22.11
ヘモグロビン
ヘモグロビンは、4つのサブユニットで構成される球状タンパク質です。これらのサブユニットのうち2つはアルファ鎖であり、他の2つはベータ鎖です。各サブユニットには、鉄原子を持ち、酸素に結合できるヘムの分子が含まれています。酸素分子が1つのヘム基に結合すると、ヘモグロビンの形状が変化し、他のヘム基も酸素と…
ヘモグロビンは、2本のアルファポリペプチド鎖と2本のベータポリペプチド鎖からなる四量体球状タンパク質です。
これらの各鎖は、酸素を可逆的に結合することができる鉄含有ヘム色素分子を有する。
酸素分子が最初のヘム基に結合すると、ヘモグロビンの立体構造変化が誘導され、残りのヘム基が酸素と容易に結合できるようになります。
4つのヘム基すべてが酸素と結合した後、酸素飽和ヘモグロビンはオキシヘモグロビンと呼ばれ、動脈血中の赤血球に真っ赤な色合いを与えます。
逆に、ヘモグロビンは組織の毛細血管を通過する際に結合した酸素を失い、デオキシヘモグロビンになります。そのため、酸素の少ない静脈血は暗赤色に見えます。
組織内の二酸化炭素濃度が高いため、ヘモグロビンは酸素を放出すると二酸化炭素と容易に結合し、血液中の二酸化炭素輸送の約20%を占めるカルバミノヘモグロビンを形成します。
この血液が高酸素濃度で肺に戻ると、ヘモグロビンは容易に二酸化炭素を放出し、酸素に結合します。
呼吸器ガス以外にも、デオキシヘモグロビンは一酸化窒素に結合し、血管拡張を引き起こし、血流を改善します。
View the full transcript and gain access to JoVE Core videos
Q1: What is the basic structure of hemoglobin?
Hemoglobin is a tetrameric globular protein composed of four polypeptide chains: two alpha chains and two beta chains. Each chain contains a heme pigment molecule with an iron atom capable of reversibly binding oxygen. This quaternary structure enables hemoglobin to transport oxygen efficiently throughout the body as part of the structure and function of erythrocytes.
Q2: How does oxygen binding to hemoglobin change its properties?
When an oxygen molecule binds to the first heme group, it induces conformational changes in hemoglobin that make it easier for the remaining heme groups to bind oxygen. Once all four heme groups bind oxygen, the resulting molecule is called oxyhemoglobin, which gives arterial blood its characteristic bright red color.
Q3: Why does venous blood appear darker than arterial blood?
As blood passes through tissue capillaries, hemoglobin releases its bound oxygen and becomes deoxyhemoglobin. This oxygen-poor form of hemoglobin gives venous blood a darker red appearance compared to the bright red of oxygen-saturated arterial blood. The color difference reflects the oxygen-binding state of hemoglobin in each blood type.
Q4: What role does hemoglobin play in carbon dioxide transport?
Due to high carbon dioxide concentration in tissues, hemoglobin readily binds carbon dioxide upon releasing oxygen, forming carbaminohemoglobin. This accounts for approximately 20% of total carbon dioxide transport in the blood. When blood returns to the lungs with high oxygen concentration, hemoglobin releases carbon dioxide and binds oxygen instead.
Q5: How does hemoglobin binding to nitric oxide affect blood flow?
Deoxyhemoglobin binds to nitric oxide, a signaling molecule that causes vasodilation, or widening of blood vessels. This binding improves blood flow and represents an additional physiological function of hemoglobin beyond oxygen and carbon dioxide transport. Nitric oxide binding demonstrates hemoglobin's role in regulating vascular function.
Q6: What is the difference between oxyhemoglobin and deoxyhemoglobin?
Oxyhemoglobin is hemoglobin with all four heme groups bound to oxygen, appearing bright red and found in arterial blood. Deoxyhemoglobin is hemoglobin that has released its oxygen in tissues, appearing darker red and found in venous blood. The two forms represent different functional states of the same protein.
Q7: Why is the iron atom in heme important for hemoglobin function?
The iron atom within each heme pigment molecule is essential because it enables reversible binding of oxygen and other gases like carbon dioxide and nitric oxide. This reversible binding capability allows hemoglobin to pick up oxygen in the lungs and release it in tissues, making iron central to hemoglobin's role in gas transport.