Protein Chemistry. Structure, Properties, Research Methods - Shendryk A.N. 2022

Amino Acids
Amino Acids
Electronic Absorption Spectra and Acid-Base Properties of Amino Acids in Solutions - Blood Buffer Systems

A unique feature of the Circulatory system is its highly stable pH level (7.37-7.44), with an average value of 7.40. Venous Blood has a pH of 7.38, while arterial blood is 7.42. The constant pH of the blood is maintained primarily by four coupled acid-Base Pairs:

> H₂CO₃ - HCO₃^-, bicarbonate

> H₂PO₄^- - HPO₄^2-, phosphate

> Hemoglobin,

> protein

The bicarbonate buffer system is the most controllable buffer system in extracellular fluid and blood, accounting for approximately 10% of the total buffering capacity of the blood. The coupled acid-base pair is:

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The phosphate buffer system constitutes only 1% of the blood's buffering capacity, yet it is one of the primary buffer systems in Tissues. It is maintained by the pair: Н₂РО₄^- as a proton donor (acid) and HPO₄^2- as a proton acceptor (base). It is capable of maintaining pH within the range of 6.1 - 7.7.

The hemoglobin buffer system is the primary one, accounting for 75% of the total buffering capacity of the blood. Its buffering properties are attributed to two forms of hemoglobin: oxyhemoglobin HHbO₂ with pKa = 6.62 and deoxyhemoglobin HHb with pKa = 8.18. At physiological pH values for HHbO₂, the basic form HbO₂^- predominates, whereas for deoxyhemoglobin, the acidic form HHb does:

The MECHANISM OF ACTION of the hemoglobin buffer system is as follows.

> When the pH drops, the efficiency of oxygen binding by hemoglobin decreases significantly (the Bohr effect), and oxyhemoglobin is converted into deoxyhemoglobin:

> СО₂ in erythrocytes is efficiently converted into Н₂СО₃ by the action of the Zn²⁺-dependent enzyme carbonate dehydratase (Carbonic anhydrase). At a physiological pH of 7.4, Н₂СО₃, having a pKa of 6.35, dissociates almost completely (90% or more), which increases [H⁺].

> The acid-base equilibrium for deoxyhemoglobin is shifted toward HbO₂^-, which ensures the binding of protons.

The simultaneous occurrence of all these processes leads to a stable equilibrium state and maintains a constant quasi-stationary concentration of H+ ions.

The figure schematically illustrates all of the above.

The binding of oxygen to hemoglobin alters the Cell/13.html">Protein Structure in a specific way, which in turn causes A change in its activity. This critically important principle of protein activity regulation is widespread in Cells.