Differential effects of two mutations at arginine-234 in the a subunit of human pyruvate dehydrogenase

The most common mutation in the a subunit of the pyruvate dehydrogenase (EI ) component of the human pyruvate dehydrogenase complex (PDQ is arginine-23 4 to glycine and glutamine in 12 and 3 patients, respectively. Interestingl y, these two mutations at the same amino acid position cause E1 (and hence PDQ deficiency by apparently different mechanisms. Recombinant human R234Q E1 had similar V-max (25.7 +/- 4.4 units/mg E1) and apparent K-m (101 +/- 4 nM) values for TPP as recombinant wild-type human El, while units/mg E1) h ad no significant change in V-max (33.6 +/- 4.7 units/mg E1) but had a 7-fo ld increase in its apparent K-m value for TPP (497 +/- 25 nM). Both of the R234 mutant proteins had similar apparent K-m values for pyruvate. Both R23 4Q and R234G mutant proteins displayed similar phosphorylation rates of sit es 1 and 2 by pyruvate dehydrogenase kinase 2 (PDK2) and site 3 by PDK1 com pared to wild-type E1. Phosphorylated R234Q E1, R234G E1, and wild-type E1 also had similar dephosphorylation rates of sites I and 2 by phosphopyruvat e dehydrogenase phosphatase 1. The rate of dephosphorylation of site 3 was about 50% for R234Q E1 and without a significant change for R234G E1 compar ed to the wild type. The data indicate that the patients with the R234G E1 mutation are symptomatic due to a decreased ability of this mutant protein to bind TPP, whereas the patients with the R234Q E1 mutation are symptomati c due to a decreased rate of dephosphorylation of site 3, hence keeping the enzyme in a phosphorylated/inactivated form. (C) 2001 Academic Press.