Category 1
This process converts toxic ammonia into a harmless compound excreted in urine.
Urea cycle
Deficiency of this urea-cycle enzyme causes elevated orotic acid in urine.
→ Ornithine transcarbamylase (OTC)
This amino acid donates one-carbon units to tetrahydrofolate when converted into glycine.
Serine
This activated form of methionine is the universal methyl donor in the body.
→ SAM (S-adenosylmethionine)
These enzymes transfer amino groups from amino acids to α-ketoglutarate, linking amino acid breakdown with ammonia generation.
→ Transaminases (ALT/AST)
A defect in this first enzyme of the urea cycle gives low orotic acid despite high ammonia.
→ CPS-I deficiency
This coenzyme carries one-carbon units for DNA synthesis and amino acid metabolism.
Tetrahydrofolate (THF)
These two vitamins are needed to convert homocysteine back into methionine.
→ Folate & vitamin B₁₂
This enzyme in the mitochondria requires N-acetylglutamate to start ammonia detox.
→ Carbamoyl phosphate synthetase I (CPS-I)
Brain swelling in hyperammonemia occurs because astrocytes accumulate this amino acid.
→ Glutamine
This vitamin deficiency interrupts one-carbon metabolism and leads to megaloblastic anemia.
Folate
This methylation-dependent reaction converts norepinephrine into adrenaline.
→ SAM-dependent methylation
A patient on a high-protein diet shows increased glutamate → α-ketoglutarate cycling. This reflects increased activity of this ammonia-handling reaction.
→ Transamination
This metabolic state during severe infections in children increases muscle protein breakdown
→ hyperammonemia.
→ Catabolic stress
A key enzyme converting serine ↔ glycine requires this vitamin-derived cofactor.
→ Vitamin B6 (PLP)
Deficiency of this enzyme causes phenylketonuria (PKU) and requires BH₄ as a cofactor.
→ Phenylalanine hydroxylase
A 3-day-old newborn with poor feeding and vomiting has very high blood glutamine — the body is using glutamine to trap this toxic compound.
→ Ammonia
A cirrhotic patient develops confusion. Ammonia inhibits the TCA cycle by depleting this key intermediate.
→ α-Ketoglutarate
A pregnant woman with folate deficiency has impaired synthesis of thymidine due to failure of this metabolic system.
→ One-carbon metabolism
A child with fair skin, seizures, musty odor, and developmental delay has this inherited metabolic disorder.
→ PKU Phenylketonuria