Which is not a constituent of enzyme cafactor?

Manganese, copper, and zinc are well-established enzyme cofactors found in metalloenzymes - Mn in superoxide dismutase and Mn-catalase, Cu in cytochrome c oxidase and plastocyanin, Zn in alcohol dehydrogenase and carbonic anhydrase. Boron, while an essential micronutrient for plants (important for cell wall structure), functions primarily through structural roles in cell walls and membrane function rather than as an enzyme cofactor.