The metal associated with haemoglobin protein is ...............

Explanation

**Step-by-Step Solution:**

1. **Understanding Hemoglobin**: Hemoglobin is a crucial protein found in red blood cells (erythrocytes) that plays a vital role in transporting oxygen from the lungs to the body's tissues.

2. **Function of Hemoglobin**: It binds to oxygen in the lungs, where the concentration of oxygen (PO2) is high, forming a complex known as oxyhemoglobin. This allows for efficient transport of oxygen throughout the body.

3. **Composition of Hemoglobin**: Hemoglobin is a protein made up of four polypeptide subunits. Each of these subunits contributes to the overall function and structure of the hemoglobin molecule.

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