What defines a coenzyme?

A coenzyme is defined as an organic substance that aids enzyme function. Coenzymes are typically derived from vitamins and play a crucial role in facilitating biochemical reactions by assisting enzymes in their catalytic activity. They can help in the transfer of electrons, specific chemical groups, or facilitate the conversion of substrates into products, thus enhancing the overall efficiency of enzyme-driven processes.

In contrast, the other options describe different types of substances or roles not directly associated with coenzymes. A vitamin that enhances cellular respiration specifically refers to vitamins used in metabolic pathways, but this does not encompass the full definition of coenzymes, which can aid various enzymatic reactions beyond respiration. Inorganic substances necessary for enzyme reactions are generally referred to as cofactors, which include minerals and metal ions, not organic compounds. Finally, a building block of proteins refers to amino acids, which are essential for protein synthesis but are not related to the role of coenzymes in enzymatic activity.