In a point mutation, a single base is substituted, inserted, or deleted. This changes the sequence of amino acids in ATP synthase. This changes the tertiary structure of ATP synthase. ATP synthase is used in oxidative phosphorylation in facilitating the diffusion of H+ ions from the inter-membrane space to the mitochondrial matrix, across the inner mitochondrial membrane, where they catalyse the formation of ATP. So the structure of the channel may change, so H+ ions cannot diffuse/less efficiently through it. So less/no ATP is produced.