| The NH4+ resulting from deamination of amino acid (such as glutamate)
is converted to ammonia either directly or indirectly. |
| In microorganisms, when need to dispose of, ammonia may be secreted
directly into environment and carried away. |
| But to many animals, ammonia is toxic, and its removal by simple diffusion
is difficult. |
| Uric acid is the major form of detoxified ammonia in nitrogen excretion
in birds. |
| Urea is the principle end product of nitrogen metabolism which also
help to maintain osmotic balance in many fishes. |
| First proposed in 1932, urea formation is a complex and costly (energy
needed) mode of ammonia detoxification in the liver of mammals. |
 |
| With the help from 5 enzymes, free ammonia formed by oxidative deamination
of glutamate is converted into carbamoyl phosphate and then, finally into
urea. |
| The urea formed in the liver cells is diffused into bloodstream and
is ultimately eliminated through the kidneys in the urine. |
| The over-all reactions of the urea cycle is: |
| CO2 + NH4+ + 3 ATP + aspartate + 2 H2O ---> |
| Urea + 2 ADP + 2 Pi + AMP + PPi + fumarate + 6 H+ |
| In each cycle of urea cycle, 2 nitrogens are eliminated, one originating
from the oxidative deamination of glutamate and the other coming from the
a-amino group of aspartate. |
| It will take the equivalent of 4 high-energy phosphates to form a single
molecule of urea. |
| Urea cycle is linked to TCA cycle. |
 |
Glutamine and alanine are also carriers of ammonia (from muscle and
other tissues to liver)
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Glutamine synthetase and its role in amino acid synthesis
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Glutaminase
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Other roles of glutamine
-
Glucose-alanine cycle and pyruvate (in contracting muscle)
|
