Heme

Heme is an iron-containing molecular component (a prosthetic group) found in hemoglobin and certain other proteins. It consists of a large ring-shaped organic molecule called a porphyrin, with an iron atom bound at its center. The iron in heme is what binds and carries oxygen.

• Heme is the reason blood is red and able to transport oxygen. Each hemoglobin protein in red blood cells contains four heme groups, and each heme's central iron atom can bind one oxygen molecule - allowing one hemoglobin to carry up to four oxygen molecules from the lungs to tissues.
• Heme is not only in hemoglobin; it's also in myoglobin (the oxygen-binding protein in muscles) and in various enzymes like cytochromes (which use heme to help in electron transport and chemical reactions). In all these cases, the heme group's iron plays a critical role in binding gases or electrons.
• Iron availability is crucial for making heme. This is why iron deficiency can lead to anemia: without enough iron, the body can't form adequate heme groups for hemoglobin, resulting in reduced oxygen-carrying capacity of the blood.

• Frequently asked: "Which part of hemoglobin binds oxygen?" The answer is the heme group (specifically, the iron ion in heme binds oxygen). Remember that without heme, hemoglobin cannot transport O2.
• Questions on iron metabolism or anemia often tie back to heme. For example, an exam might ask why iron deficiency impairs oxygen transport - the explanation is that iron is needed to form heme in hemoglobin. Recognizing heme's role will guide you to the correct reasoning.