Structure-function relationships in dihydrolipoamide acyltransferases

Altered phosphorylation state of branched-chain 2-oxo acid dehydrogenase in a branched-chain acyltransferase deficient human fibroblast cell line

The abundance and phosphorylation state of the polypeptide constituents of the human branched-chain 2-oxo acid dehydrogenase complex were examined in mitochondria from normal and maple syrup urine disease (MSUD) fibroblasts. In normal fibroblast mitochondria two forms of the E1 alpha subunit were observed: non-phosphorylated (E1 alpha) and phosphorylated (E1 alpha-P). About 40-50% of E1 alpha was present as E1 alpha-P. The ability to quantitate the two forms of E1 alpha permitted examination of the association between decreased capacity of oxidize branched-chain 2-oxo acids and the phosphorylation state of E1 alpha. Changes in phosphorylation state of E1 alpha were observed in MSUD fibroblasts as compared to control cells. Of particular interest was the absence of E1 alpha-P in an MSUD fibroblast line which lacked the dihydrolipoyl acyltransferase (E2) subunit of the dehydrogenase complex. In two MSUD cell lines deficient in E1 alpha, the abundance of E1 alpha-P appeared to be preferentially reduced. A fourth MSUD cell line contained normal quantities of E3, E2 and both forms of the E1 alpha polypeptide. Our results indicate that alterations in the abundance of dehydrogenase complex polypeptides in MSUD fibroblasts may influence the phosphorylation state of the E1 alpha polypeptide. They demonstrate the potential for examining simultaneously mutations which affect both the catalytic and regulatory components of the dehydrogenase complex.

Overexpression and mutagenesis of the catalytic domain of dihydrolipoamide acetyltransferase from Saccharomyces cerevisiae

The inner core domain (residues approximately 221-454) of the dihydrolipoamide acetyltransferase component (E2P) of the pyruvate dehydrogenase complex from Saccharomyces cerevisiae has been overexpressed in Escherichia coli strain JM105 via the expression vector pKK233-2. The truncated E2p was purified to apparent homogeneity. It exhibited catalytic activity (acetyl transfer from [1-14C]acetyl-CoA to dihydrolipoamide) very similar to that of wild-type E2p. The appearance of the truncated and wild-type E2p was also very similar, as observed by negative-stain electron microscopy, namely, a pentagonal dodecahedron. These findings demonstrate that the active site of E2p from S. cerevisiae resides in the inner core domain, i.e., catalytic domain, and that this domain alone can undergo self-assembly. The purified truncated E2p showed a tendency to aggregate. Aggregation was prevented by genetically engineered attachment of the interdomain linker segment (residues approximately 181-220) to the catalytic domain. All dihydrolipoamide acyltransferases contain the sequence His-Xaa-Xaa-Xaa-Asp-Gly near their carboxyl termini. By analogy with chloramphenicol acetyltransferase, the highly conserved His and Asp residues were postulated to be involved in the catalytic mechanism [Guest, J. R. (1987) FEMS Microbiol. Lett. 44, 417-422]. Substitution of the sole His residue in the S. cerevisiae truncated E2p, His-427, by Asn or Ala by site-directed mutagenesis did not have a significant effect on the kcat or Km values of the truncated E2p. However, the Asp-431----Asn, Ala, or Glu substitutions resulted in a 16-, 24-, and 3.7-fold reduction, respectively, in kcat, with little change in Km values.(ABSTRACT TRUNCATED AT 250 WORDS)