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Metabolism of Glycine- Part-1
- Simple amino acid
- Optically inactive
- Nutritionally non-essential
- Glucogenic in nature
Chemically glycine is amino acetic acid.
Figure-1- Structure of glycine, since it lacks an asymmetric carbon atom, hence it is optically inactive.
Synthesis of Glycine
Glycine can be synthesized in three different ways-
1) From Serine
The alpha carbon of serine becomes Alpha carbon of glycine, whereas the beta carbon is channeled to one carbon pool. The reaction can be represented as –
Figure-2- The reaction is catalyzed by Serine hydroxy methyl transferase, tetra hydrofolate is converted to N5, N10 Methylene tetra hydro folate
2) From Threonine
Glycine can also be synthesized from threonine by the action of threonine aldolase
3) De novo synthesis
Glycine can also be synthesized from its precursor molecules i.e. from CO2, NH4+ and one carbon unit. The reaction is catalyzed by Glycine synthase system. (Figure-3).
Figure-3- The reaction is reversible; the same enzyme catalyzes the degradation of glycine also.
Catabolism of Glycine
1) Oxidative deamination
Glycine undergoes oxidative deamination. The reaction is catalyzed by Glycine oxidase, an enzyme that requires FAD as a coenzyme (figure-4). The reduced form of FAD (FADH2) is not oxidized through electron transport chain, it is oxidized at the expense of molecular oxygen forming H2O2.. The decomposition of H2O2.takes place by Catalase forming water and molecular oxygen that can be reutilized. That is the reason that amino acid oxidases and catalases are found together so as to decompose H2O2 quickly as soon as it is generated.
Figure-4- The reaction proceeds through two steps, initially an imino acid is formed that undergoes hydration and deamination to produce glyoxalate.
2) Transamination- Like other amino acids, Glycine can undergo transamination to form Alpha keto acid (Glyoxalate)- Figure-5. The reaction is catalyzed by Alanine glyoxalate transaminase that requires B6-P as a coenzyme.
Fate of glyoxalate- Glyoxalate can undergo decarboxylation to produce formate that enters one carbon pool, hence this way glycine is a one carbon donor .
Alternatively glyoxalate can also be converted to oxalate by oxidation
Genetic defects in alanine-glyoxalate transaminase (either low activity or, rarely, a mutation that leads to the enzyme being in mitochondria rather than peroxisomes) results in hyperoxaluria. The glyoxalate formed by glycine oxidase cannot be recycled to glycine by transamination, but accumulates, and is a substrate for oxidation catalyzed by lactate dehydrogenase, forming oxalate. Oxalate crystallizes in the liver and kidneys, leading, to stone formation in urinary tract or in severe cases, to early death.
Figure-5- Glyoxalate is the end product of both transamination as well as oxidative deamination reactions
3) Formation of serine- Glycine can be converted to serine, which by non oxidative deamination can produce pyruvate, thus glycine can be considered glucogenic (figure-6).
Figure- 6- The first step of the reaction sequence is catalyzed by dehydratase enzyme that requires B6-P as a coenzyme. The second step is same as oxidative deamination i.e. hydration followed by deamination.
4) Glycine cleavage- Major pathway involves the cleavage of glycine to form CO2, NH4+and N5N10 Methylene tetra hydro folate. The reaction is reversible and the glycine cleavage system is a multienzyme complex comprising of-
i) Glycine decarboxylase- P protein
ii) Amino methyl transferase- T protein
iii) Hydrogen carrier protein- H protein
iv) Dihydrolipoyl dehydrogenase- L -Protein
The net reaction can be represented as follows-
Glycine + H4folate + NAD+ ↔ 5,10-methylene-H4folate + CO2 + NH3 + NADH + H+
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