Introduction to Transamination Deamination & Urea cycle
GENERAL METABOLISM OF AMINO ACIDS
- .The anabolic reactions where required proteins are synthesized. (syn = deg, N bal)
- The catabolic reactions where dietary proteins and body proteins are broken down to amino acids.
- Transfer of amino group from an amino acid to keto acid is known as Transamination.
- It involves interconversion of a pair of α-amino acid & a pair of α-keto acids and is catalyzed by transaminases or aminotransferases.
- Pyridoxal phosphate is the coenzyme essential for transaminase activity.
- Alanine-pyruvate transaminase or glutamate transaminase make a significant contribution & transfer amino group from Alanine or glutamate.
- Transamination is a reversible process.
- There is no free ammonia liberated ,only the transfer of amino group occurs.
- Each transaminase is specific for specified pair of amino acids and keto acids as one pair of substrates.
- Most amino acids are substrates for Transamination except lysine, Threonine, Proline and Hydroxy Proline.
- δ-amino group of ornithine also undergoes Transamination.
- Transamination is important for distribution of amino group and production of non-essential amino acids & involves both catabolism and anabolism of amino acids.
- Transamination diverts excess amino acids towards energy generation.
- Glutamate is the only amino acid that undergoes oxidative deamination to liberate ammonia for urea synthesis.
- Serum transaminases are important for diagnostic and prognostic purposes.
It involves two stages:
Transfer of amino group to coenzyme pyridoxal phosphate to form pyridoxamine phosphate.
The amino group of pyridoxamine phosphate is transferred to keto acid to produce new amino acid with pyridoxal phosphate is regenerated.
All transaminases require pyridoxal phosphate (PLP), active form of vitamin B6 .
Another similar reaction yields more common products:
These reactions are mediated by pyridoxal phosphate (PLP), a derivative of pyridoxine (vitamin B6):
Clinicaly important transaminases
ALT or SGPT
- Alanine-α-ketoglutarate transaminase
- Clinical marker for irreversible liver damage
AST or SGOT
- Aspartate-α-ketoglutarate transaminase
- Clinical marker for irreversibile myocardial damage
- TRANS-AMINATION( in cytoplasm of all the cells) FOLLOWED BY OXIDATIVE DEAMINATION ( in mitocho.Of hepatocytes).
- Thus the two components of the reaction are Physically Far away, but physiologically they are
- Removal of amino group from amino acids as ammonia is called Deamination
- liberation of ammonia is used for synthesis of urea cycle and the carbon skeleton of amino acids is converted to keto acids.
- Deamination my be either oxidative or non-oxidative.
- Both transamination and Deamination involve in using glutamate as central molecule.
1. oxidative Deamination:
- It is the liberation of free ammonia from the amino group of amino acids coupled with oxidation.
- It takes place mostly in liver and kidney
- the role of glutamate with enzyme glutamate dehydrogenase (GDH) serves as collection centre for amino groups to liberate ammonia.
- this enzyme is unique and it can use either NAD+ or NADP+ as coenzyme.
- Conversion of L-Glutamate to α- Ketoglutarate occurs through an intermediate product called α- iminoglutarate with help of GDH.
- GDH reaction is important as it links up glutamate metabolism with TCA Cycle through α- Ketoglutarate
- GDH is involved in both catabolic and anabolic reactions.
- L-Amino acid oxidase & D- amino acid oxidase are flavoproteins, possessing FMN & FAD.
- These act on amino acids to produce α-keto acids and ammonia.
- Oxygen is reduced to hydrogen peroxide and decomposed by catalase.
- Activity of L- amino acid oxidase is low & D-amino acid oxidase is high in liver and kidney.
- L-amino acid oxidase does not act on glycine and dicarboxylic acids.
- If enzyme catalase is absent α-keto acid produced by oxidative Deamination is dacarboxylated by hydrogen peroxide forming carboxylic acid with one carbon less.
- The enzyme L-amino acid oxidase has no effect on glycine.
- L-amino acid oxidase does not have major role in catabolism of mammalian amino acids and formation of ammonia.
The glutamate which is produced by these transaminase reactions is oxidatively deaminated by glutamate dehydrogenase to release ammonium:
2. Non-oxidative Deamination:
- Serine & Threonine are hydroxyl amino acids which undergo non-oxidative Deamination , catalyzed by PLP-dependent dehydratases.
- Histidine-..Histidase-..Urocanic acid
- Sulfur amino acids like Cysteine & Homocysteine undergo Deamination coupled with desulfhydration to give keto acids.
TRAPPING OF AMMONIA IN BRAIN CELLS
- The intracellular ammonia is immediately trapped by glutamic acid to form glutamine, especially in brain cells.
- The glutamine is then transported to liver, where the reaction is reversed by the enzyme glutaminase.
- Formation of urea by Kreb’s Henseleit urea cycle is a major route for the metabolic disposal of ammonia.
- This series of reactions occurs exclusively in the liver.
- Urea is the major end product of nitrogen metabolism in humans.
- The first two reactions of urea cycle occur in the mitochondria, whereas the remaining cycle enzymes are located in the cytosol.
The sequence of reactions involved in the biosynthesis of urea, summarized in five steps
- Formation of Carbamoyl Phosphate
- Formation of Citrulline
- Formation of Argininosuccinate
- Formation of Arginine and Fumerate
- Formation of Urea and Ornithine
- Enzymes in mitochondria:
1. Carbamoyl Phosphate synthetase 1 (CPS 1)
2. Ornithine Trans – carbamoylase (OTC)
- Enzymes in cytosol:
3. Arginino – Succinate Synthetase
4. Arginino – succinate lyase
- Ammonia is toxic
Readily ionises to ammonium ion NH4+
NH4+ converted to urea in liver (urea cycle)
Urea contains 2 x NH2
One from NH4+
One from aspartate
Urea excreted in urine
- Organisms that cannot easily and quickly remove ammonia usually have to convert it to some other substance, like urea or uric acid, which are much less toxic.
- Insufficiency of the urea cycle occurs in some genetic disorders (inborn errors of metabolism), and in liver failure.
- The overall reaction may be summerized as:
NH3 + CO2 + Aspartate –> Urea + fumerate
- During these reactions 2 ATPS are used in the first reaction.
- Another ATP is converted to AMP + PPi in third step
- Which is equivalent to 2ATPs
- The urea cycle consumes 4 high energy phosphate bonds.
- However fumarate formed in the 4 th step may be converted to malate.
6. Malate when oxidised to oxaloacetate produces 1 NADH equalent to 2.5 ATP
7. So net energy expenditure is only 1.5 high energy phosphates.
8. The urea cycle and TCA cycle are interlinked, and so it is called as “UREA BICYCLE”.
REGULATION OF UREA CYCLE
COARSE REGULATION :-
- The enzyme levels change with the protein content of diet.
- During starvation, the activity of urea cycle enzymes Is elevated to meet the increased rate of protein Catabolism.
- The major regulatory step is catalyzed by CPS-1, where the positive effector is N- Acetyl Glutamate (NAG).
- 2. It is formed from glutamate and acetyl CO-Enzyme A.
- Arginine is an activator of NAG synthetase.
- The urea cycle enzymes are located in such a way that the first two enzymes are in the Mitochondrial Matrix.
- The inhibitory effect of fumarate on its own Formation is minimized because ARGINOSUCCINATELYASE is in the cytoplasm, while Fumarase is in mitochondria.
UREA LEVEL IN BLOOD
Normal urea level in plasma 20 – 40 mg/dl.
- In disease of liver, hepatic failure can finally lead to hepatic coma and death.
- Hyperammonemia is the characteristic feature of liver failure.
- The condition is also known as portal systemic encephalopathy.
- This condition developed due to portalsystemic shunting of blood, the toxins by pass the liver.
- Concentration in systemic circulation rises.
Urea Cycle Disorders
ONE CARBON METABOLISM
- One carbon ( 1C) groups play a essential role in donating carbon atoms for synthesis of different type of compounds.
- The different one carbon groups of the one carbon pool of the body are:
- Formyl group (-CHO)
- Formimino group (-CH=NH)
- Methenyl group (-CH=N+)
- Hydroxymethyl group (-CH2OH)
- Methylene group (-CH2)
- Methyl group (-CH3)
- The one carbon groups, except methyl group, are carried by tetra hydrofolic acid (THFA).
GENERATION OF ONE CARBON GROUPS
- The one carbon groups are contributed to the one carbon pool by amino acids for example conversion of serine to glycine is the primary contributor for methylene THFA.
UTILIZATION OF ONE CARBON GROUP
- The one carbon units are used to synthesize following compounds
- C 2 of purine
- Formylation of methionyl t- RNA
- C 8 of purine
- Deoxy Thymidylic Acid
Other Biochemistry Notes
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