Classic Severe Maple Syrup Urine
Disease
Albert Yu
Introduction
• Maple Syrup Urine Disease, also known as
Branched-Chain Ketoaciduria, is a metabolic
disease inherited in an autosomal recessive
pattern1.
• Victims of Maple Syrup Urine Disease are
often diagnosed as infants or young children
at the very latest2.
Symptoms
•
•
•
•
•
•
•
•
Poor Feeding
Lethargy
Vomiting
Poor Weight Gain
Maple Syrup Scented Urine
Muscle Spasms/Seizures
Brain Damage and Coma
Death2
Normal Branched Chain Amino Acid
Metabolism
• Branched Chain Amino Acids (Valine, Leucine,
and Isoleucine) are essential in our diets since
they cannot be synthesized by the body.
• Excess Branched Chain Amino Acids (BCAA’s)
are metabolized and used for ATP generation3.
Normal Branched Chain Amino Acid
Metabolism (cont.)
• Catabolism of BCAA’s has two major steps:
– Transfer of an amino group from the amino acid to
α-ketoglutarate to form branched chain α-keto
acids.
– Oxidation of α-keto acids by branched chain
amino acid dehydrogenase complex to form CoA
derivatives4.
Branched Chain Amino Acid
Dehydrogenase Complex (BCKD)
• A complex of enzymes that forms Acyl-CoA
derivatives from α-keto acids.
– E1- α-ketoacid Dehydrogenase
– E2- Dihydrolipoyl Transacylase
– E3- Dihydrolipoamid Dehydrogenase
• Ranges in molecular mass from 4 to 10 million
daltons, depending on source and type5.
Maple Syrup Urine Disease- On the
Molecular Level
• Failure to properly catabolize excess BCAA’s
causes toxic build-ups of BCAA’s and their αketoacid derivatives2.
• The NCBI has recognizes four major genes that, if
mutated, produce Maple Syrup Urine Disease:
–
–
–
–
BCKDHA
BCKDHB
DBT
DLD6
Structural Causes of the Disease
• Any mutations resulting in structural
anomalies that result in inhibition of catalytic
functions or proper assembly of the BCKD
complex result in Maple Syrup Urine Disease6.
BCKDHA
• Gene encodes the E1 alpha subunit of the
BCKD complex.
• The E1 alpha subunit forms a heterotetramer
with the beta subunit to form the E1 catalytic
subunit of the BCKD complex.
• The E1 subunit catalyzes the decarboxylation
of branched chain α-ketoacids7.
E1 α Subunit (1DTW)
BCKDHB
• Gene encodes the E1 beta subunit of the
BCKD complex8.
E1 β Subunit (1DTW)
E1 Subunit (1DTW)
α Subunit
β Subunit
BCKDHA Mutation
• Mutations in the BCKDHA gene produce
mutant E1 α subunits.
• Mutant E1 α subunits typically show
drastically reduced assembly kinetics, inability
to form proper tetramers, improper folding,
and no enzymatic activity7.
BCKDHA Mutation- Y393N Mutation
• Caused by an A-T Transversion in the BCKDHA gene.
• Y393N mutation is most prevalent in the Old Order
Mennonite population.
• Mutation affects the assembly kinetics of the E1 subunit; it
takes over two hours to assemble the same amount of
Y393N mutant E1 as wild-type E1 forms in 10 minutes.
• Y393 mutant E1 also forms dimers instead of the normal
tetramer, thus completely abolishing enzyme activity7.
• Since the dimers are α-subunit dimers, Y393 is proposed to
be integral to proper association of the α-subunit to the βsubunit13.
α-Subunits
Y393
α-Subunits
β-Subunits
Y393
F324
Y393
W330
BCKDHB Mutation
• Mutant β subunits demonstrate improper
folding, impaired assembly with α subunits,
severely inhibited cofactor association, and
mitochondrial targeting mutations8.
BCKDHB Mutation- H156Y
• C to T transition in the BCKDHB gene results in
Histidine being replaced by Tyrosine at the 156th
residue.
• Produced polypeptides are expressed at 18% of the
wild-type. Produced polypeptides are show assembly
defects, forming dimers, trimers, or tetramers.
• Produced β subunits also show drastically reduced
affinity for thiamine, an essential cofactor8.
• Mutation introduces structural changes by interfering
with hydrogen bonding of Histidine 156 and Threonine
28414.
α-subunit
Thiamine
Diphosphate
β-subunit
H156
T284
Thiamine
Diphosphate
β-subunit
H156
T284
Thiamine
Diphosphate
β-subunit
Dihydrolipoamide Branched-Chain
Transacylase (DBT)
• Gene codes for the E2 subunit
(Dihydrolipoamide Branched-Chain
Transacylase) of BCKD.
• Catalyzes the transfer of an acyl-group from Sacyldihydrolipoamide to CoA to form an AcylCoA.
• Forms a homo-24-meric complex9.
E2 Monomer (2IHW)
E2 Trimer (2IHW)
E2 Complex (2IHW)
DBT Mutation
• Majority of DBT Mutations affect the
sequence of the produced E2 subunit. These
mutations include frameshift mutations,
deletions, and insertions9.
DBT Mutation- Single Guanine Base
Deletion
• A single Guanine base is deleted from an
intron-exon junction resulting in the inability
of splicing machinery to recognize the exon.
• Ultimately, a 78 base pair exon is excluded
from the mature mRNA transcript, resulting in
a nonfunctional BCKD Complex12.
Deleted
Region
E2 Trimer
Deleted
Region
E2 Trimer
No Interactions
with Amino
Terminal Tail
E2 Trimer
Dihydrolipoamide Dehydrogenase
(DLD)
• Gene codes for an enzyme that degrades
lipoamide and produces dihydrolipoamide.
• In the BCKD Complex, DLD processes one of
the by-products of Acyl-CoA formation from
branched chain amino acids10.
• Two E3 monomers dimerize to form the
dimeric catalytic subunit E311.
E3 Monomer
E3 Catalytic Subunit
DLD Mutation
• Mutations in the E3 subunit that cause MSUD
affect the enzyme’s ability to catalyze
lipoamide degradation by altering the active
site, cofactor binding, or dimer association10.
DLD Mutation- K37E and P453L
Mutations
• The two mutations resulted in the altered
active site of the E3 subunit and inhibited the
enzyme’s ability to bind FAD, an essential
cofactor for the enzyme’s function10.
P453
K37
P453
K37
FAD
K37
FAD
P453
Social Impact
• Maple Syrup Disease is most common in
populations with limited gene pools.
• If not properly treated, victims show low IQ’s
and mental disabilities.
• Treatment involves dietary restrictions, which,
under the proper administration, allow MSUD
patients to live full and productive lives2.
Works Cited
1.
2.
3.
4.
5.
6.
7.
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2004: n. pag. Web. 25 Mar 2010. <About.com Health's Disease and Condition>.
King, Micheal. "Introduction to Amino Acid Metabolism." The Medical Biochemistry Page. Indiana University
School of Medicine, 24 Mar 2010. Web. 29 Mar 2010. <http://themedicalbiochemistrypage.org/amino-acidmetabolism.html#valine>.
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Works Cited (cont.)
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11.
12.
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14.
"BRANCHED-CHAIN KETO ACID DEHYDROGENASE E1, BETA POLYPEPTIDE; BCKDHB." Online Mendelian
Inheritance in Man. Online Mendelian Inheritance in Man, 28 Jun 2007. Web. 30 Mar 2010.
<http://www.ncbi.nlm.nih.gov/entrez/dispomim.cgi?id=248611>.
"DIHYDROLIPOAMIDE BRANCHED-CHAIN TRANSACYLASE; DBT." Online Mendelian Inheritance in Man. Online
Mendelian Inheritance in Man, 17 Sep 2009. Web. 30 Mar 2010.
<http://www.ncbi.nlm.nih.gov/entrez/dispomim.cgi?id=248610>.
"DIHYDROLIPOAMIDE DEHYDROGENASE; DLD." Online Mendelian Inheritance in Man. Online Mendelian
Inheritance in Man, 13 Jun 2005. Web. 30 Mar 2010.
<http://www.ncbi.nlm.nih.gov/entrez/dispomim.cgi?id=238331>.
Brautigam, Chad. "Crystal Structure of Human Dihydrolipoamide Dehydrogenase: NAD+/NADH Binding and the
Structural Basis of Disease-causing Mutations." Journal of Molecular Biology 350.3 (2005): 543-52. Web. 30 Mar
2010. <http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WK7-4G7X5D71&_user=526750&_coverDate=07/15/2005&_rdoc=1&_fmt=high&_orig=search&_sort=d&_docanchor=&view=
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Mitsubuchi, H. "Maple syrup urine disease caused by a partial deletion in the inner E2 core domain of the
branched chain alpha-keto acid dehydrogenase complex due to aberrant splicing. A single base deletion at a 5'splice donor site of an intron of the E2 gene disrupts the consensus sequence in this region.." Journal of Clinical
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<http://www.ncbi.nlm.nih.gov/pmc/articles/PMC295137/?tool=pubmed>.
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