**Question:** 1 gm hemoglobin binds with: September 2007

**Core Concept:** Hemoglobin is the protein found in red blood cells that carries oxygen from the lungs to the body's tissues and removes carbon dioxide. It is composed of four globin chains: two alpha (α) and two beta (β) chains.

**Why the Correct Answer is Right:** Hemoglobin is a tetramer composed of two alpha (α) and two beta (β) globin chains. Each chain has a heme group, which is a porphyrin ring containing iron (Fe) in its center. The iron in the heme group can bind to oxygen (O2) to create oxyhemoglobin (HbO2) or bind to carbon dioxide (CO2) to create methemoglobin (HbCO2).

**Why Each Wrong Option is Incorrect:**

A. Oxyhemoglobin binding capacity is not reduced; it is the primary function of hemoglobin.
B. Deoxyhemoglobin (Hb) has a higher affinity for oxygen due to the Bohr effect, but the overall binding capacity remains the same.
C. Methemoglobin does not bind oxygen, making it irrelevant to the binding capacity of hemoglobin.
D. Deoxygenated hemoglobin (Hb) does not directly affect the binding capacity, as the primary function of hemoglobin is to bind oxygen.

**Clinical Pearl / High-Yield Fact:** Hemoglobin's binding capacity is primarily determined by the oxygen-hemoglobin dissociation curve, which is influenced by pH, carbon dioxide, and temperature. This curve illustrates the relationship between the fraction of oxygen-bound (F0) and the fraction of oxygen unbound (FU) in the plasma. The curve has a steep portion (leftward shift) for low pH (acidosis) and high CO2 levels, leading to increased O2 release from Hb, and a shallow portion (rightward shift) for high pH (alkalosis) and low CO2 levels, leading to decreased O2 release from Hb.

**Correct Answer:** Option B (Deoxyhemoglobin)

**Explanation:** Deoxygenated hemoglobin (Hb) has a higher affinity for oxygen due to the Bohr effect. This phenomenon occurs because the negative charge on the heme heme group increases as the pH decreases, making the binding of oxygen to hemoglobin tighter. This leads to a rightward shift in the oxygen-hemoglobin dissociation curve.

**Why Each Wrong Option is Incorrect:**

A. Oxyhemoglobin (HbO2) does not affect the binding capacity of hemoglobin, as its primary function is to bind oxygen.
C. Methemoglobin (HbCO) does not bind oxygen, making it irrelevant to the binding capacity of hemoglobin.
D. Deoxygenated hemoglobin (Hb) does not directly impact binding capacity, as the primary function of hemoglobin is to bind oxygen.