Defects of Electron Transfer Flavoprotein and Electron Transfer Flavoprotein-Ubiquinone Oxidoreductase: Glutaric Acidemia Type II

ABSTRACT

• Electron transfer flavoprotein (ETF) and ETF-ubiquinone oxidoreductase (ETF-QO) are nuclear encoded proteins through which electrons from flavoprotein acyl CoA dehydrogenases, dimethylglycine dehydrogenase, and sarcosine dehydrogenase enter ubiquinone in the respiratory chain. Inherited defects of either protein cause glutaric acidemia type II.

• Glutaric acidemia type II is characterized clinically by hypoketotic hypoglycemia and metabolic acidosis; pathologically by fatty infiltration of the liver, heart, and kidneys; and biochemically by a diagnostic organic aciduria. Complete enzyme defects, especially of ETF-QO, are often associated with multiple congenital anomalies, including renal cystic dysplasia, and death in infancy.

• Primary defects of ETF-QO and those of either ETF subunit are inherited as autosomal recessive traits. Several pathogenic mutations have been identified in the genes for ETF-QO and the α-ETF subunit. No single ETF-QO mutation is common, but of six that have been identified in the α-ETF gene, αT266M may account for about 40 percent of mutant alleles.

• Prenatal diagnosis of glutaric acidemia type II is possible in some cases by demonstrating increased concentrations of glutaric acid in amniotic fluid, acylcarnitine esters in maternal urine, or impaired substrate oxidation by cultured amniocytes.

• There is no effective treatment for glutaric acidemia type II patients who present in early infancy. Treatment with riboflavin, glycine, and L-carnitine and diets restricted in fat and protein may be effective in less severely affected patients.

BIOCHEMISTRY OF ELECTRON TRANSFER FLAVOPROTEIN (ETF) AND ETF-UBIQUINONE OXIDOREDUCTASE (ETF-QO)

Electron transfer flavoproteins (ETFs) in mammalian mitochondria and some bacteria act as intermediary electron carriers between primary flavoprotein dehydrogenases and terminal respiratory chains.^1-3 In mammals, ETF in the mitochondrial matrix serves as the electron acceptor for at least nine flavoprotein dehydrogenases and is reoxidized by ETF-ubiquinone oxidoreductase (ETF-QO) in the inner mitochondrial membrane.^4-6 ETF-QO in turn reduces ubiquinone, which communicates with the ubiquinone pool of the main respiratory chain.⁷ The ETF/ETF-QO system may thus be viewed as a branch of the electron transport system, with separate input sites for seven acyl CoA dehydrogenases and two N-methyl dehydrogenases.^8-14

Electron Transfer Flavoprotein

Mammalian ETFs exist in the mitochondrial matrix as heterodimers of alpha- (30-kDa) and beta- (28-kDa) subunits.^15-19 The heterodimer contains a noncovalently bound flavin adenine dinucleotide (FAD) redox cofactor and an equivalent of noncovalently bound adenosine 5′-monophosphate. The latter is associated with the β-subunit and is apparently required for protein folding and/or dimerization.^20-23

The cDNAs encoding both human subunits have been cloned^24,25 and co-expressed (from a single vector) in Escherichia coli to yield active human ETF.¹⁸ The expressed human protein has been crystallized, and its structure has been resolved to 2.1 Å.^26,27 The α-subunit (GenBank accession number J04058) is synthesized as a 35-kDa precursor that is imported into mitochondria and processed to yield the mature 30-kDa subunit.²⁸ The β-subunit (GenBank accession number X71129) has no cleavable presequence but contains a sequence in ...