Lecture Notes on Branched Chain Amino Acid Metabolism and Maple Syrup Urine Disease

Branched Chain Amino Acid Metabolism and Maple Syrup Urine Disease 

Metabolic Pathway of  Branched Chain Amino Acids (Leucine, Isoleucine, and Valine)

Transamination of Branched Chain Amino Acid

- The first step in the catabolism of branched-chain amino acids including leucine, isoleucine, and valine 

- Catalyzed by an enzyme branched-chain amino acid aminotransferase

- Conversion of branched-chain amino acids to respective branched-chain keto acids

- The amino group is transferred to alpha-ketoglutarate to form glutamate

Decarboxylation of Branched Chain Keto Acids

- Catalyzed by enzyme branch chain keto acid dehydrogenase complex (BCKDC)

- BCKDC consists of four different subunits E1-α, E1-β, E2 & E3 

- E1- α2β2 tetramer is a thiamine pyrophosphate dependent decarboxylase encoded by BCKDHA & BCKDHB 

- E2 is dihydrolipoamide branched-chain transacylase encoded by DBT

- E3 is a flavoprotein dihydrolipoamide dehydrogenase encoded by DLD

Multiple enzyme-catalyzed Steps to form Propionyl CoA, Acetyl CoA, and Acetoacetate

- Subsequent Degradative Pathway of valine results in the formation of propionyl CoA

- Subsequent Degradative Pathway of Isoleucine results in the formation of propionyl CoA and acetyl CoA

- Subsequent Degradative Pathway of leucine results in the formation of acetoacetate and acetyl CoA

Figure- Metabolic Pathway of Leucine, Isoleucine, and Valine

Maple Syrup Urine Disorder

 Prevalence 

- MSUD is a rare Genetic Disorder of  leucine, isoleucine, and valine catabolic pathway
- Incidence of 1 in 250,000 live births

- The disease is characterized by the presence of sweet-smelling urine with an order similar to that of maple syrup

Inheritance Pattern 

Autosomal Recessive

Biochemical Basis
- Deficiency of enzyme branched-chain alpha-keto acid dehydrogenase (BCKDC) complex
- Several forms of the disease occur based on the gene affected and the severity of the mutation
- BCKDC consists of four subunits designated as  E1-α, E1-β, E2 & E3

- Mutation of any of these subunits may result in defective catabolism of leucine, isoleucine, and valine 

- Increased branched-chain amino acids in plasma, and keto-acids in blood, CSF & urine 

Pathological Manifestation 
- Infants with MSUD seem normal at birth but within a week, develop lethargy, vomiting, lack of appetite, and signed of failure of thrive

- Neurological manifestation includes severe mental retardation, seizer, acidosis, and hypoglycemia. 

- The intermediate form is also reported and has less severe symptoms.

Diagnosis 
Screening test

- Guthrie Test: B. substilis Inhibition test- use of bacteria to measure the presence of high leucine in a sample. 

- 4-azaleucine is used as an inhibitor for bacterial synthesis of leucine, 

-  bacteria have restricted growth in absence of high leucine in the sample. 
Confirmatory Diagnosis

-  Confirmed by Mutation analysis of the BCKDHA, BCKDHB & DBT gene using sequencing analysis 
Other laboratory features

- Elevated plasma concentration of branched-chain amino acids and alloisoleucine

- Urinary branched-chain keto acids 

Treatment 
Diet

- Leucine restricted diet