http://aris.gusc.lv/ChemFiles/CA/CApH7-36.doc
ENZYME
homeostasis BIOLOGIC
SYSTEM pH OF BLOOD 7.36
As it was mentioned before, three main buffer systems act in blood : hemoglobin HHb/ Na Hb
C O
2 oxyhemoglobin HHb O
2
/ Na Hb O
2
– bicarbonatea H
2
O /CA/ C O
2
/ Na HC O
3
buffer systems
When these buffer systems struggle with acidic products of metabolism, more and more of the acid forms of the buffer systems are produced. For this reason, the acid forms have to be transported out of organism.
It is easy to imagine, that hemoglobin cannot be evolved out of organism, therefore there is only one buffer system, suitable for regulation of acid form’s presence by breathing out
C O
2

, that decrease the pH of blood by metabolic acid production caused of pH=7.36
decreasing problems for organism however pH of blood should be maintained constant 7.36
. Reaction is negligible slow C O
2
+2H
2
O slow→H
3
O + +HC O
3
and shifts to fastest reversible carbonic acid formation exothermic reaction C O
2
+H
2
O H
2
C O
3
+Q which provides accumulation of C O
2 gas however carbonic acid pK=6.11
value is one fold more acidic as Carbonic Anhydrase ( CA ) pK=7.0512 value.
Carbonic anhydrase make conversion of C O
2 gas to bicarbonate anion HC O
3
in to water medium fast and establish acid-base equilibrium: C O
2
+ 2H
2
O

CA

H
3
O + +HC O
3
+Q exothermic as producing right side → exothermic equilibrium reaction products H
3
O + +HC O
3
—
+Q . So Heating +Q shifts equilibrium left side ← and as soon as H + concentration grows pH<7.36 for some reason, Carbonic anhydrase equilibrium is shifted to left ← too and
C O
2 transported out together with H + as HC O
3
by respiration in lungs and acid concentration
[ H + ] decreases to homeostasis pH=7.36
level. If concentration H + decreases pH>7.36
, Carbonic anhydrase equilibrium is shifted to the right and the extra amount of HC O
3
through kidneys passes into urine and is transported out and pH returns to homeostasis pH=7.36
level according Le Chatelier’s theorem.
The Brensted acid is gaseous C O
2
, which in blood water make solution. The dissolved into water H
2
O
(into blood ) carbonic dioxide C O
2 occurring in cell converted with carbonic anhydrase CA to H + + HC O
3
. The water H
2
O and carbonic dioxide C O
2
, finally, is in direct acid-base equilibrium with its ions H + and HC O
3
.
Carbonic anhydrase equilibrium constant pK=7.0512 decreases concentration acid form C O
2
into water
H
2
O (avoid carbonic acid H
2
C O
3
formation which is one fold more acidic pK=6.11) therefore hydrogen carbonate HC O
3
and hydrogen ions H + are involved into blood pH formation according buffer solution
homeostasis : 7.36 = pH = pK+log




[ NaHCO
3
[ CO
2
]
]




= 7.0512+log




[ NaHCO
3
[ CO
2
]
]




;
[ NaHCO
3
]
[ CO
2
]
=
(pH-pK)
=
(7.36-7.0512)
=
0.3088
=
2.0263
the ratio [ Na HC O
3
]/[C O
2
] being approximately 2 / 1 .
1 usually in medical literature C O
2 amount is given, but as 1 mole C O
2 creates 1 mole H
2
O /CA/ C O
2
, it is the same.
10
9
6
5
4
8
7
3
2
HC O
3
0%
C O
2
+ 2H
2
O 100%
50%
50%
Buffer region middle point is the over inflection point in graph ○ : pH=pK=7.0512
as ratio
[ NaHCO
3
]
[ CO
2
]
=1 is one as well buffer component concentrations are equal
[ Na HC O
3
]=[C O
2
] as well as bicarbonate salt of sodium [ Na HC O
3
] concentration is equal to Brensted weak acid dissolved in blood C O
2 concentration [C O
2
] .
Alkaline reserve at 7.36 = pH is normal
[NaHCO
3
]
=
2.0263
.
[CO
2
]
1
100% salt – buffer system base
0% weak acid buffer component
1

As soon as H + pH < 7.36 concentration grows for some reason, Carbonic anhydrase equilibrium is shifted to left and C O
2 transported out together with H + as HC O
3
by respiration in lungs and blood acid concentration
[ H + ] decreases as pH returns to homeostasis pH=7.36
level according Le Chatelier’s theorem. If concentration
H + decreases pH > 7.36
, Carbonic anhydrase equilibrium is shifted to the right and the extra amount of HC O
3
through kidneys passes into urine is transported out as well blood pH returns to homeostasis pH=7.36
level according Le Chatelier’s theorem.
Bicarbonate channels in kidney cells are open at high values of pH > 7.36
from side of blood circulation, but lungs channel transport are shifted outwards to C O
2
at low values pH < 7.36
below homeostasis pH=7.36
level according Le Chatelier’s theorem.
H + +HC O
3
+ Q endothermic H
2
O + C O
2
 initial amount, concentration increase elevate C O
2
 output.
1) heating + Q shifts equilibrium right side → (Air brething human as well as animals have the lungs located inside body and equipped with heat producing cells in alveolar area as heating shifts equilibrium right side);
2) hydrogen H + ion concentration (acidity) increase shifts equilibrium right side → Hb adsorbed O
2
yield H + ;
3) bicarbonate HC O
3
concentration increase shifts equilibrium right side →.
The numerical value 7.0512
is more basic about value of carbonic acid pK= 6.11
which is almost one unit les.
This value pK=7.0512
is Carbonic anhydrase made equilibrium constant very friendly to blood pH=7.36
. As most of metabolism products are acidic, the organism has the reserve of alkalinity 2. For this reason the ratio between Na HC O
3 and C O
2 concentrations is 2 / 1 and the pH value of physiological conditions blood is 7 .
36 .
Note: Absence of CA ENZYME make 10 times more acidic concentration with pH in equation with constant pK=6.11
of carbonic acid dissociation and with 2.036/1 alkaline reserve : pH = 6.11+log


[ NaHCO
3
]
[ H
2
CO
3
]


= 6.42
This way human body undergoes to acidosis of blood which cause oxidative stress ( increase of potential for
Ox-Red)
E
O
2
O
2
+ 4
3
O
+
+ 4 e
= Eo + 0.06154/4
 lg([ O
2
-
6
2
O at blood plasma concentration [ O
2
] =
6•10 –5
M from potential
] [H
3
O + ] 4 )= 0.7008
V value at pH=7.36 to increasing oxygen oxidizing power 10000 times. Potential increases to E
O
2
= Eo + 0.06154/4
 lg([ O
2
] [H
3
O + ] 4 )= 0.7587
V at pH=6.42 making oxidative damages in body, whatever causes absence of Carbonic Anhydrase CA which are ubiquitous in all life bodies.
Active CA is strongly determined in live body to prevent acidosis , bubbling and non enzymatic oxidation .
Some molecules are tight bound to CA active site and inhibit it. The alkaline reserve 2.036/1=[ HC O
3
]/[ C O
2
] of the organism can be controlled by adding H
2
S O
4 to a sample of blood ( H
2
S O
4
reacts with HC O
3
and the C O
2
, included in salt, and is liberated). If 56.23
mL of gaseous C O
2 are liberated from 100 mL of blood , the alkaline reserve in homeostasis is normal and total alkaline reserve amount concentration 0.023M
=[ HC O
3
]+[ C O
2
] is in homeostasis normal as [ HC O
3
]= 0.0154 M , [ C O
2
]= 0.0076M
.
Controlled instructions the alkaline reserve of the organism by adding H
2
S O
4 to a sample of blood
( H
2
S O
4
reacts with Na HC O -
3 and the C O
2
, included in salt, and is liberated). If 50 60 mL of gaseous C O
2 is liberated from 100 mL of blood in homeostasis , the alkaline reserve is normal .
Two types of diseases occur, if the acid-base balance is distorted in the organism alkalosis and acidosis .
1 ) Respiratory alkalosis occurs, if lungs are hyperventilated, for example, during anesthesia. If C O
2 concentration decreases pH > 7.36
alkalose due to hyperventilation, the blood vessels are broadened and their tonus is lowered as a result of it, therefore O
2 supply to brain is shortened.
For this reason it is necessary to use mixtures of O
2 and C O
2 during anesthesia instead of pure oxygen. If respiratory alkalosis occurs for other reasons than hyperventilation of lungs , the ratio 2 / 1 of the buffer components can be re-established in a longer period of breathing normal, C O
2
-containing air 350 ppm.
2 ) Respiratory acidosis occurs in the cases, when the concentration of C O
2 in the air is increased. The result of this is that the action of breathing muscles becomes more difficult. Again, this can be canceled, if the patient starts breathing normal air. Hoverer, if increased C O
2 content in the air lasts long, a metabolic acidosis can occur pH < 7.36 acidose . In the case of metabolic acidosis the ability of hemoglobin to bound oxygen is lowered.
For this reason only the concentrations of carbonic dioxide C O
2
into water H
2
O (avoid carbonic acid H
2
C O
3 formation) and hydrogen carbonate HC O
3
+ hydrogen ions H + are included into equation for blood pH.
2

If 56.23
mL of gaseous C O
2 are liberated from 100 mL of blood , the alkaline reserve is normal 2.0263/1 =
[ HC O-
3
]/[ C O
2
]= 2.0263
; [ HC
Free energy change

G r
O-
3
] = 0.0154 M , [ HC O-
3
]+[ C O
2
]= 0.023 M and [ C O
2
]= 0.0076M .
for reaction: H
3
O + +HC O
3
-

CA

C O
2
+2 H
2
O +

G+Q

G r
= 2
 G°
H2 O
+
 G°
C O2
-
 G°
H3 O
-
 G°
HC O3
= -60.145 kJ / mol
exoergic.
Enthalpy change

H r
for reaction:

H r
=2
 H°
H2 O
+
 H°
C O2
-
 H°
H3 O
-
 H°
HC O3
= -10.06 kJ / mol
exothermic
Q= -

H r
=+ 10.06 kJ / mol
.
Entropy change

S r
>0 positive for reaction is spontaneous or favorable :

S r
=2
 S°
H2 O
+
 S°
C O2
-
 S°
H3 O
-
 S°
HC O3
= +163.012 J / mol/K
.
C O
2
+2 H
2
O + QG

CA

H
3
O + +HC O
3
-

G reverse
= -RTln(Keq)= +40.2278 kJ / mol
Carbonic anhydrase change the reaction conditions thermodynamically from

G r
= -60.145 kJ / mol
to

G r
= -40.2278 kJ / mol
via change the equilibrium constant from Keq = 10 -10.54
to Keq = 10 -7.0512
or exponent pKeq = 7,0512 constant very close to pH value of blood 7 .
36.
Membrane penetrating reaction of HC O
3
and H
3
O + is driven by exoergic conditions of free energy change

G =-60.145 kJ / mol through proton H + and bicarbonate HC O
3
channel crossing cell membrane .
For moisture membrane proton channels are protons H + permeable, unless H + impermeable for dray proton channels H + . Therefore membrane is equipped by aquaporins, which are water and solute oxygen
O=O permeable in both directions ( O=O + H
2
O aquaporin channel H
2
O + O=O ):
For protons crossing the membrane through proton channels, necessary water molecules locate both side of the membrane and aquaporins are supplier of water H
2
O molecules to moisture alveolar lungs surface.
O
O
O
H
H membrane aquaporines membrane
O
O
H
H
O
Free energy change

G= -60 kJ / mol for Reaction of H
2
C O
3
formation is exoergic

G<0 negative therefore promotes neutralization reaction
H
3
O + +HC O
3
<=> H
2
C O
3
+ H
2
O +

G
Inside the cell–cytosol
 ↔  alveolar surface in lungs consuming
+Q heat and evolving water + H
2
O
CA
C O
2 with
+2H
2 water
O

produsing heat
CA
H
3

O
H
+
3
+Q
O + +HC O
3
+ Q
+ HC O
3
-
H
H
H
O
O
O
C
H
+
O
membrane channels
O membrane
H
+
O
C
O
O
H
H
H
O
O
C
H
O
H
O
H
H calculation shows exoergic value negative

G=
 G°
H2 O
+
 G°
H2C O3
-
 G°
H3 O
H
2
C
H
O
2 supporting surface moisture
3
C
+Q
O
3
+ H
2
O
C
+
O

2
+H
G
-
 G°
HC O3
-60.145 kJ / mol and
2
O reaction H
2
C O
3
C O
2
+ H
2
O +

G;

G=
 G°
H2 O
+
 G°
C O2
-
 G°
H2C O3
=-0.0 kJ / mol is anenergic or neutral.
Enthalpy change

H for reaction H
2
C O
3 formation : H
3
O + +HC O
3
H
2
C O
3
+ H
2
O + Q .

H=
 H°
H2 O
+
 H°
H2C O3
-
 H°
H3 O
kJ / mol
H°
HC O3
= -0.1933 kJ / mol
very weak exothermic almost neutral and
Enthalpy change decomposition reaction of carbonic acid H
2
C O
3
C O
2
+ H
2
O Q

H=
 H°
H2 O
+
 H°
C O2
-
 H°
H2C O3
= +20.141 kJ / mol is endothermic exactly with the cooling effects .
3