Serine and Cysteine Synthesis| Harper's Biochemistry| Chapter 27| Lecture 5/9

Описание: Synthesis of Serine
Serine is a non-essential amino acid, meaning our body can synthesize it.
It is mainly synthesized from a glycolytic intermediate called 3-phosphoglycerate.
Steps of Serine Synthesis:
Step 1: Oxidation
3-phosphoglycerate is oxidized by the enzyme 3-phosphoglycerate dehydrogenase.
It forms 3-phosphohydroxypyruvate.
NAD⁺ is reduced to NADH.
Step 2: Transamination
3-phosphohydroxypyruvate undergoes transamination.
The enzyme phosphoserine aminotransferase transfers an amino group from glutamate.
Product formed: 3-phosphoserine.
Step 3: Dephosphorylation
3-phosphoserine is dephosphorylated by phosphoserine phosphatase.
Final product: Serine.
So, serine is synthesized from a glycolytic intermediate in three simple steps: oxidation, transamination, and dephosphorylation.

Synthesis of Cysteine
Cysteine is considered a semi-essential amino acid. Why semi-essential?
Because it requires methionine, an essential amino acid, as a sulphur source.
Cysteine synthesis occurs through the transsulfuration pathway.
Steps of Cysteine Synthesis:
Step 1: Formation of Homocysteine
Methionine is converted to homocysteine via SAM (S-adenosyl methionine).
Step 2: Formation of Cystathionine
Homocysteine combines with serine.
Enzyme: Cystathionine β-synthase
Product: Cystathionine
Step 3: Formation of Cysteine
Cystathionine is cleaved by Cystathionine γ-lyase.

Key Points to Remember
Serine is synthesized from 3-phosphoglycerate.
Cysteine is synthesized from serine + homocysteine.
Methionine provides the sulfur atom in cysteine.
Vitamin B6 (Pyridoxal phosphate) is required in the transsulfuration pathway.
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