Question

4. Amino acids are weak polyprotic acids. There they will readily form ________ systems which control the pH of a system.

a) Give an example of a biological system within the human body that is buffered.
b) What is the pH of a glutamic acid solution if the alpha carboxyl group is 1⁄4 dissociated?

c) Lysine is an amino acid with a basic side chain. Often, basic side chains accept protons and are thus positively charged. What could you do to generate a lysine that was not positively charged?

Answer 1

Amino acids are weak polyprotic acids. They will readily form Zwitter ion systems that control pH of the system.

Amino acids have both acidic carboxylic and basic amino groups.

There is internal transfer of hydrogen ion from carboxylic group(-COOH) to amino group(-NH2), forming an ion with both negative and positive charges. This form as zwitter ion.

The pH at which concentration of zwitter ions predominate is the isoelectric pH.

(a) Example of biological system that is buffered

There are many examples of buffers in human body.

In blood, the pH is maintained by the plasma proteins, bicarbonates, phosphates and carbonic acid.

Protein buffers in blood

Most proteins can function as buffers as they are made up of amino acids, because these contain positive and negative charges that can bind to hydrogen and hydroxyl ions and balance pH.

Hemoglobin as a buffer:-

In lungs, formation of oxy-hemoglobin from reduced hemoglobin release H+ ions that combine with bicarbonate to form carbonic acid.

This low CO2 tension causes shift of equilibrium towards production of CO2. This CO2 is eliminated.

In tissues, low O2 tension causes dissociation of O2 from hemoglobin, producing reduced hemoglobin.

The CO2 produced in tissues by metabolism enters blood that forms carbonic acid, which ionizes into H+ and bicarbonate.

Reduced hemoglobin acting as an anion accepts the H+ ions forming acid-reduced hemoglobin (HHb). Very little change in pH occurs because the H’ ions are buffered by formation of very weak acid.

These H+ ions are released when blood reaches lungs due to formation of strong oxy hemoglobin. These are neutralised by binding to bicarbonate and this helps in release of CO2.

(b)To calculate,pH of amino acid solution,

HA <-----> H+ + A-

According to Henderson Hasselbach equation,

pH of an amino acid solution = pKa + log[A-]/[HA]

Given that the solution is ¼ dissociated.

Therefore, ¾ of HA is left and ¼ of [H+] and [A-] is left.

pKa of glutamic acid is 2

Therefore, pH of the solution = 2 + log[1/4]/[3/4]

= 2 + log[⅓]

= 2 + (-0.477)

= 1.523

(c) Acetylation of the basic amino acid, lysine can prevent formation of positive ions.

The amino group is acetylated and is not able to accept anymore protons. Thus, lysine which do not carry positive charge may be produced.