Ascorbate as both antioxidant and enzyme cofactor
Vitamin C (ascorbate) works in two related ways. As an antioxidant it donates electrons to neutralise reactive oxygen species, and it can regenerate other antioxidants such as the tocopheroxyl radical form of vitamin E, returning it to an active state. As an enzyme cofactor it keeps the iron or copper centres of certain dioxygenases in their reduced, catalytically active form, which is the chemistry behind its more specific roles.
Those cofactor roles include prolyl and lysyl hydroxylases in collagen synthesis (without adequate ascorbate the collagen triple helix is unstable, the basis of scurvy), dopamine beta-hydroxylase in catecholamine synthesis, and enzymes in carnitine synthesis needed for fatty acid transport into mitochondria.
Saturation kinetics and what megadoses actually do
Plasma vitamin C is tightly regulated. Intestinal absorption and renal reabsorption are saturable, so above modest intakes a rising fraction of an oral dose is simply excreted and plasma levels plateau. This pharmacology means very large oral doses do not produce proportionally higher blood concentrations, and claims that megadoses cure common illnesses are not well supported by controlled trials. Routine high intake is generally tolerated but can cause gastrointestinal upset and, in susceptible people, raise the risk of oxalate kidney stones.
This is general educational information, not medical advice. Consult a qualified healthcare professional before taking high-dose vitamin C, particularly with kidney disease or a history of stones.