Question

Calculate the fraction of histidine that has its imidazole side chain protonated at pH 7.3. The pKa values for histidine are pK1 = 1.8, pK2 (imidazole) = 6.0, and pK3 = 9.2.

Answer 1

The fraction of histidine that has its imidazole side chain protonated at pH 7.3  is 0.48 or 4.8%.

Describe histidine.

An important amino acid that is crucial to the production of proteins is histidine. It is classified as a positively charged amino acid in terms of physiological pH since it contains a carboxylic acid group, a -amino group, and an imidazole side chain.

What role does imidazole have in biology?

Numerous significant biological substances contain imidazole. The amino acid histidine, which possesses an imidazole side chain, is the most common. Histidine is a crucial component of the structure and binding activities of hemoglobin and is found in numerous proteins and enzymes.

using H-H equation:-

$pH = pKa + log \frac{[A]}{[HA]}$

$7.3 = 6 + log \frac{[A]}{[HA]}$

$1.3= log \frac{[A]}{[HA]}$

$120= \frac{[A]}{[HA]}$

A:HA = 20:1

Protonated fraction is

$\frac{1}{20 +1}=0.48 =4.8$ %

The fraction of histidine that has its imidazole side chain protonated at pH 7.3  is 0.48 or 4.8%.

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