Haemoglobin

Key point: Haemoglobin consists of a tetramer of myoglobin-like subunits, with four Fe sites that bind O₂ cooperatively. Haemoglobin (Hb, 68 kg mol^–1, Fig. 27.17) is the O₂ transport protein found in special cells known as erythrocytes (red blood cells): a liter of human blood contains about 150 g of Hb. Simplistically, Hb can be thought of as a tetramer of myoglobin-like units with a cavity in the middle. There are in fact two types of Mb-like subunits, which differ slightly in their structures, and Hb is referred to as an a2 b2 tetramer. The O₂ binding curves for Mb and Hb are shown in Fig. 27.18: it is highly significant that the curve for Hb is sigmoidal, which indicates that uptake and release of successive O₂ molecules is cooperative. At low O₂ partial pressure and greater acidity (as in venous blood and muscle tissue following aggressive exercise) Hb has low affinity for O₂. This low affinity enables Hb to transfer its O₂ to Mb. As the pressure increases, so does the affinity of Hb for O₂ and as a result Hb can pick up O₂ in the lungs. This change in affinity can be attributed to there being two conformations. The tensed state (T) has a low affinity and the relaxed state (R) has a high affinity. Deoxy-Hb is T and fully loaded oxy-Hb is R. To understand the molecular basis of cooperativity we need to refer to Fig. 27.15. Binding of the first O₂ molecule to the T-state molecule is weak, but the decrease in the size of the Fe allows it to move into the plane of the porphyrin ring. This motion is particularly important for Hb because it pulls on the proximal histidine ligand and helix F moves. This movement is transmitted to the other O₂ binding sites, the effect being to move the other Fe atoms closer to their respective ring planes and thereby convert the protein into the R state. The way is thereby opened for them to bind O₂, which they now do with greater ease, although the statistical probability decreases as saturation is approached.

Figure 27.16 The orbitals used to form the Fe O2 adduct of myoglobin and haemoglobin. This model considers the O2 ligand to be in a singlet state, in which the full 2πg orbital donates an electron pair and the other 2πg orbital acts as a p-electron pair acceptor.

Figure 27.17 Haemoglobin is an α₂β₂ tetramer. Its and subunits are very similar to myoglobin.

Figure 27.18 Oxygen binding curves for myoglobin and haemoglobin showing how cooperativity between the four sites in haemoglobin gives rise to a sigmoidal curve. The binding of the first O₂ molecule to haemoglobin is unfavourable, but it results in a greatly enhanced affinity for subsequent O₂ molecules.

اخر الاخبار

اخبار العتبة العباسية المقدسة

الهيأة العليا وجمعية العميد تعقدان اجتماعا لمناقشة آليات التعاون العلمي المشترك

لتقديم الدعم النفسي والمعنوي قسم الشؤون الطبية يزور أطفال مرضى السرطان في مستشفى الإمام الحسن (عليه السلام)

قسم الشؤون الفكرية يهدي إصداراته الحديثة إلى مكاتب المراجع وعدد من المؤسسات في بابل

عبر مركز الكفيل للصحة والسلامة.. قسم الشؤون الطبية يستأنف ورش الإسعافات الأوّلية للمنتسبين

المزيد

الآخبار الصحية

سرّ بسيط لصحة القلب.. أطعمة تخفض الكوليسترول خلال أسابيع

سبب خفي لإعتام عدسة العين ؟

عوامل خفية تسرّع شيخوخة الدماغ

أسباب الرغبة الشديدة في تناول الحلويات والأطعمة النشوية

المزيد

الآخبار التكنلوجية

كندا تصنع كاسحة جليد من جيل جديد

رواد "أرتيميس 2" يكشفون تفاصيل حياتهم في الفضاء

حفريات صينية تكشف كائنات بحرية عاشت قبل 546 مليون عام

روسيا تطلق صاروخ "سويوز-2.1 إيه" من قاعدة بليسيتسك الفضائية

المزيد

مواضيع ذات صلة

Heating

Expansion against constant pressure

The general expression for work

Work, heat, and energy

Mixtures of gases

Future directions

The contributions of individual elements

Imaging agents

Anti-arthritis drugs

Cancer treatment

Chelation therapy

Biomineralization