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a co-factor is a non-protein “helper molecule” that a protein needs in order to be functional. Cofactors can be inorganic (non-hydrocarbon-based) such as metal ions or organic (carbohydron-based). When the cofactor is a metal, we call the metal-protein combo a metalloprotein – or a metalloenzyme if the protein is an enzyme. Organic cofactors are often based on vitamins and/or nucleotides. Some cofactors are participate directly in reactions – an example is NADH, which is important for electron transfer in cellular respiration. Other cofactors are have more “structural” roles – they’re needed for the protein to stay in its active shape, but they don’t participate directly in a reaction. Cofactors are often required for enzymes to be active – though not all enzymes need cofactors and some non-enzyme proteins also use cofactors (such as the heme groups in hemoglobin). An enzyme that requires a cofactor is referred to as an apoenzyme when it doesn’t have its cofactor bound and a holoenzyme when the cofactor is bound. Some cofactors are loosely-bound and come and go, switching the protein from inactive to active forms; we call such cofactors coenzymes when they’re cofactors for enzymes. Other cofactors are bound super duper tightly so they’re practically permanently stuck; we call these prosthetic groups and an example is the heme in hemoglobin.