Biochemical characterization of a variant human medium-chain acyl-CoA dehydrogenase with a disease-associated mutation localized in the active site

Medium-chain acyl-CoA dehydrogenase (MCADH) deficiency, an autosomal recess ive inherited disorder, is the most common genetic disorder in mitochondria l beta-oxidation in humans. In addition to one prevalent disease-causing mu tation (K304E), a series of rarer mutations has been reported, but none of these has yet been characterized in detail. We report here on the biochemic al characterization of the purified recombinant mutant protein in which thr eonine is replaced by alanine at position 168 of the mature protein (T168A- MCADH). It is the first mutation to be found in patients that is located in the active site of the enzyme. Thr-168 is hydrogen-bonded to the flavin N( 5) of the cofactor FAD. The thermostability of T168A-MCADH is markedly decr eased compared with human wild-type MCADH (hwt-MCADH). Catalytic activity w ith ferricenium as acceptor is lowered by 80 % and with the natural accepto r electron-transferring flavoprotein by over 90 %, compared with hwt-MCADH. In the mutant the extent of flavin semiquinone formed on reduction is appr ox. 50 % that of hwt-MCADH. The pK-reflected by the pH-dependence of V-max is shifted from approx. 8.2 (hwt-MCADH) to approx. 7 (T168A-MCADH) and the rates of enzyme flavin reduction (stopped-flow measurements) are only appro x. 1/10 those of the parent enzyme. These properties are discussed in the l ight of the possible mechanisms leading to disease in humans.