \

               H+ + NADPH        NADP+

                     O2           CO

In the second reaction, biliverdin is converted to bilirubin by biliverdin reductase (BVR):

                                BVR

                 biliverdin -----------> bilirubin

                             /      \

                     H+ + NADPH    NADP+

Bilirubin is transported into the liver bound to a protein (serum albumin), where it is conjugated with glucuronic acid to become more water soluble. The reaction is catalyzed by the enzyme UDP-glucuronide transferase (UDPGUTF).

                                                 UDPGUTF

                 bilirubin + 2 UDP-glucuronate ------------> bilirubin diglucuronide

                                                       \

                                                    2 UMP + 2 Pi

This form of bilirubin is excreted from the liver in bile. The intestinal bacteria deconjugate bilirubin diglucuronide and convert bilirubin to urobilinogens. Some urobilinogen is absorbed by intestinal cells and transported into the kidneys and excreted with urine. The remainder travels down the digestive tract and is excreted as stercobilinogen, which is responsible for the color of f***s.