2. A mutation in a mitochondrial gene encoding a component of the ATP synthase h
ID: 303161 • Letter: 2
Question
2. A mutation in a mitochondrial gene encoding a component of the ATP synthase has been identified. Individuals who have this mutation suffer from muscle weakness, ataxia and retinitis pigmentosa. A tissue biopsy was performed on each of three patients suffering from this condition, and submitochondrial particles (SMPs) competent in succinate-driven ATP synthesis (albeit at rates markedly lower than normal 'control' individuals) were isolated. The activity of the ATP synthase associated with these SMPs and the same type of membrane preparation prepared from normal individuals ('Controls') was measured on the addition of succinate in media containing an excess of ADP, Pi, NAD+ and FAD and O2, and the results shown below were obtained: ATP synthase activity (nmol ATP formed/min/mg) 3.00 0.25 0.22 0.17 Source of SMPs Controls Patient 1 Patient 2 Patient 3 A. B. What is being measured and what was the purpose of the addition of succinate? what is the effect of the mutation on the succinate-coupled synthesis of ATP (if all of the SMP preparations assayed - both those from controls and those from patients were capable of similar rates of succinate-dependent electron transport)? Next the ATPase activity of the enzyme was measured by incubating the SMPs with ATP in the absence of succinate, ATP hydrolysis (nmol ATP hydrolyzed/min/mg) Source of SMPs Controls Patient 1 Patient 2 Patient 3 30 25 31Explanation / Answer
A. Effect on ATP synthesis was measured. Succinate is added because it is oxidized by the complex 2 (succinate dehydrogenase). The resulting electrons are transfered via FADH2 and a group of Fe-S proteins into the ubiquinone pool. This complex does not pump electrons.
B. The effect of mutation on succinate dependent ATP synthesis is reduced . If both the controls and the patients show similiar rate of electron transport then the mutation has no effect on succinate dependent ATP Synthesis.
C. The ATPase activity of the enzyme is measured .Here succinate is omitted because here we are measuring ATP hydrolysis if we add succinate in presence of ATP then there would be no reaction due to respiratory control.
D. The mutation has only little or no effect on the ability of ATP hydrolysis enzyme.
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