Phosphate buffer
The
pKa for phosphate, H2PO4 , is 6.8, close to the desired
blood pH of 7.4, so phosphate is a good buffer.
In the ECF, phosphate is present as inorganic
phosphate in a very low concentration (about 1 – 2 mmol/L), so it plays a minor role
in extracellular buffering.
Phosphate
is an important intracellular buffer for
the reasons:
1.
Cells contain large amounts of phosphate in
such organic compounds as adenosine triphosphate (ATP), adenosine diphosphate
(ADP), and creatine phosphate. Although these compounds primarily function in
energy metabolism, they also act as pH buffers.
2.
Intracellular pH is generally lower than the
pH of ECF and is closer to the pKa of phosphate which is 6.8 (e.g.The cytosol
of skeletal muscle has a pH of 6.9.).
Phosphoric acid is
triprotic weak acid and has a pKa value for each of the three dissociations:
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pKa1 = 2
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pKa2 = 6.8
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pKa3 = 12
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H3PO4
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<= = = >
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H+ + H2PO4-
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<= = =>
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H+ + HPO4-2
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< = = = >
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PO4-3 +
H+
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The three pKa values
are sufficiently different so that at any one pH only the members of a single
conjugate pair are present in significant concentrations.
The pKa2 is 6.8 and this makes the closed phosphate buffer
system a good buffer intracellularly and in urine. The pH of glomerular
ultrafiltrate is 7.4 and therefore phosphate is predominantly in the monohydrogen form so as to combine with more H+ in
the renal tubules efficiently. This makes the phosphate buffer more effective in buffering
against a drop in pH than a rise in pH.
Protein buffer
Proteins act as efficient buffers
because of their amino acid structure which have a central carbon with
four groups off of it.
The carboxyl and amino groups are what enable proteins to act as
buffers.
Protein buffers in blood include haemoglobin (150g/l) and plasma proteins (70g/l). Buffering
is by the imidazole group of the histidine residues which has a pKa of about
6.8. This is suitable for effective buffering at physiological pH (7.4).
Haemoglobin is quantitatively about 6 times more important than
the plasma proteins as it is present in about twice the concentration and
contains about three times the number of histidine residues per molecule. For
example if blood pH changed from 7.5 to 6.5, haemoglobin would buffer 27.5
mmol/l of H+ and total
plasma protein buffering would account for only 4.2 mmol/l of H+.
Deoxyhaemoglobin is a more
effective buffer than oxyhaemoglobin and this change in buffer capacity
contributes about 30% of the Haldane effect. The major factor accounting for
the Haldane effect in CO2 transport
is the much greater ability of deoxyhaemoglobin to form carbamino compounds.
Haemoglobin
has a special place in the pHstabilizing mechanisms of blood because
(a) Haemoglobin
and oxyhaemoglobin have different iso-electric points and different ionization constants
so that at the pH of blood, absorption of about 0.7 g. of hydrogen ion [from
carbonic acid mainly] (and the release of an equivalent amount of potassium ion
in exchange) takes place during liberation of oxygen from 1 g molecule of
oxyhemoglobin.
(b) (b)
Haemoglobin, more easily than other proteins, reacts reversibly with carbonic acid
to form a still weaker carbaminoacid; and an increasing accumulation of carbamino-haemoglobin
as the blood passes through the capillaries and a sharp reversal when, in the lungs,
the equilibrium is disturbed, carbon dioxide being excreted, carbonic acid
converted to carbon dioxide under the influence of carbonic anhydrase, and,
consequently, carbaminohaemoglobin is decomposed.
(Frazer &Stewart, J. clin. Path. (1959), 12, 195-206)
For more details readers are advised to go
through the above mentioned paper and then directed to the link http://www.madsci.org/posts
