Humans and other species that cannot synthesize their own vitamin C carry a mutated and ineffective form of the enzyme L-gulonolactone oxidase, the fourth and last step in the ascorbate-producing machinery.
Humans and other organisms incapable of producing their own vitamin C possess an altered form of the L-gulonolactone oxidase enzyme. This enzyme performs the fourth and concluding function in the ascorbate biosynthesis pathway.