Buffers in biological systems

Buffers is biological systems
By Jwala Jayadeep
KUFOS

1. BUFFERS IN BIOLOGICAL SYSTEMS BY JWALA 1 ST MSc MARINE MICROBIOLOGY

2. INTRODUCTION A biological buffer is an organic substance that has a neutralizing effect on hydrogen ions. B iological buffers helps maintaining the body at the correct pH so that biochemical processes continue to run optimally. Most buffers consist of a weak acid and a weak base. They help maintain a given pH even after the addition of an acid or a base. Biological buffers help maintain a steady pH around the physiological pH.

3. BUFFER Buffer is a mixture of weak acid and salt of conjugate base that resist the change in pH upon the addition of acid or base. Buffering results two reversible reaction equilibrium in a solution wherein the concentration of proton donor and its conjugate proton acceptor are equal. Buffers have both intensive and extensive properties. Buffer capacity generally depends on the concentration of buffer solutions. Buffer capacity is a term used to describe the ability of a given buffer to resist changes in pH on addition of acid and base.

4. How do Buffers Act Buffers acts as “ Shock absorbers ” against sudden changes of pH by converting injurious strong acids and bases into harmless weak acid salts. If a buffer solution is composed of weak acid HA and its salt BA , they ionize as follows:- HA  H + + A - BA  B + + A -

5. Handerson-Hasselbalch Equation The pH of a solution containing a weak acid is related to its acid dissociation constant. The relationship can be stated in the convenient form of the “ Handerson - Hasselbalch equation ” pH = Pk a + log 10 [A - ] [HA] The Handerson - Hasselbalch equation is an expression of great predictive value in protonic equilibria .

6. BIOLOGICAL BUFFERS Biological buffers should meet the following criteria: Their pK a should reside between 6.0 to 8.0 They should exhibit high water solubility and minimal solubilty in organic solvents. They should not permeate cell membranes. They should not exhibit any toxicity towards cells. They should not interfere with any biological process. The salt effect should be minimal, however, salts can be added as required. Ionic composition of the medium and temperature should have minimal effect of buffering capacity. Buffers should be stable and resistant to enzymatic degradation. Buffers should not absorb either in the visible or in the UV region.

7. Importance of Biological Buffer To maintain homeostasis. To regulate enzymatic function. To control pH in biochemical reaction.

8. Types of B iological Buffers Bicarbonate buffer Phosphate buffer Protein buffer Haemoglobin

9. BICARBONATE BUFFER The bicarbonate buffer system is an acid- base homeostatic mechanism involving the balance of carbonic acid, bicarbonate ion and carbon dioxide in order to maintain pH in the blood and duodenum, among other tissues, to support proper metabolic function. This process is catalyzed by carbonic anhydrase . CO 2 + H 2 O H 2 CO 3 HCO 3 - + H + As with any buffer system, the pH is balanced by the presence of both weak acid and its conjugate base so that any excess acid or base introduced to the system is neutralised. Failure of this system to function properly results in acid-base imbalance, such as acidemia and alkalemia in the blood.

10. The Bicarbonate Buffer

11. Disorders of acid-base balance in respiratory system

12. PHOSPHATE BUFFER It plays a major important role in buffering renal tubular fluid and intracellular fluids. The main elements of phosphate buffer system are H 2 PO 4 - and HPO 4 - . When a strong acid such as HCl is added to a mixture of these two substances, the hydrogen is accepted by the base HPO 4 - and converted to H 2 PO 4 - . HCl + Na 2 HPO 4  NaH 2 PO 4 + NaCl The phosphate buffer system has a pK of 6.8. This allows the system to operate near its maximum buffering power. The phosphate buffer is important in the tubular fluids of the kidneys. The phosphate buffer is also important in buffering intracellular fluid.

13. PROTEIN BUFFER Protein buffer system consists of the plasma proteins ( such as albumins) and certain proteins in cells, including the haemoglobin uses freely exposed amino acids groups of an amino acid or protein; combines with a hydrogen ion in the presence of excess acid. Proteins are made up of amino acids, which contain positively charged amino groups and negatively charged carboxyl groups. The charged regions of these molecules can bind hydrogen and hydroxyl ions, and thus function as buffers. Protein buffers interact extensively with other buffer systems. They helps in regulating pH in extracellular fluid and intracellular fluid.

14. Intracellular buffers:- proteins, phosphates and the pottassium -hydrogen ion exchange .

15. HAEMOGLOBIN BUFFER Transports oxygen from lungs to peripheral tissues. Transports carbon dioxide from tissues to lungs for exhalation. Haemoglobin is a buffer for both carbon dioxide and H +. Carbon dioxide diffuses across RBC membrane from tissues. The CO 2 can bind directly with haemoglobin and be released in the lungs. The CO 2 that reacts with water forms carbonic acid that then dissociates into bicarbonate in RBC. Bicarbonate ions diffuses into plasma in exchange for chloride ions. H + binds to haemoglobin and released in RBC’s in lungs to combine with bicarbonate and reform CO 2 for exhalation.

16. Buffering in Cells and Tissues Amino acids present in proteins in cells and tissues contain functional groups that act as weak acid and bases. The phosphate and bicarbonate buffer systems are most predominant in biological systems. The bicarbonate buffer system plays an important role in buffering the blood system where in carbonic acid acts as a weak acid (proton donor) and bicarbonate act as a conjugate base (proton acceptor). Their relationship can be expressed as follows:- K 1 = [H + ] [ HCO 3 - ] [ H 2 CO 3 ]

17. Relationship between bicarbonate buffer system and carbon dioxide

18. Effect of temperature on pH Generally when we consider the use of buffers we make the following two assumptions:- The activity coefficients of the buffer ions is approximately equal to 1 over the useful range of buffer concentrations. The value of K a is constant over the working range of temperature. The pH changes slightly with change in temperature. This is very critical in biological systems where a precise hydrogen ion cocncentration is required for reaction systems to operate with maximum efficiency. The mathematical relationship of activty and temperature may be complictaed , the actual change of pK a with temperature is approximately linear.

19. Effect of Temperature on pH of Phosphate Buffer

20. pK a and ∆ pK a ∕ o C of Selected Buffers

21. Effects of Buffers on Factors Other than pH Some weak acids and bases may interfere with the reaction system. For example, citrate and phosphate buffers are not recommended for systems that are highly calcium- dependent. Phosphate ions in buffers can inhibit the activity of some enzymes, such as carboxypeptidase , fumeras , carboxylase, and phosphoglucomutase . Trisaminomethane can chelate copper and also acts as a competitive inhibitor of some enzymes. Tris based buffers are not recommended when studying the metabolic effects of insulin. Borate buffers are not suitable for gel electrophoresis of protien , they can cause spreading of the zones if polyols are present in the medium.

22. Effect of pH on Solubility Changes in pH can affect the solubility of partially soluble ionic compounds. As a result of the common ion effect, the solubility of Mg(OH) 2 can be increased or decreased. When a base is added to the concentration of OH - increases and shifts the solubilty equillibrium to the left causing a dimunition in the solubility of Mg(OH) 2. . When an acid is added to the solution, it neutralises OH - and shifts the solubility equilibrium to the right. This results in increased dissolution of Mg(OH) 2 .

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