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Metabolism of Serine
Serine is a hydroxyl group containing, nutritionally non-essential, one carbon donor and glucogenic amino acid.
Structure– Chemically it is α- Amino –β- Hydroxy Propionic acid (figure-1)
Figure-1- Chemical structure of serine
1) From Glycine
Glycine and serine can be inter- converted by the action of serine hydroxy methyl transferase (figure-2).
Figure-2– Serine to glycine interconversion
2) From 3- Phospho Glycerate (figure-3)
This is the major source of serine in the body. The reactions involved are as follows
The reaction is catalyzed by dehydrogenase enzyme
3- Phosphoglycerate——–> 3-Phosphohyroxy pyruvate
The reaction is catalyzed by phospho serine transaminase enzyme; the alpha amino group is donated by glutamate
3-Phosphohyroxy pyruvate <———–> Phosphoserine
The reaction is catalyzed by phosphatase enzyme
Phosphoserine——–> Serine + H3PO4
Figure-3- Synthesis of serine from 3-Phosphoglycerate
3) From Hydroxy pyruvate
Hydroxy pyruvate can be transaminated to form serine
Alanine + Hydroxy pyruvate——–> Pyruvate +Serine
Catabolism of Serine
1) Non oxidative deamination- Serine can be non oxidatively deaminated to form pyruvate, hence it is glucogenic amino acid (figure-4)
Figure-4- 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.
2) Conversion to Glycine- It can also be converted to glycine depending upon the cellular requirement.
3) Transamination– Serine can be transaminated to form hydroxy pyruvate
Metabolic role of serine
1) One carbon donor- During the conversion of serine to glycine one carbon fragment is transferred to THF forming N5N10 Methylene THF (figure-2).
2) Synthesis of Cysteine- Serine contributes its carbon skeleton for the synthesis of cysteine, the- SH group is donated by Methionine (figure-5). It is a two-step process and the reactions are as follows-
Figure-5- Synthesis of Cysteine from Serine and Methionine. Homocysteine is a metabolic product of Methionine
3) Synthesis of phosphoproteins- Serine acts as a carrier of phosphate group in phosphoproteins- like casein, vitellin etc.
4) Synthesis of phospholipids- Phosphatidyl serine is biologically an important phospholipid (figure-6).
Figure-6-Chemical structure of Phosphatidyl serine
5) Synthesis of Sphingosine- Sphingosine the alcohol present in sphingolipids is synthesized by the condensation of Palmitic acid and serine.
6) Synthesis of Ethanolamine- Serine is decarboxylated to form ethanolamine. Ethanolamine can be used either for the synthesis of choline by subsequent methylation reactions or it is as such used for the synthesis of Phosphatidyl ethanolamine, an important phospholipid and a lipotropic agent.
7) Regulation of enzyme activity- The hydroxyl group of serine can be reversibly phosphorylated or dephosphorylated to regulate the enzyme activity (Figure-7).This is covalent modification and is an important mechanism to regulate the activity of many enzymes. For example- Glycogen synthase a key regulatory enzyme for glycogen synthesis gets activated upon dephosphorylation, whereas phosphorylase, an enzyme of glycogen degradation becomes active upon phosphorylation. All the enzymes under the influence of Insulin are active in the dephosphorylated form while the enzymes under the influence of glucagon are active in the phosphorylated form.
Figure-7- Reversible phosphorylation and dephosphorylation of serine residues for regulation of enzyme activity (covalent modification)
Apart from that serine is also found at the active site of many enzymes- Serine proteases- Coagulation factors and trypsin.
8) Formation of O- glycosidic linkages- In glycoproteins the carbohydrate groups are generally linked either by O-Glycosidic linkages or by N- Glycosidic linkages. The O-Glycosidic linkages are provided by –OH group of either serine or threonine, whereas the N-glycosidic linkages are provided by NH2 group of Asparagine (figure-8).
Figure-8- O and N-Glycosidic linkages
9) Incorporation in to tissue proteins- Like other amino acids serine is also incorporated in to tissue proteins.
10) Glucogenic- Serine undergoes non oxidative deamination to form Pyruvic acid that can be channeled towards pathway of gluconeogenesis.
Serine analogues- Azaserine and Cycloserine are serine analogues. They are used as drugs to inhibit nucleotide biosynthesis. Azaserine is an anticancer drug, whereas Cycloserine is used as an antitubercular drug.Please help "Biochemistry for Medics" by CLICKING ON THE ADVERTISEMENTS above!