Serine

Flashcard 1 Q: What is the specific enzyme deficiency in the rare congenital disorder affecting serine biosynthesis? Give the full name and abbreviation.

A: 3-Phosphoglycerate dehydrogenase (3-PGDH).

Flashcard 2 Q: Write the first step of serine biosynthesis from 3-phosphoglycerate, including the enzyme and coenzyme.

A: 3-Phosphoglycerate + NAD⁺ → 3-Phosphohydroxypyruvate + NADH + H⁺. Enzyme: 3-Phosphoglycerate dehydrogenase (3-PGDH). Coenzyme: NAD⁺.

Flashcard 3 Q: Write the second step of serine biosynthesis, including the enzyme and coenzyme.

A: 3-Phosphohydroxypyruvate + Glutamate → 3-Phosphoserine + α-Ketoglutarate. Enzyme: Phosphoserine aminotransferase (PSAT). Coenzyme: Pyridoxal phosphate (PLP – vitamin B₆).

Flashcard 4 Q: Write the final step of serine biosynthesis, including the enzyme.

A: 3-Phosphoserine + H₂O → Serine + Pi. Enzyme: Phosphoserine phosphatase (PSP).

Flashcard 5 Q: What is the pattern of inheritance for 3-PGDH deficiency?

A: Autosomal recessive.

Flashcard 6 Q: What are the three enzymes in the serine biosynthesis pathway from 3-phosphoglycerate?

A: 1) 3-Phosphoglycerate dehydrogenase (3-PGDH).

Phosphoserine aminotransferase (PSAT).

Phosphoserine phosphatase (PSP).

Flashcard 7 Q: Why does 3-PGDH deficiency cause low serine levels in blood and CSF?

A: The first step of serine biosynthesis is blocked, so serine cannot be synthesized de novo. Serine becomes a conditionally essential amino acid.

Flashcard 8 Q: List four reasons why serine is essential for brain development and function.

A: 1) Precursor for glycine (inhibitory neurotransmitter).

Precursor for D-serine (NMDA receptor co-agonist).

Precursor for phosphatidylserine and sphingolipids (myelin/membranes).

Involved in one-carbon metabolism (nucleotide synthesis, methylation).

Flashcard 9 Q: Write the reaction for the conversion of serine to glycine, including the enzyme and coenzyme.

A: Serine + THF → Glycine + N⁵,N¹⁰-Methylene-THF + H₂O. Enzyme: Serine hydroxymethyltransferase (SHMT). Coenzyme: PLP (vitamin B₆).

Flashcard 10 Q: What is the other product of the serine-to-glycine reaction besides glycine?

A: N⁵,N¹⁰-Methylenetetrahydrofolate (N⁵,N¹⁰-methylene-THF) – a one-carbon donor.

Flashcard 11 Q: How does serine deficiency affect glycine levels?

A: Serine is the major precursor for glycine via SHMT. Serine deficiency leads to glycine deficiency.

Flashcard 12 Q: List three metabolic processes that require glycine.

A: 1) Heme synthesis.

Purine synthesis.

Glutathione synthesis.

Flashcard 13 Q: What is the role of serine in one-carbon metabolism?

A: Serine donates its β-carbon to THF via SHMT, producing N⁵,N¹⁰-methylene-THF, which is used for nucleotide synthesis and methylation reactions.

Flashcard 14 Q: Write the reaction for the conversion of serine to serine phosphate.

A: Serine + ATP → Serine phosphate + ADP. Enzyme: Serine kinase (or specific serine phosphorylating enzyme).

Flashcard 15 Q: Write the reaction for the conversion of serine to phosphatidylserine.

A: Serine + Phosphatidylcholine (or CDP-DAG) → Phosphatidylserine + Choline (or CMP). Enzyme: Phosphatidylserine synthase.

Flashcard 16 Q: Which nucleotides require serine-derived one-carbon units?

A: Purines (adenine and guanine) and pyrimidines (thymine) – via the folate cycle using N⁵,N¹⁰-methylene-THF.

Flashcard 17 Q: Write the transamination reaction of serine to hydroxypyruvate, including the enzyme and coenzyme.

A: Serine + α-Ketoglutarate ⇌ Hydroxypyruvate + Glutamate. Enzyme: Serine transaminase (or serine:pyruvate aminotransferase). Coenzyme: PLP.

Flashcard 18 Q: What is the relationship between serine and alanine metabolism?

A: Both are transaminated to their corresponding α-ketoacids (hydroxypyruvate and pyruvate) by PLP-dependent transaminases, connecting amino acid metabolism to gluconeogenesis and energy production.

Flashcard 19 Q: Why might patients with 3-PGDH deficiency have reduced glutathione levels?

A: Glutathione is a tripeptide (γ-Glu-Cys-Gly). Glycine is one of its three constituent amino acids. Serine deficiency → glycine deficiency → impaired glutathione synthesis.

Flashcard 20 Q: What are the three amino acids that make up glutathione, and which one is affected in 3-PGDH deficiency?

A: Glutamate, Cysteine, and Glycine. Glycine is affected (due to serine deficiency reducing glycine production).

Flashcard 21 Q: Name the three cofactors/coenzymes required in the serine biosynthesis pathway.

A: NAD⁺ (step 1), Pyridoxal phosphate / PLP (step 2), and no cofactor for step 3 (PSP).

Flashcard 22 Q: What is the role of phosphatidylserine in the brain?

A: It is a key component of neuronal membranes and myelin; involved in synaptic function, neurotransmission, and cell signaling.

Flashcard 23 Q: What is the role of D-serine in the brain?

A: D-serine is a co-agonist at NMDA (N-methyl-D-aspartate) receptors, which are critical for synaptic plasticity, learning, and memory.

Flashcard 24 Q: What is the role of glycine as a neurotransmitter?

A: Glycine is an inhibitory neurotransmitter in the spinal cord and brainstem (via glycine receptors), and also acts as a co-agonist at NMDA receptors.

Flashcard 25 Q: How does serine contribute to myelin formation?

A: Serine is a precursor for sphingolipids (including sphingomyelin) and phosphatidylserine, which are essential components of myelin sheaths.

Flashcard 26 Q: What is the clinical presentation of 3-PGDH deficiency in a newborn?

A: Microcephaly, intractable seizures, psychomotor retardation, and severe neurological symptoms.

Flashcard 27 Q: Why does 3-PGDH deficiency cause seizures?

A: Due to low glycine (inhibitory neurotransmitter deficiency), low D-serine (NMDA dysfunction), and impaired phospholipid/myelination (structural brain disruption).

Flashcard 28 Q: What is the role of serine in nucleotide synthesis?

A: Serine provides one-carbon units (via N⁵,N¹⁰-methylene-THF) for the synthesis of purine nucleotides and thymidylate (dTMP).

Flashcard 29 Q: Why is serine considered a "conditionally essential" amino acid in 3-PGDH deficiency?

A: Because the body cannot synthesize enough serine due to the enzymatic block, so it must be obtained from the diet to meet physiological demands.

Flashcard 30 Q: What is the biochemical link between serine, glycine, and one-carbon metabolism?

A: Serine → (SHMT, PLP) → Glycine + N⁵,N¹⁰-methylene-THF. The THF derivative donates one-carbon units for nucleotide synthesis, methylation, and amino acid metabolism