Isolation, characterization and chromosomal localization of cDNA clones for the E1 beta subunit of the pyruvate dehydrogenase complex

Pyruvate dehydrogenase complex deficiency and absence of subunit X

The pyruvate dehydrogenase complex (PDHc) is a multienzyme complex consisting of three catalytic and two regulatory enzymes, as well as a less well defined subunit called protein X. PDHc deficiency is a common cause of congenital lactic acidosis. Most patients with PDH deficiency have a mutation in the alpha chain of the PDH E1 enzyme. Very few patients have been described in whom the basic defect of a PDH deficiency is situated in the X protein. We studied a boy with severe lactic acidosis and developmental delay in whom a deficiency of PDH activity led to further investigations. Immunochemical analysis with anti-PDHc antibodies demonstrated an absence of the X component. This report is the fourth family in which an abnormal protein X has been found. In cases with PDH deficiency where no mutation of the PDHE1 alpha gene is found, further investigations by means of immunoblotting with specific antibodies against the different subunits should be performed.

The relationships between transketolase, yeast pyruvate decarboxylase and pyruvate dehydrogenase of the pyruvate dehydrogenase complex

The amino acid sequences of four thiamine pyrophosphate-requiring enzymes were aligned with the published amino acid sequence of the transketolase of Hansenula polymorpha. Sequences of the combined alpha and beta subunits of the E1 enzyme of the pyruvate dehydrogenase complexes of Homo sapiens and Bacillus stearothermophilus aligned well with the transketolase while the E1 of the pyruvate dehydrogenase complex of Escherichia coli aligned easily provided a non-aligning segment of 77 amino acids was omitted. The non-acetylating pyruvate decarboxylase of Saccharomyces cerevisiae could only be aligned if the sequence was cut in two with the C-terminus corresponding to the N-terminus of the other TPP-dependent enzymes. Using the published 2.5 A resolution of the X-ray crystal structure of Saccharomyces cerevisiae transketolase as a template we show that a hydrophobic region of the beta-subunit of the PDH E1 alpha beta enzymes likely contains a binding site for the thiazolium ring of TPP and key motifs are retained in common by all the TPP-dependent enzymes considered, which are essential for catalysis.

Mutation of E1 alpha gene in a female patient with pyruvate dehydrogenase deficiency due to rapid degradation of E1 protein

A mutation of an insertion of 4 bp in the gene for the alpha subunit of pyruvate dehydrogenase (E1 alpha) was found in a female with pyruvate dehydrogenase deficiency due to the rapid degradation of alpha and beta subunit proteins of pyruvate dehydrogenase. This mutation caused a frameshift that altered the amino acid sequence and created a premature stop codon. This 4-bp insertion has been found in an unrelated female patient with E1 alpha deficiency. It is rare that the same mutation is found in unrelated patients with this rare inborn error of metabolism. Furthermore, short deletions or duplications in the E1 alpha gene of patients with E1 alpha deficiency have been found only in exons 10 and 11. These exons may be hot spots for the mutations by the recombinational processes. This patient was heterozygous for the normal and a mutant allele. However, in most of the cultured skin fibroblasts from this patient, the mutant allele was expressed. These observations suggest that the X chromosome containing the normal allele was predominantly inactivated so that she developed lactic acidaemia and neurological abnormalities despite being heterozygous. The mutant alpha subunit protein failed to form a stable structure of pyruvate dehydrogenase, so that both alpha and beta subunit proteins were degraded rapidly.