$61.$ Succinyl$-$CoA synthetase is a dimer of an and a subunit. A single gene encodes the subunit protein. Two genes code for two different subunit proteins. One subunit, which is specific for ADP, is expressed in $$catabolic tissues$$ such as brain and muscle, whereas the other subunit, which is specific for GDP$,$ is expressed in $$anabolic tissues$$ such as liver and kidney. Propose a hypothesis to explain this observation.

$62.$ Individuals with a mutation in the gene for the subunit of succinyl$-$CoA synthetase $($see Problem $61)$ experience severe lactic acidosis and usually die within a few days of birth, but individuals with a mutation in the gene for the ADP$-$specific subunit of the enzyme experience only moderately elevated concentrations of lac$-$ tate and usually survive to their early twenties. Why is the prognosis for patients with the subunit mutation better than for patients with the subunit mutation?