Glycine disorders, also known as glycine encephalopathy or nonketotic hyperglycinemia (NKH), are a group of rare neurometabolic disorders where glycine, an amino acid, accumulates in the body, particularly in the brain, due to a defect in glycine metabolism
- Primary Glycine Disorder: Nonketotic hyperglycinemia (NKH), also known as glycine encephalopathy
- Caused by deficiency of the glycine cleavage enzyme
- Leads to accumulation of glycine in the body
- All forms result in cerebral dysfunction
- Function of the Glycine Cleavage Enzyme:
- Breaks glycine into carbon dioxide and ammonia
- Transfers a methyl group to tetrahydrofolate → forms methylenetetrahydrofolate
- Consists of four subunits:
- P-protein: Pyridoxal-phosphate dependent
- H-protein: Hydrogen carrier, requires lipoylation
- T-protein: Requires tetrahydrofolate
- L-protein: Lipoamide dehydrogenase
- Secondary Enzyme Deficiencies:
- Disorders affecting pyridoxal-phosphate (e.g., PNPO deficiency) can cause secondary NKH
- Disorders in protein lipoylation result in variant forms of NKH
- Neonatal Presentation of NKH:
- Typically occurs in the first week of life
- Symptoms include:
- Lethargy and poor feeding
- Progression to coma
- Frequent hiccupping and seizures
- Severe hypotonia
- Characteristic burst suppression pattern on EEG
- Spontaneous recovery of respiratory function typically occurs within 3 weeks
- Infantile Presentation of NKH:
- Symptoms include:
- Hypotonia
- Lethargy
- Seizures (spasms or myoclonic)
- EEG findings:
- Multifocal epilepsy
- Hypsarrhythmia
- Information
- Published:22 June 2025
- Modified: 22 June 2025