Deficiency of enzyme α–keto acid decarboxylase leading to a block in the metabolism of branch chain amino acids is observed in –
Maple Syrup urine disease 

It is an inherited (autosomal recessive) disorder of branched-chain amino acid i.e. - Mine, Leucine and Isoleucine.
Maple syrup urine disease (MSUD) is d/t defect in enzyme - α-keto acid dehydrogenase.
It is a mitochondrial, enzyme complex consisting of
α-ketoacid decarboxylase
Dihydrolipoyl dehydrogenase and    
Transacylase               
The early steps in the metabolism of these three amino-acids are similar. One of the steps - decarboxylation is accomplished by a complex enzyme system i.e. branched chain α-keto acid dehydrogenase using thiamine pyrophosphate (vitamin B1) as a coenzyme. Deficiency of this enzyme system causes the disease, named after the sweet odour of maple syrup found in the body fluids - urine, sweat etc.

Clinical features

Affected infants are initially normal at birth but develop poor feeding and vomiting during the Pt week of life; lethargy and coma may ensue within a few days.
Physical examination reveals hypertonicity and muscular rigidity with opisthotonos (spasm in which the body forms a bow like a figure)
Periods of hypertonicity may alternate with bouts of flaccidity.
Convulsion occurs in most infants, and hypoglycemia is common (However in contrast to most hypoglycemic states, correction of the blood glucose level does not improve the clinical condition).
Maple syrup urine disease
Hartnup's disease
Alkaptonuria
Phenylketonuria