How can cofactors be bound to proteins?

Cofactors can bind to proteins through both covalent and non-covalent interactions, making the option indicating this flexibility the correct choice.

Covalent binding often involves a strong and stable attachment, usually forming a permanent link between the cofactor and the protein. This is commonly seen in many enzymes where the cofactor is integral to the active site and necessary for enzymatic activity. An example of this would be the covalent attachment of biotin to carboxylase enzymes.

On the other hand, non-covalent interactions include various types of reversible bindings such as hydrogen bonds, ionic bonds, hydrophobic interactions, and van der Waals forces. These interactions allow the cofactor to associate and dissociate as needed, which is particularly important for regulatory molecules that need to turn enzyme activity on or off.

Thus, the ability of cofactors to bind either covalently or non-covalently provides proteins with a broad range of functionality and regulatory mechanisms, which is essential for various biochemical processes.