Human Molecular Genetics, Vol 6, 1457-1464, Copyright © 1997 by Oxford University Press
J Janata, N Kogekar and WA Fenton
L-Methylmalonyl-CoA mutase (MUT) is an adenosylcobalamin (AdoCbl)-
requiring mitochondrial matrix enzyme that catalyzes the isomerization of
L-methylmalonyl-CoA to succinyl-CoA. Inherited defects in the gene encoding
this enzyme result in the mut forms of methylmalonic acidemia. Expression
of mature human MUT cDNA in Escherichia coli at a post- induction
cultivation temperature of 12 degrees C, rather than 37 degrees C, led to
the folding of the majority of the synthesized protein to a soluble form,
with an activity of 0.2-0.3 U/mg protein in the cell-free extract, 10-15
times higher than that in human liver homogenate. Six missense mutations,
producing the amino acid changes G94V, Y231N, R369H, G623R, H678R and
G717V, were detected in MUT cDNA of patients suffering from the mut- form
of methylmalonic acidemia, resulting from defective AdoCbl binding. Two
(G623R and G717V) had been reported in other patients. Three (G94V, Y231N
and R369H) are the first changes in the NH2-terminal part of the enzyme
reported to cause the mut- phenotype. Enzymes with the mutations were
individually expressed, and their kinetic parameters were generally in
accord with published biochemical data from extracts of fibroblasts from
these patients. The mutations increased the K(m) for AdoCbl by 40- to
900-fold, while V(max) values varied from 0.2% to nearly 100% of that of
wild-type protein. In one case of a doubly heterozygous cell line, however,
neither of the constituent mutant enzymes had a K(m) corresponding to the
lower of the two estimated from the extract data. This finding may reflect
the natural occurrence of interallelic complementation in vivo in this cell
line.
ARTICLES
Expression and kinetic characterization of methylmalonyl-CoA mutase from patients with the mut- phenotype: evidence for naturally occurring interallelic complementation
Department of Genetics, Yale School of Medicine, New Haven, CT 06510, USA.
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