Vitamins are substances essential for the functioning of certain proteins (cofactors). For example, vitamin K is essential for endowing proteins with the ability to bind calcium ions. Vitamin C (ascorbate) is a reducing agent often acting to keep the coordinated metal ions of enzymes in their proper state of oxidation. Scurvy, vitamin C deficiency, for example, is caused when there is insufficient ascorbate to maintain the iron atom of prolyl hydroxylate in the ferrous state (prolyl hydroxylase carries out the hydroxylation of collagen, which gives the fibers their strength).