haemoglobin structure

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Haemoglobin is a quaternary structure protein consisting of multiple polypeptide chains. It has non-covalent interactions among the chains, including hydrogen bonds, electrostatic attractions, and hydrophobic bonds, which allow small structural changes in one site to affect another—a characteristic of allosteric proteins.

As the oxygen-binding protein in red blood cells, haemoglobin differs from myoglobin as it has a quaternary structure with two identical alpha chains (141 amino acids each) and two identical beta chains (146 amino acids each). This arrangement results in what is referred to as two alpha-beta dimers. In contrast, fetal haemoglobin has two alpha chains and two gamma chains.

Each chain contains an iron-containing heme unit, allowing haemoglobin to bind up to four molecules of oxygen, compared to myoglobin's one. Both proteins exhibit reversible oxygen binding. The cooperative reaction means that once one heme group binds oxygen, it facilitates binding in the others.

Haemoglobin exists in two states: T state (tense, more stable when deoxygenated) and R state (relaxed, stabilized by oxygen binding). This structural change, referred to as protein breathing, demonstrates how heme groups interact indirectly through surface interactions. The gap in the T state is wider, indicating that when oxygen is absent, the structure becomes tense, akin to experiencing stress without enough oxygen. Understanding these two states is crucial as they relate to the oxygen-binding capacity of haemoglobin and its functional implications for oxygen transport.